Differential Recruitment of Medial and Lateral Gastrocnemius During Heel Raises: Role of Ankle ROM, Unilateral Execution, and Limb Dominance

Abstract

Background: The medial (MG) and lateral gastrocnemius (LG) muscles exhibit differential activation patterns during plantar flexion tasks. However, the influence of range of motion (ROM), exercise type (unilateral vs. bilateral), and limb dominance on muscle activity during heel raise exercises (HREs) remains unclear. Methods: Nineteen physically active adults performed unilateral and bilateral HREs under two ankle ROM conditions: neutral (NROM) and full (FROM). Surface electromyography (EMG) was collected from both legs during each condition and normalized to peak values recorded during overground sprinting. Results: MG activity was significantly higher during the FROM than the NROM, in both the dominant leg (F = 11.55, p < 0.01, η² = 0.47) and the non-dominant leg (F = 6.63, p < 0.05, η² = 0.31), and was not affected by exercise type. In contrast, LG activity increased significantly during unilateral versus bilateral HREs, especially in the dominant leg during the FROM (F = 17.47, p < 0.01, η² = 0.52) and in the non-dominant leg (F = 5.44, p < 0.05, η² = 0.25). Activation ratios (MG:LG) differed significantly between dominant and non-dominant legs only in the unilateral FROM (p = 0.03). MG activation during the unilateral FROM was comparable to sprinting values, highlighting its high neuromuscular demand. Conclusion: The MG and LG respond differently to exercise parameters. The MG is primarily influenced by ROM, whereas the LG is sensitive to both exercise type and limb dominance. These findings emphasize the importance of ROM manipulation and unilateral training to target specific gastrocnemius regions. FROM and unilateral execution optimize gastrocnemius activation, with implications for rehabilitation and performance programs targeting calf musculature.

1. Introduction

Anatomical and physiological differences between the medial (MG) and lateral (LG) heads of the gastrocnemius muscle are well-documented, with the MG typically exhibiting a 20–30% greater cross-sectional area and volume compared to the LG [1,2]. These morphological differences are accompanied by divergent neural activation patterns, as the MG and LG can be independently recruited depending on the contraction type and joint configuration [3,4]. Such task-specific activation has been observed during a variety of functional activities, including postural swaying [5,6], reactive perturbations [7], walking [8], running [9], and sprinting [10].

Manipulating resistance training variables is essential to optimizing muscular activation, with volume, load, and frequency being the most commonly examined factors [11]. Within the context of resistance training, range of motion (ROM) is a critical but often underexplored variable, despite its established relevance for modulating muscle activation and promoting hypertrophic adaptations [12]. While training through a full ROM has been shown to enhance lower-limb muscle development [12], the specific interaction between ROM and exercise execution type—namely, bilateral versus unilateral movement—in MG and LG activity during heel raise exercises (HREs) remains insufficiently understood. Dynamic plantar flexion exercises such as heel raises are widely used to enhance running and jumping performance, assess muscle function through calf raise tests [13], and prevent or rehabilitate stroke-related impairments and lower leg injuries, including Achilles tendon damage [14]. In this context, the precise assessment of muscle activation patterns, including coactivation around key joints like the knee, has become crucial for understanding joint stability and developing effective injury prevention strategies [15,16].

Performing HREs under different foot, knee angle, and ankle ROM conditions has been proposed to target region-specific gastrocnemius activity. Evidence remains inconclusive regarding the effect of foot position on MG and LG activation during dynamic, weight-bearing tasks [17,18,19,20,21,22]. Knee flexion appears to differentially influence MG and LG activity under controlled conditions (e.g., seated or prone), with the LG being less affected [23,24,25]. In functional movements (e.g., unilateral weight-bearing HREs at 0° and 45°), electromyography (EMG) amplitudes are similar, indicating limited differentiation between muscles [26]. With the knee extended, the MG and LG exhibit distinct neuromuscular responses during HREs performed on the ground [2,27] and at full ankle ROM [28]. While LG and soleus activation correlates across various exercise conditions, the MG does not show such consistency [28]. A longitudinal study revealed that the MG and LG respond differently to full- vs. half-ROM training in terms of muscle thickness gains [29], suggesting task-dependent activation patterns. Given the limited evidence on MG and LG responses across the ankle ROM during bilateral and unilateral HREs, further investigation is warranted.

