Do Worn-In Tactical Boots Affect Lower-Extremity Biomechanics During Walking and Running?

Abstract

Background: Much of the research conducted on tactical-style boots has evaluated the biomechanical effect of boots in brand-new condition; however, the extent to which changes due to wear influence lower-extremity biomechanics remains uninvestigated. The purpose of this study was to compare lower-extremity biomechanics with worn-in boots and running shoes during both walking and running. Methods: Lower-extremity biomechanical parameters were evaluated during walking and running in 12 individuals with previous tactical experience. Participants were asked to complete one 5 min bout of walking and running at a self-selected pace in both self-selected athletic shoes and their own worn-in standard-issue tactical boots while lower-extremity spatiotemporal, joint kinematic, kinetic, and ground reaction force data were collected. Differences between conditions were evaluated using the Wilcoxon signed-rank test. Results: Spatiotemporal measures of gait, as well as ankle and hip kinematics, were different between shoes and boots during walking. During running, no spatiotemporal differences existed. However, significant differences were found for the ankle, knee, and hip kinematics between shoe and boot conditions during both walking and running. Conclusions: The worn-in boots in our sample performed similarly to running shoes during both walking and running tasks. Though there were several biomechanical differences between boots and shoes during both tasks, small mean differences suggest that these differences may not be large enough to create substantive or relevant changes in performance. This information could aid in developing future tactical boot design strategies to help aid in lower-extremity injury as well as allowing for optimal performance when wearing boots.

1. Introduction

Non-combative musculoskeletal injuries (MSKIs) are prevalent and costly and often have long-term impacts on service members and tactical athletes. MSKIs are a primary cause of outpatient treatment and hospitalizations, accounting for approximately 2 million clinic visits and 25 million limited duty days annually [1]. The knee is most susceptible to overuse MSKIs (19%), closely followed by the ankle (16%) and the lower leg (8%) [2]. Load carriage, high-intensity strength and conditioning programs, and the influence of footwear have been identified as major contributors of lower-extremity MSKIs [1,3,4].

One of the most important factors contributing to lower-extremity MSKIs is footwear [5,6,7]. With their intended design, tactical boots offer protection for the foot while simultaneously attenuating shock and controlling frontal plane foot motion. New boots, those with no long-term wear history, have been shown to affect ground reaction forces (GRFs), kinematics, and kinetics, thus altering the gait patterns of typical athletic shoes and increasing the risk of injury [5]. In a recent study, Bini et al. (2021) reported that brand-new tactical boots produced increased loading rate and second peak GRFs compared to an athletic shoe during overground walking [8]. Kinematic analyses of new tactical boots have reported reduced ankle dorsiflexion [9], and recent work by Sinclair et al. (2014) with different styles of tactical boots demonstrated increased eversion of the ankle, greater internal rotation of the knee, and reduced hip and knee adduction [10]. They subsequently also reported increased plantarflexion and knee extension moments, increased patellofemoral compressive forces, and reduced knee abduction moments in tactical boots compared to athletic shoes, all of which are linked to mechanisms of injury [10,11].

The majority of research has been conducted on the effects and implications of brand-new tactical boots on biomechanics of the foot and ankle. The protocols of these research studies have had participants familiar with such boot wear to perform tasks in either a brand-new pair of boots [8,9,10,11,12] or have had participants who do not regularly wear boots perform tasks in brand-new boots or athletic shoes [10]. These research limitations make the translation of results to current tactical athletes wearing worn-in or boots that have a long-term wear history challenging. In recreational running, it has been suggested that shoes should be replaced after 500–700 km of use as the force-attenuating properties of the shoe degrade [13]. Currently, there is no recommendation for the replacement of tactical boots. Interestingly, more contemporary investigations of the force-attenuating properties of worn-in running shoes have demonstrated reduced peak plantar pressures compared to brand-new running shoes [14,15].

From a kinematic perspective, boots have a noticeable impact on the plantarflexion range of motion in large part due to increased collar height and boot shaft stiffness. These properties have been shown to have significant effects on the plantarflexor muscle group’s ability to generate joint power during gait [16]. Cikajlo et al. (2007) reported that participants who walked in boots made with a more compliant shaft and sole were able to produce increased sagittal plane range of motion and ankle joint power [17]. In a comparison of tactical boots and athletic shoes, Sinclair et al. (2014) found significant kinematic changes in ankle eversion, tibial internal rotation, hip abduction, and knee adduction when running [10]. Understandably, the material properties of the boot that have been shown to affect force attenuation and ankle range of motion will degrade with use. The extent to which those changes influence lower extremity biomechanics remains in question.

