Biomechanical and Physiological Comparison Between a Conventional Cyclist and a Paralympic Cyclist with an Optimized Transtibial Prosthesis Design

Abstract

Background/Objectives: This study aimed to identify the functional adaptations that enable competitive performance in a Paralympic cyclist with optimized bilateral transtibial prostheses compared to a conventional cyclist. Additionally, it describes the development of the prosthesis, designed through a user-centered engineering process incorporating Quality Function Deployment (QFD), Computer-Aided Design (CAD), Finite Element Analysis (FEA), Computational Fluid Dynamics (CFD), and topological optimization, with the final design (Design 1.4) achieving optimal structural integrity, aerodynamic efficiency, and anatomical fit. Methods: Both athletes performed a progressive cycling test with 50-watt increments every three minutes until exhaustion. Cardiorespiratory metrics, lactate thresholds, and joint kinematics were assessed. Results: Although the conventional cyclist demonstrated higher Maximal Oxygen Uptake (VO₂max) and anaerobic threshold, the Paralympic cyclist exceeded 120% of his predicted VO₂max, had a higher Respiratory Exchange Ratio (RER) [1.32 vs. 1.11], and displayed greater joint ranges of motion with lower trunk angular variability. Lactate thresholds were similar between athletes. Conclusions: These findings illustrate, in this specific case, that despite lower aerobic capacity, the Paralympic cyclist achieved comparable performance through efficient biomechanical and physiological adaptations. Integrating advanced prosthetic design with individualized evaluation appears essential to optimizing performance in elite adaptive cycling.

1. Introduction

Cycling performance in athletes with bilateral transtibial amputation poses unique biomechanical and physiological challenges due to the absence of native ankle and foot joints. Prostheses used in this context significantly influence pedaling kinematics, joint load distribution, and metabolic efficiency, often requiring neuromuscular and cardiorespiratory adaptations to maintain competitive performance levels [1,2,3]. Despite growing interest in adaptive sports, few studies have directly compared the performance profiles of Paralympic and conventional cyclists under standardized laboratory conditions.

Previous research has emphasized the role of prosthetic design in influencing motion economy and energy expenditure, highlighting the need for tailored engineering solutions that accommodate both anatomical and performance demands [4,5]. These factors are extensively discussed in the literature on cycling performance [6]. Yet, controversies remain regarding the extent to which advanced prostheses can compensate for the absence of active musculature, and whether high-level para-athletes can achieve physiological profiles comparable to their able-bodied peers [7].

This study contributes to this field by comparing the physiological and biomechanical performance of a Paralympic cyclist using optimized bilateral transtibial prostheses with that of a conventional cyclist, under a controlled incremental workload protocol. In parallel, it presents the engineering and clinical development of the transtibial prosthesis (Design 1.4) used by the Paralympic athlete. This design was based on user-centered principles and advanced simulation techniques, including Quality Function Deployment (QFD), Computer-Aided Design (CAD), Finite Element Analysis (FEA), Computational Fluid Dynamics (CFD), and topological optimization [8,9,10].

The findings reveal that, although the conventional cyclist demonstrated superior aerobic capacity and movement symmetry, the Paralympic cyclist achieved comparable workloads, higher joint mobility, and effective postural control. These results underscore the potential of combining individualized prosthetic design with tailored physiological training to close the performance gap in elite-level adaptive cycling.

Therefore, this study aims to assess the physiological and biomechanical differences between a Paralympic and a conventional cyclist under standardized conditions, while highlighting the impact of optimized prosthetic design.

2. Materials and Methods

This study aimed to compare the physiological and biomechanical performance of a Paralympic cyclist with optimized bilateral transtibial prostheses and a conventional cyclist, in order to identify functional adaptations that support competitive performance and inform prosthetic optimization strategies. Additionally, the study presents the design and validation process of the prosthesis used by the Paralympic athlete, integrating engineering and clinical reasoning. To achieve this, a comparative case study design with a quantitative and biomechanical approach was adopted, enabling in-depth characterization of individual physiological and kinematic responses under controlled experimental conditions.

This exploratory case-comparative design is appropriate for detailed profiling of elite athletes with unique physiological and biomechanical characteristics and is commonly used for high-resolution comparisons in specialized athletic populations [7].

2.1. Development and Evaluation of the Optimized Transtibial Prosthesis

The transtibial prosthesis employed by the Paralympic cyclist (Design 1.4) was developed through a user-centered engineering framework. Quality Function Deployment (QFD) was first used to establish functional modules and design requirements [1]. Three conceptual sockets were digitally modeled using Computer-Aided Design (CAD) software. These were structurally assessed using Finite Element Analysis (FEA) in ANSYS Workbench^® (Version 2023 R1, ANSYS Inc., Canonsburg, PA, USA).evaluating mechanical performance under simulated pedaling forces, including stress distribution, buckling, fatigue resistance, and drop–impact response [5]. Simultaneously, Computational Fluid Dynamics (CFD) simulations were performed to examine aerodynamic behavior. Topological optimization was implemented to reduce structural mass without compromising stiffness [3]. Design 1.4 exhibited optimal stress dissipation, aerodynamic efficiency, and anatomical conformity. Modal analysis confirmed its natural frequencies were outside the typical pedaling cadence range (90–120 rpm), minimizing resonance risk. Drop–impact simulations indicated deformations under 1.6 mm, validating the prosthesis’s energy absorption capacity and impact tolerance.