EMG studies during plantar flexion suggest that limb dominance influences muscle activation [27,30,31]. However, no prior studies have compared MG and LG activity between dominant and non-dominant legs during HREs, justifying bilateral analysis. Such insights may inform performance and rehabilitation strategies targeting muscular asymmetries. Concerning EMG normalization, previous studies have compared maximal voluntary isometric contractions with a dynamic normalization method [32,33]. Sprint running has been recommended over isometric tasks due to its simplicity and higher normalization values. Dynamic tasks (e.g., sprint running, squat jump) elicit similar neural drive and muscle coordination to the target exercise, with consistent relative activation levels [33]. Sprint running is also a reliable and reproducible EMG reference for the triceps surae. Moreover, the resulting EMG percentages offer exercise-specific insights relative to maximal sprint activity, supporting their use in function-oriented assessments of plantar flexion, as supported by previous findings linking inter-limb strength asymmetries with sprint and change-of-direction performance in elite soccer players [34].

Although ankle strengthening exercises are commonly used for therapeutic and performance purposes, the literature shows low adherence to evidence-based prescription and poor reporting quality. A recent scoping review [35] found that only 2% of exercises met the American College of Sports Medicine (ACSM) recommended intensity threshold (≥60% 1RM) for strength gains. Moreover, the reporting completeness was critically low, averaging just 31% on the Consensus on Exercise Reporting Template (CERT) checklist. These deficiencies hinder replicability and effective implementation, limiting clinical and research outcomes. This highlights the need for research on precise triceps surae exercise prescription, given the complex compensatory muscle interactions involved. For example, individuals with Achilles tendinopathy adopt strategies that reduce tendon stress—such as lowering the peak load, the loading rate, ankle dorsiflexion, and knee flexion—during rehabilitation [36]. Additionally, muscle hardness increases with length (e.g., the MG from rest to mid-range), influencing activation and functional output [37].

This study aimed to examine the effects of ankle ROM and exercise type (bilateral vs. unilateral) on EMG activity and the MG:LG activation ratio during non-weight-bearing heel raises, using overground sprinting for EMG normalization. We hypothesized that (i) the MG and LG would show distinct activation patterns and (ii) their activation ratio would vary with the ROM and exercise type. Additionally, we explored the influence of limb dominance on gastrocnemius activation. The findings may inform exercise prescription and rehabilitation strategies targeting region-specific muscle activation.

2. Materials and Methods

2.1. Study Design

A cross-sectional design compared maximal EMG activation of the MG and LG in both legs. The EMG was normalized to each muscle’s peak activity during maximal sprinting. This dynamic normalization approach, utilizing peak EMG during maximal sprinting, is preferred over traditional maximal voluntary isometric contractions (MVICs) for highly dynamic tasks like heel raise exercises, as it offers greater ecological validity and can elicit higher peak activations more representative of intense, functional movements [32,38,39]. This aligns with evidence indicating that maximal muscle activation during dynamic, high-velocity tasks, such as sprinting, often exceeds values obtained during isometric contractions, providing a more appropriate reference [40,41]. Furthermore, understanding complex biomechanical mechanisms, including those relevant to injury risk, increasingly benefits from data-driven approaches, supporting the use of ecologically relevant normalization methods [42]. Despite its advantages, dynamic normalization using a maximal sprint can present challenges in consistently ensuring a truly maximal and representative reference value, given the inherent variability in maximal dynamic efforts [32,36] and the general limitations of surface electromyography signals, such as amplitude cancellation and sensitivity to electrode placement [32,43]. Two HREs were analyzed: one with a neutral stance and one with maximal dorsiflexion. All EMG data were collected in a single session to ensure consistent electrode placement. This study adhered to the Declaration of Helsinki and was approved by the University of Valencia Research Ethics Committee (Ref. 1973315, date: 10 May 2022). All participants gave written informed consent.

