Effects of using exposure footwear for firemen and fitness training shoes on lower limb biomechanics during walking – Scientific Reports

This study investigated how EFF influences lower limb joint kinematics, kinetics, energy, and muscle activation compared to FTS during walking. To ensure relevance, only studies examining level walking were included, as other tasks involve different mechanics. Additionally, walking velocity was not statistically different between walking with EEF and with FTS, ensuring that it would not affect our gait variables.

The results showed that wearing EFF increased the ROM of the hip and knee joints in the sagittal plane, while reducing the ROM of the ankle and MTP joints in the same plane. Furthermore, EFF use was associated with increased peak hip flexion/extension moments and ankle dorsiflexion moments, but a decrease in peak dorsiflexion moments at the MTP joint. Energy analysis revealed that wearing EFF resulted in more positive work performed by the hips, increased negative work by the ankles, reduced positive work by the ankles, and increased negative work at the MTP joint. Additionally, muscle activation levels in the rectus femoris and tibialis anterior were higher when wearing EFF compared to FTS.

Figure 2 revealed that MTP joint angles were significantly greater in FTS than in EFF between 60% and 66.8% of the gait cycle. This finding aligns with Dobson’s study, which suggests that less flexible soles alter ankle and foot movement, potentially affecting dorsiflexion angles in the MTP joints²⁰. This may be attributed to the stiffer soles and uppers of EFF, which restrict dorsiflexion in the MTP joints during the late stance and pre-swing phases. The upward curvature of the sole at the front of the shoe, known as the “toe spring,” elevates the toes above the ground in a dorsiflexed position, facilitating the forward roll of the forefoot during the propulsive phase of walking (heel-to-toe). This rolling motion has been shown to reduce mechanical work in shoes with curved rocker soles^19,21,22. Sichting et al.²³ found that toe springs alter joint moments and toe work, with greater curvature requiring less muscular effort during walking. Therefore, the smaller toe spring angle in the EFF used by firefighters may lead to increased muscular work compared to the FTS.

Figure 3 indicates that the ROM of the hip and knee joints in the sagittal plane was significantly greater in the EFF group than in the FTS group. In contrast, the sagittal plane ROM of the ankle and MTP joints was reduced in the EFF group compared to the FTS group. This finding is consistent with the study by Park et al.²⁴, who reported similar results for rubber firefighter boots. Additionally, other studies have shown that leather lace-up boots reduce ankle ROM throughout the gait cycle^15,16,17,18. These results suggest that the high shaft and stiff structure of EFF can limit the dorsiflexion of the ankle and MTP joints during walking, and the increased hip and knee ROM observed in the EFF may serve as a compensatory mechanism for these ankle and MTP joint limitations. The ankle ROM was reduced because the EFF boots increased the ankle plantarflexion angle but reduced the dorsiflexion angle to a larger extent. Previous research has indicated that wearing boots that restrict ankle dorsiflexion can reduce reaction time, thereby impairing agility¹⁸.

The joint moment results showed that the peak hip flexion and extension moments were greater in EFF compared to those in FTS. This finding contrasts with the results of Farzaneh et al.²⁵, who compared boots and casual shoes and found no significant difference in hip flexion and extension moments on the sagittal plane between the two types of footwear. The discrepancy may arise from the differences in study populations (college students in their study) and the use of leather lace-up boots. Furthermore, in the present study, the effect of the different footwear on knee moments and power during the gait cycle was not significant, a result that is consistent with those of previous studies^15,16,25. The large hip flexion and extension moments observed in EFF suggest increased muscle activation and power in the relevant muscle groups. This finding is confirmed by the present study’s surface EMG results (Fig. 7), which align with the findings of previous research^13,14. Increased hip joint moments may lead to heightened patellar pressure and potential pain, as well as increased hip contact force, which could contribute to the development of degenerative hip joint diseases over time²⁵. Tateuchi et al.²⁶ demonstrated a link between the accumulation of hip joint moments and the progression of radiographic signs of hip osteoarthritis.

The peak ankle dorsiflexion moment was greater in the firefighters wearing EFF boots than in those wearing FTS. This result is similar to the findings of Huang et al.²⁷, who reported that rubberized protective boots exhibit higher peak ankle dorsiflexion moments compared to leather ones. Farzaneh et al., discovered that during the early gait cycle, the ankle dorsiflexor muscle groups experience considerably greater moments, power, and work, particularly during eccentric and concentric contraction phases, when boots are worn compared to the situation where athletic shoes are utilized. This suggests increased activation and loading of the ankle dorsiflexor muscles, especially the tibialis anterior²⁵, as confirmed by the EMG results in this study. This finding is consistent with Schulze et al.‘s findings¹². Such increased loading may elevate the risk of conditions such as tibialis anterior muscle strain, shin splints, and chronic exertional compartment syndrome^12,28. No significant difference in ankle plantarflexion moment was found between the two types of footwear, a result that aligns with Böhm et al.‘s findings, although they compared different boot stiffnesses in lace-up leather boots.

The peak dorsiflexion moment of the MTP joint in the EFF group was lower than that in the FTS group. This result may be attributed to the material construction of EFF boots, which include 6061 aluminum for the toe lining, natural rubber for the upper and outer soles, and a puncture-resistant insole layer. This construction results in a rigid boot body and sole, making the boot difficult to flex during walking. To mitigate this stiffness, EFF boots are designed with increased vertical space inside the toe box, reducing resistance during MTP dorsiflexion. This design feature leads to a smaller peak moment of MTP dorsiflexion during walking, thereby helping to minimize the negative effects of EFF boots on firefighters’ lower limb biomechanics.