The prosthesis was entirely designed, developed, and optimized by the research team, specifically tailored to the characteristics of the Paralympic cyclist who participated in the experiments. The design process followed a structured engineering approach, consisting of four main stages: conceptual design, preliminary design, detailed design, and CAD modeling. In the conceptual stage, the user’s needs were identified, the scope of the research was defined, the product architecture was outlined, and initial sketches were created along with statistical analyses. In the preliminary design phase, a review of the state of the art on aerodynamic profiles and materials was conducted, the everyday training scenario of the Paralympic cyclist was defined, 3D scans of the endo-socket were performed, and the socket geometry was established. The detailed design stage included analyses of the cyclist’s kinematic behavior during training, as well as evaluation of mechanical parameters of the crank link, crank rotation angle, force distribution, kinematic modeling of the cyclist, and dynamic modeling of the transtibial prosthesis. The final design was rigid at the ankle joint, fully customized to the athlete’s anthropometric measurements, optimized for competitive track cycling, and specifically tailored to his pre-competitive training phase to enhance performance during the subsequent competitive period.

Following these stages, four CAD versions (1.1, 1.2, 1.3, and 1.4) were developed and evaluated using FEA. The evaluations included static, frequency, topology, buckling, fatigue, drop, and aerodynamic performance analyses. Among all prototypes, CAD 1.4 demonstrated superior mechanical, aerodynamic, and anatomical performance and was therefore selected as the final optimized design. The prostheses were designed to be rigid at the ankle joint, as the athlete presented with bilateral transtibial amputations, functionally preserving the hip and knee joints, while replacing the ankle–foot complex. This configuration made it possible to assess the kinematic and dynamic behavior of the prostheses compared to the equivalent joints of a conventional cyclist, with the aim of determining to what extent the prosthetic limbs could approximate the biomechanics and physiology of human extremities.

2.2. Physiological and Biomechanical Evaluation

Both athletes performed a standardized incremental cycling test on a Cyclus 2 ergometer. The protocol began at 100 W with increases of 50 W every 3 min until volitional exhaustion. Environmental conditions were held constant (23.6 °C, 2600 mbar, 68% RH). Cardiopulmonary data were recorded with the Metamax 3B system (version 3.1.96) and processed using the Wasserman algorithm [11]. Interpretation of exercise test results followed established principles of cardiopulmonary exercise testing [12]. The test followed Astrand’s progressive model and was terminated upon fatigue, withdrawal, or clinical indication of risk [2].

2.3. Motion Capture and Lactate Testing

Kinematic data were collected using eight optoelectronic cameras (100 Hz) and two VIXTA cameras (25 Hz). Reflective markers were positioned on standard anatomical landmarks (trunk, hip, knee, ankle) and processed in BTS Workstation software [8]. Joint angles and angular variability were analyzed across all test stages.

Capillary lactate samples (0.7 µL) were obtained at three time points: rest, ventilatory threshold, and immediately after exercise. This allowed determination of the anaerobic threshold [10].

2.4. Data Analysis

All data were processed using RStudio (version 2023.06). Descriptive statistics were applied, including mean, standard deviation, and absolute and relative differences. These statistical descriptors align with standard practices in physical fitness testing and prescription [13]. Due to the limited sample size (n = 2), no inferential analyses (e.g., t-tests or effect size estimates) were conducted, which is acknowledged as a methodological limitation. Interpretation focused on individual performance profiles and standardized comparisons under equal conditions. It is also acknowledged that individual factors such as training history, muscle fiber composition, and adaptation time to prosthetic use were not controlled and may have influenced the observed outcomes.

3. Results

The results of this study are presented in two analytical moments. The first focuses on the Comparative Physiological and Ventilatory Performance in a Paralympic and a Conventional Cyclist, addressing variables such as maximal oxygen uptake (VO₂max), anaerobic and ventilatory thresholds, Respiratory Exchange Ratio (RER), heart rate response, and lactate tolerance under progressive cycling effort. This section aims to highlight functional cardiorespiratory adaptations in both athletes and assess the physiological efficiency of the transtibial prosthesis user under high-intensity conditions.

The second moment, addressed in the following subsection, consists of a Kinematic Comparison of Pedaling Technique, in which joint angles, ranges of motion, and postural variability of trunk, hips, knees, and ankles are analyzed using 3D motion capture. This biomechanical evaluation provides insights into movement strategies, compensatory mechanisms, and segmental coordination specific to each athlete. Below, we present the results corresponding to the first analytical moment.

3.1. Comparative Physiological and Ventilatory Performance in a Paralympic and a Conventional Cyclist

The comparison between a Paralympic cyclist with bilateral transtibial amputation and a conventional cyclist reveals both similarities and significant differences in the physiological parameters evaluated during a maximal exercise test. In terms of body composition, the Paralympic cyclist was taller (1.75 m vs. 1.69 m) and heavier (72 kg vs. 61.6 kg), although with a healthy body mass index (24) and a lower body fat percentage (10% vs. 15%), suggesting a higher proportion of lean muscle mass—likely the result of compensatory adaptations and high-level training.