2.2. Participants

Nineteen physically active male university students (24.4 ± 5.9 years; 178.8 ± 5.7 cm and 73.7 ± 10.4 kg) participated. All participants exercised more than 3 h per week, with 16 identified as right-leg dominant and 3 as left-leg dominant based on their preferred kicking leg. A priori power analysis indicated that 18 participants would yield 80% power (α = 0.05, f = 0.40) (G*Power, version 3.1.9.3, University of Dusseldorf, Dusseldorf, Germany) [44]. Volunteers were recruited via social media and campus flyers. All were familiar with resistance training and sprinting (>3 h/week), and none reported lower-limb injuries requiring medical intervention. Participants avoided caffeine and intense or unfamiliar exercise for 48 h prior to testing. Leg dominance was assessed by the preferred kicking leg [33]. Specifically, the preferred limb refers to the leg most frequently used to kick a ball, while the non-preferred limb typically functions as the support leg during dynamic movements such as sprinting and directional changes.

2.3. Procedures

All procedures were completed in a single 1 h session. Prior to data collection, participants were familiarized with unilateral and bilateral HREs under both ROM conditions: neutral (NROM) and full (FROM) (Figure 1).

During familiarization, 1 cm wedges ensured an FROM in both bilateral and unilateral exercises. Foot placement was standardized by marking the shoe sole so that one-third of the foot remained on the step. Once the participant reached full ankle dorsiflexion, wedges were placed under the calcaneus to set the FROM heel raise. In order to maintain lower leg synchronization during the whole ROM in bilateral exercises, if greater dorsiflexion was observed in one ankle compared to the other, the measurement with the highest wedges was maintained for both legs. Participants lowered their heels to touch the wedges before initiating the lift. A bar at upper-thigh height guided vertical push-off; participants raised their heels without contacting the bar, though hand support was allowed for balance. All the participants received a brief pre-session explanation and a visual demonstration of all exercises.

An experienced researcher placed the EMG sensors (see the EMG measurement section). Participants then performed a standardized warm-up: 5 min of self-paced jogging, followed by 10 min of dynamic stretches (6 reps per muscle group) and 5 min of explosive movements (i.e., jumps and hops) targeting the primary muscles involved (gluteus maximus, quadriceps, hamstrings, hip abductors and adductors, and triceps surae). Subsequently, participants underwent a familiarization time with the HRE and the wedge set-up procedure. Finally, participants completed two 40 m runs at ~80% of their perceived maximal speed.

2.3.1. Unilateral and Bilateral Heel Exercises with Neutral Range of Motion

For the unilateral HRE, participants stood on one foot with the knee extended, neutral foot alignment, and the non-stance leg flexed. They executed plantar flexion to raise the body. For the bilateral HRE, they adopted a shoulder-width stance with extended knees and neutral foot position, resting their hands lightly on the bar [26].

2.3.2. Unilateral and Bilateral Heel Exercises with Full Range of Motion

Both HREs were performed on a 20 cm step, with metatarsal supports [17]. For the unilateral condition, participants stood with standard alignment (knee extended, toes forward, heel in maximal dorsiflexion against the wedge), the non-stance leg flexed, and hands resting on the bar. For both conditions, body elevation was achieved via plantar flexion. For the bilateral condition, participants adopted a shoulder-width stance with similar alignment.

Invalid repetitions were repeated if participants contacted the bar with their thighs, deviated from the prescribed tempo, or improperly used the bar for support. Participants performed five repetitions per exercise, each lasting 5 s (1 s concentric, 1 s isometric, 3 s eccentric), paced by a metronome. The exercise order was randomized by ROM and leg dominance using four shuffled cards. Bilateral exercises preceded unilateral ones. Rest periods were 1 min within the same ROM and 3 min between different ROMs. All HREs were performed barefoot.