Muscle work is the only source of mechanical energy for human walking, and the mechanism by which EFF alters the biomechanical characteristics of the lower limbs during walking results from the combined effects of “human” (lower limb muscle activity) and “boot” (EFF characteristics) in the human–boot system. EFF influences lower limb muscle activity and joint kinematics and dynamics, which in turn interact with each other, leading to changes in joint energy. The high weight, stiffness, and tall boot shaft of EEF increase muscle activation in the rectus femoris and tibialis anterior during walking, as these muscles work to overcome the added boot weight and joint resistance. Additionally, the high stiffness of boot restricts the ROM of the ankle and MTP joints. To compensate for these limitations, increased muscle activity results in greater hip flexion and extension moments, as well as elevated ankle flexion moments. Together, these changes in joint moments and angles lead to alterations in joint energy. This change in kinematics and kinetics is accompanied by changes in muscle activity.

A study by Gordon et al.²⁹ highlighted the crucial role of joint energy transfer in the mechanical energy changes of body segments during walking. From a work–energy perspective, the work done by muscle moments influences the system’s mechanical energy: centripetal contraction of joint muscles generates energy for positive work, while centrifugal contraction absorbs energy for negative work. As shown in Fig. 6, the hip joint performs positive work through the extensor muscles, followed by negative work through the flexor muscles and positive work by the flexor muscles. Table 1 shows that the energy generated by the positive work of the hip joint in EFF boots is greater than that in FTS, with no significant difference in negative work. This result suggests that hip muscles must perform more positive work to achieve the same walking maneuver in EFF boots, a finding consistent with the results of Farzaneh et al.²⁵.

No significant difference in the work done by the knee joints was found between the two types of footwear, a result consistent with the findings of Farzaneh and Cikajlo et al.^17,26 but contrasting with those of Kersting et al.¹⁷, who reported a considerable increase in negative work (energy absorption) by the knee during the gait cycle when stiff boots are used for walking. However, their study focused on different boot stiffnesses.

This study also found that, compared to FTS, EFF decreased the positive work done by the ankle plantarflexors and increased the negative work done by the dorsiflexors. Figure 6 shows that the ankle joint underwent the negative work by the dorsiflexors, absorbing a substantial amount of energy, followed by the positive work by the plantarflexors, which generated minimal energy. Farzaneh et al., demonstrated that during the gait cycle, the power and work done by the ankle dorsiflexors increase considerably when boots are worn compared to casual shoe; this increase may elevate the loading on the anterior tibialis muscle and the risk of injury. Moreover, when boots are worn, plantarflexors perform slightly less work than when casual shoes are used, but the difference is not statistically significant²⁵. This discrepancy may be due to differences in the footwear used in their study (leather lace-up boots and casual shoes) and in the present study.

The study results showed that the positive work done by the MTP joints during the gait cycle in EFF was greater than that in FTS. The power curve in Fig. 6 indicates that the MTP joints perform negative work to absorb energy throughout the entire gait cycle in FTS, whereas in EFF, they primarily perform positive work to generate stomping energy at the end of the stance phase, This effect may be attributed to the hard soles of EFF, which cause the toes to produce high stomping energy when they lifted off the ground at the end of the stance phase.

In this study, the EMG signals of key lower limb muscles, including the rectus femoris, lateral femoris, medial femoris, gluteus maximus, biceps femoris, lateral head of the gastrocnemius, and tibialis anterior, were analyzed. The results indicated that the RMS of the rectus femoris and tibialis anterior muscles during the gait cycle was higher in the EFF group than in the FTS group. This suggests that wearing firefighting boots increases the activation levels of these muscles, a finding consistent with the results of Schulze et al.²⁵, although their study used jogging shoes and leather lace-up military boots. Other studies have reported varying results. For instance, Kim et al.¹³ found that medial femoral muscle activity increases sequentially when when wearing Converse sneakers, rain boots, and military combat boots. Dobson et al., reported that muscle activity in the lateral femoris and biceps femoris during walking on an inclined surface in leather lace-up boots is higher than that during walking in rubber boots. This increased muscle activity may serve as a preventive measure against slips and trips in response to challenging surfaces and varying boot characteristics¹⁴. The differences in study results may be due to the varying choice of test shoes and maneuvers.

High muscle activation levels imply considerable energy expenditure, and prolonged high-intensity muscle activation can lead to neuromuscular fatigue. Garner et al.¹⁰ found that rubberized firefighting boots cause greater muscular fatigue in the lower limbs compared to leather boots. Increased activation of the rectus femoris muscle while wearing firefighting boots may also elevate patellar pressure, potentially leading to pain²⁵. Furthermore, heightened activation of the tibialis anterior muscle could increase the risk of overuse injuries, such as shin splints and chronic exertional compartment syndrome^12,28.

During the late gait cycle, which primarily involves plantarflexion of the MTP joints, the deep plantarflexor muscles (e.g., flexor digitorum longus) are likely engaged. However, their activity cannot be measured by surface EMG due to their deep location within the calf muscles. Boot stiffness may influence these deep muscles, and this aspect could be further explored in future studies using alternative methods, such as computer simulations.

Limitations of the study

This study has several limitations. First, although a multisegmental foot model was used to analyze the biomechanical characteristics of the MTP joint while walking in EFF, the stiffness of the toes and soles, combined with the large vertical space inside the toe of the boots, resulted in substantial relative displacement between the toes and the boots. This displacement likely introduced some errors when optical sensors were used to collect kinematic data on the MTP joint in EFF. Second, the study did not strictly control walking speed during data collection, allowing participants to maintain their natural walking habits. This lack of strict control may have influenced the biomechanical parameters measured. Third, this study focused solely on the effect of EFF on the biomechanical characteristics of walking. Future research could extend this analysis to other activities, such as jogging, stair climbing, and obstacle crossing, to obtain a more comprehensive understanding of the boots’ effects.