Both athletes reached the same peak workload (350 W), enabling a valid comparison under identical physical conditions. However, the conventional cyclist showed a higher absolute and relative maximal oxygen consumption (4.26 L/min and 69 mL/kg/min vs. 3.53 L/min and 52 mL/kg/min). Still, the Paralympic athlete exceeded his predicted value by 120%, compared to 148% for the conventional cyclist—demonstrating excellent physiological efficiency in both, particularly in the athlete using prostheses, given the mechanical and metabolic challenges imposed by assistive devices [4].

Regarding ventilatory efficiency, the respiratory exchange ratio (RER) was higher in the Paralympic cyclist (1.32 vs. 1.11), indicating greater anaerobic contribution at peak effort, potentially due to the additional energetic cost of using heavier or less efficient prosthetic limbs [14]. Both athletes reached high maximum heart rates (192 and 204 bpm), exceeding 110% of the predicted values, confirming maximal effort during the test.

The Anaerobic Threshold (AT) and the Ventilatory Threshold 1 (VT1) were also higher in the conventional cyclist (3.22 L/min and 52 mL/kg/min) compared to the Paralympic cyclist (2.57 L/min and 38 mL/kg/min), reflecting greater tolerance to submaximal sustained exertion. Nevertheless, both athletes showed similar lactate thresholds (8.0 vs. 8.1 mmol), suggesting comparable tolerance to metabolic acidosis during intense exercise, despite differences in limb structure and movement mechanics.

These findings indicate that while the conventional cyclist demonstrates superior physiological performance in several indicators, the Paralympic cyclist displays remarkable adaptations to exercise, with high cardiorespiratory efficiency and lactate tolerance. These results could be further optimized through improvements in prosthetic design—such as weight reduction and aerodynamic enhancement—which may reduce the energy cost of locomotion and extend the athlete’s submaximal performance capacity [15].

Based on the data presented in

Table 1. Physiological comparison between a paralympic cyclist and a conventional cyclist.

Variable	Paralympic Cyclist	Conventional Cyclist
Height	1.75 m	1.69
Weight	72 kg	61.6 kg
Body Mass Index (BMI)	24	22
Body Fat Percentage	10%	15%
Workload	350 watts	350 watts
Peak VO2max	3.53 L/min	4.26 L/min
Predicted Peak Vo2max	2.95 L/min	2.55 L/min
Predicted Efficiency Value	120%	148%
VO2max	52 mL/kg/min 1	69 mL/kg/min 1
Respiratory Exchange Ratio (RER)	1.32	1.11
Heart rate (HR)	192	204
HR efficiency percentage	111%	114%
Anaerobic threshold (AT-VT1)	2.57 L/min	3.22 L/min
Anaerobic threshold (AT-VT1) as a percentage of VO2max	38 mL/kg/min 1	52 mL/kg/min 1
Lactate threshold	8.1 mmol	8.0 mmol

Notes: ¹ Relative maximal oxygen uptake (VO₂max), expressed in milliliters of oxygen per kilogram of body mass per minute.

, the conventional cyclist demonstrated higher absolute and relative VO₂max (69 vs. 52 mL/kg/min), anaerobic threshold, and body fat percentage, while the Paralympic cyclist showed greater predicted efficiency and comparable lactate tolerance despite mechanical constraints. Although the 24.6% lower relative VO₂max observed in the Paralympic athlete represents a considerable gap in endurance sports, he still falls within the “excellent” category according to the American Heart Association, underscoring his competitive physiological profile. See Table 1 for full details.

Regarding absolute VO₂max, the difference was 0.73 L/min (3.53 vs. 4.26 L/min), which equates to a 17.1% lower value for the Paralympic cyclist. However, he exceeded his predicted value by 120%, demonstrating high individual physiological efficiency. For the relative anaerobic threshold, the Paralympic cyclist reached 38 mL/kg/min compared to 52 mL/kg/min for the conventional cyclist, revealing a gap of 14 mL/kg/min, or a 26.9% difference. This suggests that the conventional cyclist can sustain submaximal efforts for longer before reaching anaerobic conditions.

In terms of the Respiratory Exchange Ratio (RER), the Paralympic cyclist recorded a value of 1.32 compared to 1.11 in the conventional cyclist. While not an indicator of inferior performance per se, it reflects greater anaerobic metabolic stress required by the Paralympic athlete to sustain the same workload—likely due to mechanical limitations or the increased energy cost associated with prosthetic use. Additionally, the maximum heart rate differed by 12 beats per minute (192 vs. 204 bpm), though both athletes exceeded 110% of their predicted HR, indicating equivalent maximum intensity in relative terms.

Lastly, in the lactate threshold, both cyclists produced nearly identical results (8.1 vs. 8.0 mmol), showing that despite structural and oxygen consumption differences, the Paralympic athlete has a comparable tolerance to lactic stress, indicating meaningful physiological adaptations to high-intensity competition.

Table 2. Comparison of anthropometric and physiological variables between a paralympic cyclist and a conventional cyclist.