Currently, there is no biomechanical assessment of worn-in tactical boots compared to athletic shoes. Given that tactical personnel often wear their boots for many months and years, a biomechanical analysis of boots with a history of wear may indicate areas of susceptibility for lower-extremity MSKIs in tactical athletes. With the association of boot wear to MSKI, and the lack of recommendation for replacement of worn-in boots, an investigation into the lower-extremity biomechanics when wearing worn-in tactical boots is warranted. Such an investigation offers a practical assessment with direct implications for MSKIs, not only for the foot and ankle but also for the entire lower extremity.

Thus, the purpose of this study was to compare GRF, temporal–spatial parameters, and lower extremity kinematics and kinetics (ankle, knee, and hip) with worn-in boots and running shoes during both walking and running. We hypothesize that differences will persist between running shoes and worn-in boots. Specifically, we hypothesize that (1) participants will demonstrate reduced peak ankle, knee, and hip joint angles and joint moments in worn-in boots; (2) worn-in boots will still promote increased spatiotemporal parameters (e.g., greater step length and step width); and (3) worn-in boots will promote greater peak GRF in all three directions. Primary variables of interest include GRF in all three planes, spatiotemporal parameters of gait (e.g., step length and step width), peak, heel strike and toe-off joint angles, and peak stance-phase joint moments in the sagittal and frontal planes.

2. Materials and Methods

2.1. Experimental Design and Subjects

Twelve recreationally active males and females with previous tactical (military or public service) experience and in possession of standard-issue tactical boots with a greater than 3-month history of dedicated wear (

Table 1. Mean ± SD of participant descriptive characteristics.

Variable	Total (n = 12)
Age (yrs)	23.4 ± 2.5
Height (m)	1.69 ± 0.10
Weight (kg)	72.53 ± 11.49
Walking Velocity (m/s)	1.16 ± 0.11
Running Velocity (m/s)	2.04 ± 0.17

) volunteered to take part in this study. Participants were excluded from the study if there was any history of surgical procedures or injury to the lower body in the previous year. Prior to participating in the study, all participants provided informed consent approved by the institutional review board.

2.2. Experimental Protocol

Upon arrival at the laboratory, participant height and mass were recorded. Retroreflective anatomic markers were placed bilaterally on the iliac crest, greater trochanter, medial and lateral femoral epicondyles, medial and lateral malleoli, distal end of the second toe, and the first and fifth metatarsal heads. Rigid thermoplastic segmental tracking clusters were placed on the trunk and pelvis and bilaterally on the thighs, shanks, and heel. Participants then performed 5 min bouts of walking and running at a self-selected pace in their personal athletic shoes and boots (Figure 1). Marker trajectory and GRF data were simultaneously recorded during the final 15 s of each bout of walking and running. Footwear condition was randomized among participants; however, all participants performed the walking condition immediately preceding the running condition. Between shoe conditions, participants were given sufficient time to rest and change shoes. All walking and running trials were performed on a tandem belt instrumented treadmill (1200 Hz, American Mechanical Technology Inc., Watertown, MA, USA) within a 10-camera motion capture volume (240 Hz, Qualisys, Göteborg, Sweden).

2.3. Data Processing

Raw marker trajectories and GRF were exported into Visual 3D for analysis (Version 6, C-Motion, Inc., Germantown, MD, USA). Data were then low-pass filtered using a recursive zero-lag fourth-order Butterworth filter with a cutoff frequency of 10 Hz [18]. Angular computations were completed using a Cardan rotational sequence (X-Y-Z). Ankle dorsiflexion and inversion, knee extension and adduction, and hip flexion and adduction defined positive joint rotations, and GRFs were normalized to body mass. Heel strike and toe-off of each step defined the stance phase and were determined using a 10 N threshold of the vertical GRF. All variables were analyzed during the stance phase of the right leg, and participant averages were computed from each stride completed.