2.3.3. sEMG Measurements

EMG signals for the LG and MG were recorded bilaterally during HREs and overground sprints. The skin was shaved and cleaned with alcohol to reduce impedance and ensure optimal signal quality. Electrodes were positioned over the belly of the MG and LG of both legs, placed parallel to the muscle fibers following SENIAM guidelines [43]. Pre-gelled silver chloride bipolar electrodes (KendallTM Medi-Trace, Covidien, Barcelona, Spain) were used with a 10 mm interelectrode distance with the reference electrode placed on the lateral malleolus of the fibula. Two synchronized two-channel units (Shimmer, 16-bit analog-to-digital) recorded EMG at 1024 Hz—one per leg (Realtime Technologies Ltd., Dublin, Ireland). Sensors were positioned on the mid-medial tibia, aligned with the electrodes. A compressive tubular bandage was placed over the lower leg and the ankle to prevent electrode and sensor displacement.

EMG signals were acquired via validated mDurance software (mDurance Solutions S.L., version 2.4.2, Granada, Spain) [45]. Raw data were stored for subsequent evaluation and digitally filtered using a fourth-order “Butterworth” bandpass filter (20 and 450 Hz) to attenuate movement artifacts [46]. Signals of <0.05 V or >2 V were flagged as dropouts or spikes. After normalization, values of <5% or >100% were also flagged [38]. All flagged signals were visually inspected and retained if artifact-free. In addition, full-wave rectification was applied to preserve amplitude characteristics, and signal smoothing was performed using a moving average (5–10 Hz) to extract the linear envelope, following validated procedures from previous studies using the mDurance system.

After completing the HREs, participants performed two 40 m sprints from a standing start at maximal effort, with a 5 min rest between attempts. Peak EMG values for each muscle were extracted from both sprints and averaged. These served as reference values to normalize EMG activity from the HREs, calculated as the mean of the three middle repetitions. Consequently, all EMG values are expressed as a percentage of sprint-based maximal activation [10]. This dynamic normalization followed standardized procedures [38].

2.4. Statistical Analysis

Statistical analyses were conducted using SPSS v26 (Inc. IBM., version 29.0.2.0, Chicago, IL, USA). Data are expressed as the mean ± standard deviation (SD) and 95% confidence interval (CI). Reliability was not assessed, given prior studies reporting adequate-to-excellent test reliability [47,48], with intraclass correlation coefficients (ICCs) ranging from 0.78 to 0.99 for heel raise performance [49] and 0.82 to 0.99 for triceps surae EMG during dynamic tasks [50]. The Shapiro–Wilk test confirmed normality (p > 0.05). For the repeated measures ANOVAs, the assumption of sphericity was assessed. Given that all within-subject factors (ROM and type) had only two levels, the assumption of sphericity was automatically met and could not be violated (Mauchly’s test: df = 0, ε = 1.000). Therefore, no corrections to the degrees of freedom (such as Greenhouse–Geisser or Huynh–Feldt) were required for the within-subject effects tests. A 2-way repeated measures ANOVA with muscle as the within-participants factor was applied to compare maximum LG and MG activation across exercises. Pairwise t-tests examined differences in MG:LG ratios by ROM and exercise type. For all pairwise comparisons, Bonferroni correction was applied to adjust for multiple comparisons and control the Family-Wise Error Rate. To assess the magnitude of the significant pair differences, effect sizes (ESs) were computed using the package “effectsize”. Effect sizes were calculated for all key comparisons to support interpretations of practical relevance. Partial eta-squared (η²) values are reported for ANOVA-based effects, and Hedges’ g was used for pairwise comparisons. The effect size magnitude was interpreted following Cohen’s guidelines as small (0.2–0.5), moderate (0.5–0.8), or large (>0.8). [51]. Significance was set at p ≤ 0.05.

3. Results

3.1. Medial Gastrocnemius Muscular Activity During Unilateral and Bilateral Heel Raise Exercises

The MG showed a significant increase in maximum EMG activation when the HRE was performed with the FROM compared to the NROM, in both the dominant (F = 11.55, p = 0.005, η² = 0.47) and non-dominant legs (F = 6.63, p = 0.021, η² = 0.31).

Furthermore, the mean MG muscular activity during the bilateral and unilateral FROM HREs (96.1% and 104.4%, respectively) was similar to the muscular activity obtained in the sprint. This similarity highlights the high neuromuscular demand of FROM HREs, comparable to that of high-intensity activities. Mean differences of 13% and 14% MG EMG activity were found between the NROM and the FROM for bilateral and unilateral HREs, respectively (Figure 2 and Figure 3).