	Paralympic Cyclist	Conventional Cyclist	Absolute Difference	% Difference Paralympic vs. Conventional
Height (m)	1.75	1.69	0.06	3.60
Weight (kg)	72	61.6	10.39	16.93
BMI	24	22	2.0	9.14
Body Fat (%)	10	15	5.0	−33.38
Workload (W)	350	350	0	0,0.5
Peak VO2max (L/min)	3.53	4.26	0.73	−17.19
Predicted Peak VO2max (L/min)	2.95	2.55	0.4	15.74
Predicted Efficiency (%)	120	1.48	28.0	−18.97
VO2max (mL/kg/min)	52	69	17.0	−24.69
RER	1.32	1.11	20.99	18.97
Heart Rate (bpm)	192	204	0,12.0	−5.93
HR Efficiency (%)	111	1.14	3.0	−2.68
Anaerobic Threshold (L/min)	2.57	3.22	0.65	−20.24
VO2max at AT (mL/kg/min)	38	52	14.0	−26.97
Lactate Threshold	8.1	8	0.09	1.30

provides a detailed comparison of fifteen key biomechanical variables between a Paralympic cyclist with bilateral transtibial amputation and a conventional cyclist, both tested under the same workload conditions (350 W). The analysis highlights greater ranges of motion and lower trunk variability in the Paralympic cyclist, indicative of adaptive postural strategies, whereas the conventional cyclist showed greater intersegmental symmetry and lower overall Range of Motion (ROM), reflecting technical efficiency. See Table 2 for full details.

In anthropometric terms, the Paralympic cyclist is taller (1.75 m vs. 1.69 m) and heavier (72 kg vs. 61.6 kg), resulting in a higher body mass index (24 vs. 22). However, his body fat percentage is significantly lower (10% vs. 15%), suggesting a greater proportion of lean mass, functionally optimized for performance.

Regarding maximal oxygen consumption (VO₂max), the conventional cyclist showed a 17.19% higher value in absolute terms (4.26 L/min vs. 3.53 L/min) and a 24.69% higher relative value (69 vs. 52 mL/kg/min). This indicates greater aerobic capacity, likely due to his lower body mass and more efficient cardiorespiratory economy.

Nevertheless, the Paralympic cyclist exceeded his predicted Vo₂max by 120%, demonstrating remarkable efficiency. In fact, he surpassed the predicted value more than the conventional cyclist did (+15.74%), indicating high relative efficiency within his physiological constraints.

For the anaerobic threshold, the conventional cyclist performed better both in absolute (3.22 vs. 2.57 L/min) and relative terms (52 vs. 38 mL/kg/min), reflecting a greater capacity to sustain exercise without critical lactate accumulation. However, both athletes showed almost identical lactate thresholds (8.1 vs. 8.0 mmol), indicating that the Paralympic cyclist has a comparable tolerance to metabolic acidosis, which is a key marker of anaerobic endurance.

Additionally, the Paralympic cyclist reached a higher Respiratory Exchange Ratio (RER) (1.32 vs. 1.11), suggesting greater reliance on anaerobic metabolism at peak effort, possibly due to the higher energetic cost associated with prosthetic use. His maximum heart rate was slightly lower (192 vs. 204 bpm), though both exceeded 110% of their predicted HR, confirming maximal voluntary effort.

Therefore, although the Paralympic cyclist presents significant physiological differences compared to the conventional cyclist—particularly in aerobic capacity and anaerobic threshold—he maintains highly competitive levels of lactate tolerance, relative efficiency, and exercise adaptation. These findings support the need to consider the redesign of lighter and more functional prostheses, in order to reduce compensatory effort and optimize athletic performance in high-level competitive settings.

Figure 1 compares the Anaerobic Threshold (AT) response between a Paralympic cyclist and a conventional cyclist during a progressive exercise test. This threshold represents the point at which aerobic metabolism can no longer fully meet energy demands, requiring the body to increase anaerobic energy production, leading to greater lactate accumulation.

The Paralympic cyclist reached his AT at an oxygen consumption of 2.57 L/min, equivalent to 38 mL/kg/min, representing approximately 73% of his Vo₂max. He was able to sustain this level for 13 min, demonstrating a high tolerance to submaximal effort, possibly due to muscular adaptations and compensatory strategies related to the use of transtibial prostheses.

In contrast, the conventional cyclist presented an AT of 3.22 L/min (52 mL/kg/min), equivalent to 76% of his Vo₂max, which he sustained for 12 min. Although this threshold is higher in both absolute and relative terms, the Paralympic cyclist maintained it for one minute longer, highlighting his ability to sustain efficient aerobic performance during extended workloads.

This comparison suggests that, despite structural and functional differences, the Paralympic cyclist demonstrates a remarkable adaptive response, which could be further optimized through improvements in the biomechanical design of the prostheses.

Figure 2 presents the Anaerobic Threshold (AT or VT1) in two cyclists—one Paralympic (left) and one conventional (right)—during a graded exercise test, determined by the inflection point in the VO₂, VCO₂, and VE curves, along with a sustained increase in the Respiratory Exchange Ratio (RER).

Figure 2 shows the comparison of the Anaerobic Threshold (AT/VT1) between a Paralympic cyclist (a) and a conventional cyclist (b) during a graded exercise test. In the upper graph, the scatter plot displays VCO₂ values (red points, L/min) as a function of VO₂ (x-axis, L/min), showing a clear positive linear correlation in the early stages of the test, reflecting aerobic metabolic balance. However, at a specific point—identified as the inflection point—the slope of the relationship increases, indicating a disproportionate rise in CO₂ production due to anaerobic activation. This inflection point corresponds to the anaerobic threshold and is marked by the central green line. In the middle panel, the time-course curves of oxygen uptake (VO₂, blue line, L/min) and carbon dioxide output (VCO₂, red line, L/min) initially rise gradually and in parallel, followed by a divergence near the AT, where the VCO₂ curve increases more steeply than VO₂, confirming the metabolic shift. In the lower panel, the ventilatory equivalents support this physiological transition.