2.4. Statistical Analyses

All variables were compared between conditions using a Wilcoxon signed-rank test with a significance level set a priori at α = 0.05. All statistical tests were performed using SPSS (v28.0, SPSS Inc., Chicago, IL, USA). Cohen’s d effect sizes were also calculated using Hopkin’s interpretation [19,20]. Effect size was interpreted as trivial (0–0.19), small (0.20–0.49), moderate (0.50–0.79), and large (0.80 and greater).

3. Results

3.1. Shoe vs. Boot Walking Mechanics

Statistically significant differences in spatiotemporal parameters were found between shoes and boots during walking. Both step length (shoe: 0.63 ± 0.05 m, boot: 0.65 ± 0.06 m, p = 0.013, d = 0.36) and step width (shoe: 0.13 ± 0.02 m, boot: 0.15 ± 0.03 m, p = 0.031, d = 0.78, Table S1) were found to be significantly larger while walking in boots compared to shoes.

The sagittal-plane ankle angle at heel strike and toe-off during stance was significantly different between shoes and boots (Table S2). While wearing shoes, individuals were in a plantarflexed position at heel strike compared to boots, which exhibited ankle dorsiflexion (shoe: −1.59 ± 4.44°; boot: 0.56 ± 3.54°, p = 0.012, d = 0.53). At toe-off, boots demonstrated significantly less plantarflexion compared to shoes (shoe: −13.79 ± 7.31°; boot: −7.70 ± 5.11°, p = 0.034, d = 0.97). Mean peak ankle dorsiflexion was 2.09° greater while walking in boots compared to shoes (shoe: 6.28 ± 5.24°; boot: 8.37 ± 3.30°, p = 0.028, d = 0.48). Conversely, mean peak ankle plantarflexion was 3.46° greater in the shoe condition compared to boots (shoe: −15.10 ± 4.44°; boot: −11.64 ± 2.83°, p = 0.019, d = 0.93). At the hip, boots created greater mean peak hip flexion angle compared to shoes while walking (shoe: 24.28 ± 7.14°; boot: 28.83 ± 8.25°, p = 0.015, d = 0.59). No statistically significant differences existed in knee joint angles between shoes and boots during walking. Kinematic waveforms during walking are presented in Figure 2.

We observed a statistically significant difference in frontal-plane ankle moment between shoes and boots during walking (Table S4). Individuals had greater ankle eversion moment when walking in boots compared to walking in shoes (shoe: −0.10 ± 0.07 Nm/kg, boot: −0.15 ± 0.17 Nm/kg, p = 0.031, d = 0.58). No other joint kinetic differences were found. We also did not observe any statistically significant differences in ground reaction forces between shoes and boots during walking (Table S5).

3.2. Shoe vs. Boot Running Mechanics

No statistically significant differences were found in spatiotemporal parameters between shoes and boots during running (Table S1). However, we did observe statistically significant differences in ankle, knee, and hip joint angles between the shoe and boot conditions. During running, sagittal-plane hip angle and frontal-plane ankle angle at heel strike were significantly different between shoes and boots (Table S2). Hip flexion angle at heel strike was 5.01° greater while running in boots compared to running in shoes (shoe: −25.43 ± 11.10°; boot: −30.44 ± 12.19°, p = 0.034, d = 0.43). While wearing shoes, individuals exhibited greater ankle eversion at heel strike compared to boots, which demonstrated greater ankle inversion (shoe: 2.19 ± 3.09°; boot: −0.03 ± 4.07°, p = 0.041, d = 0.61, Table S3).

We also observed that sagittal-plane ankle angle was significantly different during toe off. Greater ankle plantarflexion was seen in the shoe condition compared to the boot condition (shoe: −15.45 ± 5.04°; boot: −10.35 ± 4.62°, p = 0.019, d = 1.05, Table S2, Figure 3). Differences between mean peak ankle inversion and eversion were also observed. While running in shoes, individuals experienced 2.25° more of mean peak ankle inversion compared to running in boots (shoe: 3.42 ± 3.62°; boot: 1.17 ± 4.04°, p = 0.05, d = 0.59, Table S3). Individuals also experienced 2.19° more of peak ankle eversion while running in shoes compared to boots (shoe: −11.23 ± 3.53°; boot: −9.04 ± 3.03°, p = 0.019, d = 0.67). Mean peak knee flexion angle was significantly different between shoes and boots, with the boot condition exhibiting 3.31° greater knee flexion angle compared to shoes (shoe: −40.90 ± 4.99°; boot: −44.21 ± 5.46°, p = 0.05, d = 0.63). Kinematic waveforms during running are presented in Figure 4.