On the other hand, the type of exercise (unilateral vs. bilateral) did not significantly affect the muscular activation of the MG (p > 0.05). This highlights that, for the MG, ROM is a more determinant factor of muscle activation than the exercise modality (unilateral or bilateral).

3.2. Lateral Gastrocnemius Muscular Activity During Unilateral and Bilateral Heel Raise Exercises

During HREs, the LG exhibits a distinct activation pattern from the MG, with the former influenced by exercise type (unilateral vs. bilateral) and limb dominance and the latter primarily modulated by ROM. In this regard, the LG showed a significant increase in maximum EMG activation when the unilateral HRE was performed with the FROM compared to the NROM. However, this difference was significant only in the dominant leg (F = 8.35, p = 0.011, η² = 0.34) (Figure 4).

Significant differences in the maximum EMG activation of the LG were obtained in the unilateral and bilateral HREs for the dominant (F = 17.47, p < 0.001, η² = 0.52) and non-dominant legs (F = 5.44, p = 0.033, η² = 0.25). Post hoc comparisons showed that mean maximum EMG activation of the LG was significantly higher in the unilateral compared to the bilateral HRE. Mean differences of 10 and 9% LG EMG activity were found between the NROM and the FROM for unilateral and bilateral HREs, respectively (Figure 5).

3.3. Activation Ratio of Medial and Lateral Gastrocnemius During Unilateral and Bilateral Heel Raise Exercises

The activation ratio of the MG and LG was significantly different between the dominant and non-dominant legs when performed unilaterally and with an FROM (M = 0.19, SE = 0.10, t (18) = 2.04, p = 0.03, g = 0.42) or NROM (M = 0.13, SE = 0.09, t (18) = 1.40, p = 0.09, g = 0.32).

On the other hand, no statistically significant differences were found during bilateral HREs with an FROM (p = 0.32) or bilateral HREs with an NROM (p = 0.48) (

Table 1. Results from the paired t-test activation ratios of the MG and LG muscles of both exercises (bilateral and unilateral) performed with full and neutral ranges of motion.

Type of Exercise	ROM	Mean ± SD	SE	95% CI		t	p	g
				Low	Upper
Unilateral	FROM	0.19 ± 0.42	0.10	−0.01	0.40	2.04	0.03 *	0.42
	NROM	0.13 ± 0.39	0.09	−0.06	0.31	1.40	0.09 *	0.32
Bilateral	FROM	0.06 ± 0.45	0.13	−0.21	0.34	0.49	0.32	0.10
	NROM	0.01 ± 0.45	0.13	−0.27	0.28	0.06	0.48	0.02

FROM: full range of motion; NROM: neutral range of motion; SD: standard deviation; SE: standard error; CI: confidence interval; t: t value; * statistically significant difference (p < 0.05); g: Hedge’s effect size.

). Thus, the second hypothesis—predicting general MG:LG ratio variation by ROM and exercise type—was not broadly supported, although a distinct effect was observed under unilateral HREs with an FROM.

4. Discussion

Regarding the physiological basis for our joint angle settings, the HREs in our study were consistently performed with the knee extended (approximately 0° extension). This posture optimally stretches the bi-articular gastrocnemius muscles (medial and lateral heads), enabling superior force production [4,24]. In contrast, a flexed knee, as seen in tests like those by Silfverskiöld et al. [52], would slacken the gastrocnemius, shifting mechanical advantage to the mono-articular soleus [25,26]. For the ankle, we evaluated two ranges of motion: NROM and FROM. The FROM increases the length of both the gastrocnemius and soleus muscles, significantly enhancing MG activation [2] and increasing Achilles tendon mechanical loading [36]. This wider ROM also supports longitudinal muscle growth [53]. Therefore, specific joint angle and ROM choices are critical for muscle activation, force generation, and tendon loading in lower-limb biomechanics.