Overall, the variables observed in Figure 2 demonstrate that the Paralympic cyclist reached the AT with lower absolute oxygen consumption and workload, yet with an efficient and competitive response, whereas the conventional cyclist reached it at higher intensity with a more gradual ventilatory transition, reflecting differences in metabolic economy and adaptation strategies. Although these differences are physiologically relevant, they are not markedly significant in terms of overall performance, suggesting that both exhibit highly competitive functional profiles within their respective conditions.

Figure 3 shows the comparison of the Respiratory Compensation Point (RCP) between a Paralympic cyclist (a) and a conventional cyclist (b) during a graded exercise test. In the upper graph, the scatter plot displays VCO₂ values (red points, L/min) plotted against VO₂ (x-axis, L/min), showing a positive linear correlation during the early stages of exercise until, at an inflection point, the slope increases disproportionately, indicating the transition to predominantly anaerobic lactic metabolism. This point corresponds to the RCP and is marked by the central green line. In the middle panel, the time-course curves of oxygen uptake (VO₂, blue line, L/min) and carbon dioxide output (VCO₂, red line, L/min) rise in parallel initially, but near the RCP the VCO₂ curve shows a steeper and sustained increase compared to VO₂, reflecting lactate accumulation and the increased ventilation required to compensate for metabolic acidosis. In the lower panel, ventilatory equivalents confirm this physiological transition.

Overall, the variables observed in Figure 3 demonstrate that the Paralympic cyclist reached the RCP with lower absolute VO₂, heart rate, and workload compared to the conventional cyclist, but with a competitive, albeit more abrupt, ventilatory response. The conventional cyclist reached the RCP at higher intensity with a more gradual ventilatory transition, indicating greater cardiorespiratory efficiency. Although these differences are physiologically relevant, they are not critical to overall performance, suggesting that both display highly competitive adaptive profiles within their respective conditions.

Figure 4 shows the evolution of maximum oxygen uptake (VO₂) during a graded exercise test in a Paralympic cyclist (a) and a conventional cyclist (b). The blue curve represents VO₂ in liters per minute (L/min) over time, illustrating the maximal cardiorespiratory response of each athlete. Both curves rise progressively, reaching a peak in the final phase of the test, followed by an abrupt drop after exercise cessation. The Paralympic cyclist achieved a maximum VO₂ of 3.53 L/min, exceeding 120% of his predicted theoretical value, with a maximum heart rate of 192 bpm and an RER of 1.32, indicating a valid maximal effort. In contrast, the conventional cyclist achieved a higher absolute maximum VO₂ of 4.26 L/min, equivalent to 148% of his predicted value, with a maximum heart rate of 204 bpm and an RER of 1.11, also confirming maximal effort. Overall, Figure 4 demonstrates that while the conventional cyclist exhibits higher absolute aerobic capacity, the Paralympic cyclist displays outstanding relative efficiency and the ability to sustain elevated oxygen consumption despite the biomechanical constraints imposed by prosthetic use.

3.2. Kinematic Comparison of Pedaling Technique

As part of this study, a kinematic comparison of the pedaling technique was conducted between a Paralympic cyclist and a conventional cyclist, aiming to identify biomechanical differences in movement patterns during incremental exercise. Both athletes were evaluated using a standardized cycling protocol, beginning at 100 watts and increasing by 50 watts every three minutes until reaching voluntary exhaustion.

Throughout the test, a kinematic analysis of key body segments involved in pedaling—trunk, hip, knee, and ankle—was performed. Motion capture was carried out at intervals corresponding to each workload increment, allowing for the observation of joint angle progression and intersegmental coordination under progressively demanding conditions. Prolonged cycling may affect force distribution during these phases [16]. This approach facilitates the evaluation of both mechanical efficiency and neuromuscular adaptation in response to increasing intensity [17,18].

In the case of the Paralympic cyclist, the analysis accounted for potential restrictions resulting from the use of bilateral transtibial prostheses, which may alter the range of motion and force distribution during the power and recovery phases of the pedaling cycle [14]. The conventional cyclist served as a biomechanical reference to contrast the pedaling pattern within a typical performance model. This comparison not only highlights technical differences but also identifies opportunities for prosthetic design optimization and individualized training strategies.

Figure 5 shows the trunk inclination curves (flexion–extension) throughout the full pedaling cycle (0–100%) in the Paralympic cyclist (upper panel) and the conventional cyclist (lower panel), differentiating between right and left pedaling cycles. This analysis is complemented by measurements of maximum and minimum angles, as well as the total Range Of Motion (ROM) of trunk inclination, expressed in degrees.

Figure 5 shows trunk inclination in degrees (°) throughout a complete pedaling cycle (0–100%). In the Paralympic cyclist (upper panel), maximum angles during the right and left cycles were 47.4° ± 0.1°, and minimum angles were 45.0° ± 0.1°, yielding a total range of motion (ROM) of approximately 2.4°. This pattern remained consistent across the cycle, with very low standard deviation, indicating stable and highly consistent postural control, likely due to compensatory adaptations to bilateral prosthetic use. In contrast, the conventional cyclist (lower panel) exhibited maximum angles of 46.1° ± 1.7° and minimum angles of 45.3° ± 1.7° in both right and left cycles, with a ROM of approximately 0.9°. The higher standard deviations observed suggest greater variability between cycles and less consistent postural patterns, albeit accompanied by greater stiffness and intersegmental symmetry.