4. Discussion

The main findings of this investigation support those previously published that tactical boots have an impact on the lower-extremity biomechanics during both walking and running. These findings are novel to those previously reported as participants completed all walking and running trials in their personal, worn-in footwear for both the athletic shoe and tactical boot conditions. Anecdotally, it is common knowledge that boots are not replaced frequently in tactical populations for a variety of reasons (i.e., time to break in, comfort, cost, and deployment status); therefore, it is important to understand if biomechanical variables related to injury are present after a period of use. From a biomechanical perspective, the worn-in tactical boots in our sample performed similarly to running shoes during both walking and running tasks. Though there were several statistically significant differences between boots and shoes during both tasks, small mean differences suggest that these differences may not be large enough to create substantive or relevant differences in performance.

Our hypotheses regarding joint kinematics were partially supported, as small differences in joint kinematics were observed during walking between shoe conditions (Figure 2). At both heel strike and toe-off, there was decreased plantarflexion while wearing the boot than compared to the shoe. This supports previous findings by Schulze et al. (2014) which also found reductions in plantarflexion when wearing boots compared to other shoe and barefoot conditions [21]. Additionally, statistically significant differences were seen in peak dorsiflexion and plantarflexion values between shoe conditions. Though statistically different, these differences are likely not functionally meaningful at the ankle joint, which produces a large (>50°) joint range of motion. An illustrative example of this is provided with the sagittal-plane ankle angle at toe-off during walking and running in Figure 3. It should be noted that the mean difference between the two footwear conditions is between 5 and 7 degrees of sagittal-plane rotation during both walking and running. Although “statistically” different by our a priori definition, with a moderate effect size, a 5-degree difference is likely not “functionally” different for the ankle joint with a large sagittal-plane range of motion. The collar of the boot rising above the talocrural joint and extending to the mid-shank restricts ankle joint movement in the sagittal plane of the ankle [17,22,23]. It should be noted that both shoe conditions’ total ankle joint range of motion in the sagittal plane (the difference between peak plantarflexion and peak dorsiflexion) was similar in magnitude, achieving a total range of approximately 20 degrees. The boot, however, keeps the ankle in a less plantarflexed state and promotes greater dorsiflexion as the stance phase progresses.

The constraints of the collar of the boot play a significant role in the frontal-plane motion of the ankle. In the walking condition, kinematic differences between the boot and shoe conditions were present in the sagittal plane, whereas, in the frontal plane, they were only present during the running condition (Figure 4). Running in boots constrains the ankle in the frontal plane. This reduction in peak joint angles and range of motion may be indicative of frontal plane ankle stability afforded by the higher collar of the boots.

During the walking condition, greater hip flexion was present when wearing the boot. This increased hip flexion may likely serve to assist in toe clearance during a swing. Previous investigations in footwear worn by firefighters have demonstrated that the increased mass of the boot and the reduction in the ankle joint range of motion can lead to a more likely occurrence of a trip when attempting to clear a 30 cm tall obstacle [24]. Thus, to improve toe clearance, participants likely exhibited greater hip flexion, thereby elevating the foot during a swing. Collectively, the changes in both hip and ankle kinematics may also serve to explain the changes in step length between shoes and boots seen in the current study. Increased hip flexion and reduced ankle plantarflexion would potentially allow for the foot to travel further during each step. Future investigations should explore the relationship between hip and ankle kinematics and spatiotemporal parameters in tactical boots conditions.

During the running condition, the findings in the current study both supported and contrasted those previously reported [10,25]. We found that hip flexion angle at heel strike was significantly greater while running in boots compared to shoes. Schulze et al. (2014) attributed increases in hip flexion when wearing tactical boots to an increase in step length. This increase in step length was driven by the increased weight of the boot. While we did not see significant differences in step length between shoes and boots during the stance phase of running, it is plausible that the weight of the boot influenced swing-phase mechanics prior to heel strike. Shamsodinni et al. (2022) [25] reported the kinematics and joint kinetics of 17 healthy males while running in new pairs of boots and running shoes. They reported significantly greater lower-extremity joint moments during running in boots, concluding that running in new boots in more physically demanding and associated with a greater contribution of lower-extremity joint torque production. Conversely, we found comparable plantarflexion moments, and reduced knee extension and hip abduction moments when running in boots. It is likely that with the degradation of the integrity of the boot, the physical and metabolic cost of running in boots is reduced.