No studies have assessed MG and LG muscle activity during weight-bearing HREs using dynamic normalization. In this study, the FROM increased EMG activity by ~10% compared to the NROM in both bilateral and unilateral exercises. Hébert-Losier et al. [26] reported 4–5% differences between overground heel raises at 0° and 45° knee flexion, whereas our study observed a 12% increase between unilateral NROM and FROM conditions. Sequential knee flexion and ankle dorsiflexion may support early retraining, although differing normalization methods warrant cautious interpretation. Further studies should assess the clinical relevance of these differences.

Our study showed 13.5 and 9.5% increases in MG and LG activity, respectively, consistent with prior evidence of greater MG activation during plantar flexion exercises [2,24,25,26,28]. Kinugasa et al. [2] reported greater differences (MG: 26.7–31.2%, LG: 10.2–19.5%) across bilateral and unilateral HREs at an NROM. In contrast, Gentil et al. [28] and Hébert-Loisier et al. [26] observed smaller differences (~2–4%) under different conditions. These findings align with the MG’s larger cross-sectional area [46]. Given the importance of task-specific normalization, this study is the first to provide normative EMG data for both unilateral and bilateral HREs using dynamic normalization methods, offering a reference for future research.

MG and LG EMG amplitudes during unilateral weight-bearing HREs matched those observed during sprinting [40], highlighting the high muscular demands of this exercise. The elevated MG activation supports its application in the final phase of rehabilitation. Moreover, the higher incidence of MG injuries compared to LG injuries (58–65% vs. 8–38%, respectively) in athletes [53] further supports its use in late-stage MG rehabilitation protocols. The MG and LG showed different peak EMG activity across HRE types, consistent with previously reported activation differences during dynamic tasks [7,10,54]. Among the mechanisms proposed to explain these results is neuromuscular compartmentalization, where each gastrocnemius head has its own moment arms and length tension curves during ankle movements [7,54,55]. These findings support our first hypothesis: the MG and LG exhibit different activation profiles between unilateral and bilateral HREs but respond similarly to ROM changes.

Our results support the role of muscle length in promoting longitudinal growth [53]. Although EMG differences across the ROM cannot predict morphological adaptations, they offer initial evidence to guide gastrocnemius exercise selection. Unlike our findings, Hébert-Loisier et al. [26] reported similar MG and LG activation during standing HREs at 0° and 45° knee flexion. This may be due to reduced gastrocnemius tension when the knee is flexed, shifting mechanical advantage to the soleus [56]. Thus, increasing muscle length via ankle dorsiflexion more effectively enhances MG and LG activation than altering the knee angle.

Comparison with previous studies is limited, as most focused on the knee rather than the ankle angle. Similarly, Hébert-Loisier et al. [26] found no MG:LG activation differences when modifying the ROM during standing HREs at 0° and 45° knee flexion. In contrast, other studies reported greater MG activation with lower knee angles, while the LG remained uncharged [23,24,25]. These findings, derived from non-weight-bearing, open-chain tasks with isometric EMG normalization, limit generalizability to standard weight-bearing HREs. Further research is needed to clarify whether increased ankle dorsiflexion alters MG:LG activation patterns. Still, our findings support using an extended ROM in HREs to enhance gastrocnemius activation, in line with evidence favoring training at longer muscle lengths [24,27,28].

The LG showed greater activation in unilateral than bilateral exercises, unlike the MG. This may be due to the full-body-weight support, reduced base of support, and greater neuromuscular demands in unilateral tasks [55]. Additionally, the LG’s lower passive stiffness and smaller volume compared to the MG [53,54] may make it more responsive to these demands. This effect was not observed in the non-dominant leg, suggesting inter-limb asymmetries [57]. However, activation alone does not clarify whether differences are due to strength or neuromuscular control. Given the ankle’s key role in functional tasks and the link between leg dominance and lower-limb injury risk [58], future studies should explore whether these asymmetries contribute to injury susceptibility.