Overall, the differences observed throughout the pedaling cycle reflect distinct biomechanical strategies for maintaining trunk stability: the Paralympic cyclist shows greater range of motion and lower variability, while the conventional cyclist shows reduced range and higher variability, both functional within their respective contexts.

Figure 6 shows hip flexion–extension angles in degrees (°) throughout a complete pedaling cycle (0–100%), with the Paralympic cyclist in the upper panel and the conventional cyclist in the lower panel. The graphs display the right (RT Hip) and left (LT Hip) joints. In the Paralympic cyclist, the right hip reached a maximum angle of 95.0° ± 0.3° and a minimum of 47.6° ± 0.1°, with a Range Of Motion (ROM) of approximately 47.4°. The left hip reached a maximum of 100.1° ± 0.2° and a minimum of 49.3° ± 0.1°, with a ROM of 50.8°. This shows a slightly asymmetrical pattern, with greater amplitude in the left hip, likely due to compensatory strategies related to bilateral prosthetic use. In the conventional cyclist, the right hip reached a maximum of 104.7° ± 1.4° and a minimum of 63.4° ± 1.5°, with a ROM of 41.3°, while the left hip showed a maximum of 108.8° ± 1.8° and a minimum of 66.6° ± 1.8°, with a ROM of 42.1°. This pattern was more symmetrical and exhibited smaller joint amplitudes compared to the Paralympic cyclist, reflecting greater stiffness and intersegmental control.

Overall, the differences observed indicate distinct functional adaptations: the Paralympic cyclist displays greater joint amplitude and lower variability, while the conventional cyclist exhibits a more controlled and symmetrical pattern, both adequate for the demands of their respective conditions.

Figure 7 shows the flexion–extension angles of the right (RT Knee) and left (LT Knee) knees in degrees (°) throughout a complete pedaling cycle (0–100%), with the Paralympic cyclist in the upper panel and the conventional cyclist in the lower panel. In the Paralympic cyclist, the right knee reached a maximum angle of 130.8° ± 0.3° and a minimum of 62.2° ± 2.5°, resulting in a Range Of Motion (ROM) of 68.6°, while the left knee reached a maximum of 138.7° ± 0.4° and a minimum of 69.8° ± 0.2°, with a ROM of 68.9°. This pattern reflects slight functional asymmetry, with greater amplitude in the left knee, likely due to biomechanical adaptations to bilateral prosthetic use. In contrast, the conventional cyclist showed a more symmetrical pattern: the right knee reached a maximum of 132.4° ± 1.1° and a minimum of 67.7° ± 0.4°, with a ROM of 64.7°, while the left knee reached a maximum of 133.3° ± 0.9° and a minimum of 68.0° ± 0.5°, with a ROM of 65.3°. The conventional cyclist demonstrated smaller joint ranges but greater symmetry between knees.

Overall, these differences reflect distinct biomechanical strategies: the Paralympic cyclist exhibits greater range of motion and slight asymmetry as a compensatory mechanism, whereas the conventional cyclist maintains a more symmetrical and controlled pattern, both functional within their respective demands.

Figure 8 shows the dorsiflexion and plantarflexion angles of the right (Rt Ankle) and left (Lt Ankle) ankles in degrees (°) throughout a complete pedaling cycle (0–100%), with the Paralympic cyclist in the upper panel and the conventional cyclist in the lower panel. In the Paralympic cyclist, the right ankle reached a maximum angle of 30.5° ± 2.1° and a minimum of 8.7° ± 0.1°, resulting in a Range Of Motion (ROM) of approximately 21.8°. The left ankle reached a maximum of 33.7° ± 0.2° and a minimum of 14.5° ± 1.3°, with a ROM of 19.1°. These values reflect greater joint amplitude and lower variability, suggesting an adaptive and stable pattern to compensate for the limitations imposed by bilateral prostheses. In contrast, the conventional cyclist showed lower ROM and higher variability: the right ankle reached a maximum of 22.8° ± 2.0° and a minimum of 9.8° ± 1.8°, with an ROM of 13.0°, while the left ankle reached a maximum of 17.1° ± 2.4° and a minimum of 3.0° ± 1.7°, with an ROM of 14.1°.

Overall, these differences reflect distinct biomechanical strategies: the Paralympic cyclist demonstrates greater mobility and consistency to maintain cadence and technical execution of pedaling, whereas the conventional cyclist shows reduced amplitude and greater angular dispersion, both functional within their respective competitive contexts.

4. Discussion

This study contributes valuable insights into the functional adaptations in elite Paralympic cycling, specifically in an athlete with bilateral transtibial amputation using optimized prosthetic technology. The results align with and expand upon prior findings regarding performance limitations and compensatory strategies in amputee athletes [14,15]. Despite the conventional cyclist exhibiting superior aerobic capacity—both absolute and relative (VO₂peak: 4.26 L/min and 69 mL/kg/min)—the Paralympic cyclist exceeded 120% of his predicted VO₂max (3.53 L/min and 52 mL/kg/min). This suggests remarkable individual efficiency, likely enhanced by training adaptations and advanced prosthetic integration.