Our hypothesis regarding GRF in worn-in boots was not supported. A secondary finding of this work was that peak GRFs in all directions were comparable between shoe conditions during both tasks. This is similar to the results reported by Shamsoddini et al. (2022), who reported no significant differences between peak GRFs in all directions [25]. Thus, it seems that worn-in boots attenuate force to the lower extremities with similar effectiveness as running shoes. In our study, however, we did observe an increase in peak knee flexion during running while wearing the boot. This may be a compensatory strategy to reduce the magnitude of peak GRF when running. The greater knee flexion seen in the boot also supports the findings of Sinclair and Taylor (2014) [11], which could aid in the shock attenuation needed with running in the boot condition. It has previously been proposed that the changes in both sagittal-plane knee and ankle mechanics were to aid in this shock attenuation. As the current study found sagittal-plane differences at the knee and hip and not the ankle, this should be further investigated.

In the walking condition, step length was greater while wearing boots than in the shoe condition. This supports previous findings demonstrating greater stride lengths in boot conditions compared to both barefoot and shoe conditions [21,26]. This finding is not unique to tactical footwear as increases in stride length have been reported when comparing barefoot walking to shod conditions [27]. While running, no differences were observed in any spatiotemporal measures between shoe conditions. The single-limb support and adduction of the hip during running may eliminate the step width differences that were seen during walking. This supports previous findings where no differences were observed in step width and length between boot and shoe conditions during running [25]. It is important to note that all previous investigations used a standardized boot and shoe, whereas the current investigation allowed participants to use their boots and shoes that had been broken in. This would suggest that footwear age and footwear mileage may not influence spatiotemporal parameters between boots and running shoes. However, future research is warranted in investigating tactical boot lifespan and its influence on lower-extremity mechanics.

This study is not without its limitations. In the current investigation, participants used their boots and shoes with self-reported usage histories. Though we believe that utilizing worn-in boots added a level of ecological validity to our investigation, future studies should act to quantify specific attributes of the wear history and condition of each article. These characterizations were not obtained for the current work. Additionally, using the within-subject study design allows for more flexibility as to the specifics of the footwear being used. As the sample used for this investigation were experienced tactical athletes, the footwear tested met current military boot standards [28]. Further investigations should examine the impact of new vs. worn-in boots on lower-extremity mechanics, as several differences exist between the current investigation and those previously reported. This would be especially useful in aiding in the development of when tactical footwear should be replaced. Finally, the self-selected running velocity determined by participants may be lower than their true self-selected velocity. All participants were to self-select the pace at which they could run comfortably for 2 miles, thus simulating a real-life tactical physical training session. This may be explained by the limitations of the instrumented treadmill, which does not allow for the control of treadmill parameters such as belt speed, incline, and belt direction by the participant. Rather, these parameters are controlled by a researcher via control software installed on the computer in which the treadmill is wired. Thus, this experimental protocol required self-selected velocities to be identified by the participant by producing hand-signs to the research staff (i.e., thumbs up for “faster” and thumbs down for “slower”). This constraint without visual feedback of speed given to the participants may have lowered the self-selected running pace of the participants.

5. Conclusions

In conclusion, data from this investigation show that wearing worn-in tactical boots has a small impact on the mechanics of the lower extremity in both walking and running conditions. While this has been previously shown, the use of personnel wearing their own worn-in footwear provides further evidence that these differences do exist potentially across the lifespan of the boot and, thus, should be further investigated. These results suggest that worn-in tactical boots promote similar lower-extremity biomechanics to athletic shoes during walking and running, while offering necessary protection for the foot. Thus, restrictions to range of motion, shock attenuation, or GRF loading rate inherent with a new tactical boot and indicative of MSKI risk may be abated as the boot wears in. Even though there were several statistically significant differences between boots and shoes during both tasks, small mean differences suggest that these differences may not be large enough to create substantive or relevant differences in performance. Future iterations of tactical boots may seek to provide functional performance similar to athletic shoes—while maintaining protective integrity—sooner in the lifespan of the boot, or even when it is brand new. This information would be impactful when developing strategies in reducing the burden that lower-extremity injuries have on tactical athletes, as well as the design of future boots to allow for optimal performance while protecting the foot.