Regarding leg dominance, MG EMG activity was unaffected by any HRE condition. This finding is relevant for two reasons. First, it provides normative data for unilateral and bilateral weight-bearing HREs, supporting its application as a clinical screening tool to assess muscle and soft tissue recovery. Second, it suggests that MG EMG data from the dominant leg during unilateral FROM HREs should not be extrapolated to the non-dominant leg. The trend toward greater MG activation in the non-dominant limb observed in our study requires cautious interpretation. This finding may reflect a stabilization bias, as the non-preferred leg often acts as the support limb during unilateral tasks, potentially demanding enhanced postural control and neuromuscular recruitment. Alternatively, this asymmetry could stem from individual motor strategies or sampling variability inherent in small cohorts (n = 19). While previous studies have shown variable inter-limb EMG patterns during balance-related and functional activities [59], the observed trend in MG activation emphasizes the need for future research exploring neuromechanical roles of the support leg in loaded plantarflexion tasks.

Our second hypothesis was not supported, as MG:LG activation ratios did not differ significantly by ROM or exercise type. Only the unilateral FROM HRE, the most demanding condition, showed a distinct MG:LG ratio. Promsri et al. [59] emphasized the role of leg dominance in motor behavior during balance-challenging tasks, recommending its consideration in neuromuscular control assessments. The elevated MG involvement may reflect the high coordination and stabilization demands of this exercise. While the elevated MG involvement may suggest increased stabilization demands in unilateral FROM HREs, this interpretation should be considered speculative and context-dependent, given the lack of direct kinematic or coordination assessments.

Several study design factors limit the results’ generalizability. The participants were moderately active young adults, experienced in HREs and free from musculoskeletal impairments affecting MG or LG activation. Generalization to older adults or individuals with a gastrocnemius pathology requires further investigation. Activation strategies varied substantially across individuals, as evidenced by large standard deviations. As all HREs were performed unweighted, outcomes may differ under loaded conditions. Future research should examine the effects of external loads and surface inclination on muscle activation.

The present findings have several important applications. LG and especially MG activation during unilateral weight-bearing HREs was comparable to the maximal EMG during sprinting, supporting its use in athletic training and late-stage lower-leg rehabilitation. In this context, the effectiveness of eccentric exercises in managing gastrocnemius tightness has been demonstrated by studies such as that by Hamza et al. [60], who observed that stretching and eccentric exercise (Stanish protocol) successfully normalized gait parameters, reducing compensatory knee flexion in individuals with isolated gastrocnemius tightness. HREs are also employed to enhance gastrocnemius hypertrophy and strength. The MG and LG showed functional differences depending on ankle ROM and exercise type. MG activation increased consistently with the ROM, whereas LG activation depended on dorsiflexion and execution. Additionally, LG activation during unilateral FROM HREs varied with limb dominance, which is relevant for assessing muscular asymmetries. These distinctions should guide individualized rehabilitation programming. The differential activation patterns of the medial and lateral gastrocnemius observed during FROM-based unilateral heel raise exercises have meaningful clinical implications. Targeted loading of the medial head may benefit post-operative rehabilitation following Achilles tendon rupture or medial calf strain, particularly given its high involvement and injury incidence in athletes [13]. Furthermore, unilateral FROM HREs may contribute to functional retraining in individuals recovering from anterior cruciate ligament (ACL) reconstruction and stroke-related gait asymmetries, where selective activation and ankle control are essential. Implementing region-specific gastrocnemius stimulation strategies may support hypertrophic adaptations, enhance postural stability, and assist in late-stage return-to-sport decision-making.

5. Conclusions

This study confirmed differential recruitment of the MG and LG during HREs with varying ROM, execution type, and limb dominance. The MG was more responsive to ROM increases, while the LG was influenced by unilateral execution and dominant leg use. Activation during FROM unilateral HREs reached sprint-level intensities, highlighting their utility in advanced rehabilitation. These findings support individualized programming based on neuromuscular profiles, particularly for late-stage return-to-play or populations at risk for triceps surae dysfunction. Incorporating unilateral HREs with maximal dorsiflexion is recommended to optimize selective activation and address inter-limb asymmetries. Based on these findings, future research should explicitly investigate how different EMG normalization methodologies, as well as advanced techniques for evaluating muscle activity and coordination, influence the observed triceps surae recruitment patterns and their physiological interpretation. This will allow for further refinement of individualized training prescriptions for optimizing both performance and rehabilitation.