The findings of this study align with prior research indicating that elite-level Paralympic cyclists can achieve competitive performance through remarkable physiological adaptations and biomechanically efficient movement strategies, despite inherent mechanical constraints [14,15]. The observation of high relative VO₂max, efficient lactate tolerance, and stable kinematic patterns in the Paralympic cyclist is consistent with literature reporting that targeted training and tailored prosthetic design enable para-athletes to narrow the performance gap with able-bodied peers. Moreover, the engineering principles applied in this study—Finite Element Analysis (FEA), Computational Fluid Dynamics (CFD), and topological optimization—are not limited to transtibial cycling prostheses and could be generalized to the design of other sport-specific prosthetic devices, such as running blades or upper-limb prostheses for adaptive rowing or climbing, by optimizing mechanical, aerodynamic, and anatomical factors.

From a practical perspective, the study provides valuable insights for coaches, physiotherapists, and engineers: it highlights the importance of integrating biomechanical assessment, individualized training regimens, and advanced engineering techniques to maximize performance and minimize injury risk in adaptive sports. This multidisciplinary approach can inform the development of evidence-based protocols for athlete preparation and prosthetic innovation.

From a ventilatory standpoint, the higher Respiratory Exchange Ratio (RER) in the Paralympic athlete (1.32 vs. 1.11) indicates a greater reliance on anaerobic pathways at maximal exertion. This outcome supports earlier hypotheses about increased metabolic cost linked to prosthetic use, particularly in lower-limb amputees [4]. Nonetheless, both cyclists achieved nearly identical lactate thresholds (8.1 vs. 8.0 mmol), which implies comparable resilience to metabolic acidosis, a vital component in endurance performance [10]. This aligns with discussions on the validity of lactate threshold concepts in endurance performance [9].

Kinematic results further reinforce the hypothesis of functional compensation. The Paralympic cyclist demonstrated greater joint range of motion (ankle, knee, and hip), as well as reduced trunk angular variability (2.4° ± 0.1°), pointing to adaptive motor control strategies. These findings are consistent with previous research suggesting that increased proximal mobility may compensate for distal joint limitations in amputee athletes [17]. Such physiological adaptations are also reported in endurance athletes with varied anthropometric characteristics [19]. In contrast, the conventional cyclist showed more symmetrical but less dynamic joint patterns, reflecting biomechanical efficiency supported by anatomical integrity [18].

The observed kinematic differences, such as the increased joint Range Of Motion (ROM) and reduced trunk variability in the Paralympic cyclist, have relevant functional and clinical implications. Greater ROM at the hip, knee, and ankle may reflect adaptive strategies to compensate for the lack of ankle musculature, enabling efficient power transfer despite prosthetic constraints. However, this increased mobility could also lead to higher energy expenditure and potentially greater fatigue over prolonged efforts. This observation is supported by studies showing that prosthetic alignment influences the energy cost of cycling in transtibial amputees [20]. Conversely, the reduced angular variability of the trunk suggests effective postural control, which may help stabilize the pedaling motion and prevent compensatory movements that could increase injury risk. These insights highlight the importance of monitoring kinematic patterns in Paralympic cyclists to optimize performance, design targeted strength and conditioning programs, and minimize the likelihood of overuse injuries associated with altered biomechanics.

While the Anaerobic Threshold (AT/VT1) was higher in absolute terms in the conventional cyclist (3.22 L/min vs. 2.57 L/min), the Paralympic cyclist sustained it longer (13 min vs. 12 min), supporting the hypothesis of enhanced tolerance to sustained submaximal exertion. This endurance may be associated with his lower body fat percentage (10% vs. 15%) and higher lean muscle mass, which facilitates metabolic efficiency.

A key strength of this study lies in the integration of clinical and engineering approaches for prosthetic development. Design 1.4 was developed using Quality Function Deployment, CAD, FEA, and CFD methods, resulting in a prosthesis optimized for stiffness-to-weight ratio, vibration damping, and aerodynamic performance. These mechanical enhancements likely contributed to the stable, energy-efficient movement patterns observed.

However, this study also presents notable limitations. Due to the small sample size (n = 2), inferential statistics were not applied, and conclusions are primarily descriptive. Nonetheless, single-subject comparative studies are widely recognized in sports medicine for their capacity to explore performance profiles in highly specialized populations [21]. Future studies should validate these findings in larger cohorts and include matched groups of Paralympic and able-bodied athletes using standardized testing protocols.

It is important to emphasize that the findings of this study should be interpreted within the inherent limitations of its comparative case design. While the physiological and biomechanical profiles of the Paralympic and conventional cyclists provide valuable insights into functional adaptations and the potential of optimized prosthetic design, including only one athlete per category does not allow for broad generalizations about these populations. Rather, this study is presented as an exploratory analysis and a methodological validation of the measurement protocols and simulation frameworks employed, which is consistent with the use of single-subject designs in sports medicine for highly specialized populations [21]. Both participants were confirmed as professional cyclists, belonging to the national and international Olympic and Paralympic systems, which ensures a high-performance level and competitive relevance of the observed data. Nevertheless, the limited sample size precludes inferential analysis and requires cautious interpretation of these results when extrapolating to other athletes. Future studies are recommended to recruit larger and more homogeneous cohorts to confirm these findings and better isolate the effects of prosthetic design, training history, and individual variability on performance outcomes. These findings should be understood in the context of a technological innovation case study, rather than as generalizable results.

In conclusion, these results support the hypothesis that optimized prosthetic design, combined with targeted training, can minimize the physiological and biomechanical gap between Paralympic and conventional athletes. Continued innovation in prosthetic engineering, guided by functional performance data, represents a promising direction for enhancing competitive outcomes and reducing energy costs in adaptive sports.

Based on the comparative performance analysis, future developments of the transtibial prosthesis could focus on further reducing its weight and improving energy transfer efficiency, while maintaining structural integrity and stability. The incorporation of advanced lightweight composite materials, as well as the optimization of internal geometry to minimize unnecessary mass, could help reduce the metabolic cost of pedaling. Additionally, exploring semi-active or adaptive components, such as adjustable stiffness modules at the socket or footplate interface, could enhance comfort and dynamic responsiveness under different cycling conditions. Refining the aerodynamics of the prosthetic shape, especially for road cycling with high physical demands, as reflected in the anthropometric and physiological profiles of the Paralympic cyclist, also represents a promising direction. Finally, testing and validating these improvements in larger cohorts of Paralympic athletes would contribute to a stronger evidence base to guide future prosthetic design optimizations.

When comparing the different versions, CAD 1.4 showed significant advantages that justified its selection as the final solution. This design demonstrated greater structural strength in static and fatigue analyses, as well as better stress distribution, ensuring increased durability and safety under repetitive loads. Topological optimization allowed for weight reduction without sacrificing stiffness or stability, thereby improving the cyclist’s efficiency. Additionally, it offered superior aerodynamic performance, with reduced air resistance and lower deformations in impact and buckling tests, contributing to more stable and safer performance. It also achieved a more precise anatomical fit to the cyclist’s stump, enhancing comfort, reducing pressure points, and providing greater dynamic stability by keeping its natural frequencies outside the pedaling cadence range, thereby avoiding unwanted resonances.

It is important to note that the findings of this study should not be interpreted as broadly generalizable. The comparative case study design allowed for an in-depth characterization of two elite cyclists with unique physiological and biomechanical profiles under controlled conditions, which is valuable for hypothesis generation and methodological validation. However, due to the inclusion of only one Paralympic and one conventional athlete, the results are specific to these individuals and serve primarily to illustrate potential functional adaptations and inform future research directions. Further studies with larger, more representative samples are needed to confirm the observed trends and strengthen the evidence base.

However, these advantages were accompanied by some drawbacks. The high level of specialization and detail in CAD 1.4 increased manufacturing complexity and costs, due to the need for more advanced production processes. Furthermore, being highly customized to the characteristics of the evaluated cyclist, its versatility for adaptation to other users without additional modifications is limited. These considerations were carefully weighed, and, despite its limitations, CAD 1.4 was validated as the most effective and suitable solution for the purposes of this study.

Beyond the scope of this comparative case study, the optimized transtibial prosthesis (Design 1.4) has undergone preliminary field testing during the athlete’s regular training sessions to assess its durability and functional performance under real-world conditions. These extended tests have corroborated the laboratory findings, showing sustained mechanical integrity and user-reported comfort during prolonged use. However, larger-scale clinical validation involving more athletes is still needed. It should be noted that including more participants would necessarily require an individualized study for each one, as the prosthesis design depends on the specific characteristics of each athlete, such as height, weight, body composition (fat mass and lean mass), as well as the phase of their training cycle (general, specific, pre-competitive, or competitive). These variables may entail variations or adjustments in the prosthetic design to optimize performance according to individual needs. This requirement for personalized studies was precisely one of the reasons why only one Paralympic cyclist was included in the present work, as involving more subjects would have required significantly more time and much higher costs, for which the available resources were very limited.

In parallel, the design is currently undergoing the initial stages of regulatory review and intellectual property protection. A national patent application for Design 1.4 has been filed with the Colombian Superintendence of Industry and Commerce, and the research team is exploring pathways for further development and commercialization in collaboration with local biomedical engineering firms. Including these translational efforts underscores the potential of this work to inform both clinical practice and innovation in adaptive sports technology.

5. Conclusions

This study demonstrates that a Paralympic cyclist with bilateral transtibial amputation using an optimized prosthesis (Design 1.4) can achieve physiological and biomechanical performance comparable to that of a conventional cyclist. Despite lower absolute VO₂peak values, the Paralympic athlete exceeded 120% of his predicted VO₂max, showing remarkable metabolic efficiency under high workloads. Kinematic analyses revealed greater joint ranges of motion and reduced trunk variability, suggesting effective compensatory strategies and postural control. These adaptations are likely supported by the individualized engineering of the prosthesis, which integrated FEA, CFD, and topological optimization to enhance mechanical and aerodynamic performance.

Although the conventional cyclist showed superior aerobic capacity and greater intersegmental symmetry, both athletes achieved similar maximal workloads and lactate thresholds, indicating that personalized prosthetic design combined with targeted training can help bridge the performance gap in elite adaptive cycling.

Given the small sample size, these findings should be interpreted with caution, as they are exploratory and specific to the athletes studied. Nevertheless, the study provides valuable insights and a methodological framework for future research on adaptive cycling performance and prosthetic optimization, highlighting the importance of refining prosthetic designs to further reduce energy cost and improve biomechanical efficiency.