How boots affect the kinematics and kinetics of lower limb joints during walking compared to casual footwear

Boots are widely used by many people for various purposes, but their impact on gait biomechanics and injury risk is not well understood. This study investigated the effects of boots on walking biomechanics, compared to casual footwear. The lower limb joint kinematics and kinetics of 20 healthy male participants aged 20 to 30 years old were compared during self-paced walking with boots and shoes. The results showed that walking with boots is associated with greater hip extensor (P = 0.009) and ankle dorsiflexor (P < 0.001) moments in early stance, hip power generation (P < 0.001) and knee power absorption (P < 0.001) in early swing phase, hip abductor (P < 0.001) and knee adduction (P < 0.001) moments in the entire stance, net concentric work for the hip joint in sagittal (13.9%, P = 0.001) and frontal (21.7%, P = 0.002) planes. In contrast, the subtalar supinator moment in the entire stance (P < 0.001), ankle angular velocity in late stance (P < 0.001), and net concentric (− 42.7%, P < 0.001) and eccentric (− 44.6%, P = 0.004) works of subtalar joint were significantly lower in the boot condition. The compensatory adjustments in the hip and knee joints may result from ankle restrictions. While boots may aid those with ankle disorders, lower limb loading and the risk of musculoskeletal injuries and osteoarthritis could be increased. This study offers new perspectives on the biomechanical impact of boots on gait, potential prevention and treatment strategies of related injuries, and advancing footwear design.


Study participants
In total, twenty healthy young male university students with an average age of 23 ± 2.43 years, mass of 71.55 ± 10.47 kg, and height of 1.77 ± 0.08 m participated in this study.The required sample size was estimated based on the parameters of 80% power, 5% significance level and 0.72 effect size derived from ankle joint eccentric power reported by Böhm and Hösl 5 .The inclusion criteria were body mass index (BMI) less than 31 kg/m 2 , no history of any pain or orthopedics or neurological pathologies in lower limb that may influence walking, and being physically active.Each participant signed an informed consent prior to data collection, and they were not involved in the study design, analysis and interpretation of the findings.

Footwear
The two footwear conditions tested in this study, a boot and a casual shoe (Fig. 1), were provided locally in three European sizes ranging from 41 to 43.The shoes, which were made of Nubuck, were not as hard as the leather lace-up boots.Compared to the shoes, the boots had more mass (~ 600 vs. 1200 g), shaft height (12.5 cm), and heel-to-toe drop (5 vs. 20 cm).Also, the outsole of the shoes was Polyurethane with a hardness of 50 ShoreA and a density of 450 g/l, respectively.In contrast, the boots had a dual-density outsole made of the same Polyurethane as the shoes and Rubber with a hardness of 65 ShoreA and a density of 1150 g/l, increasing the stiffness and durability.

Study procedures
Retro-reflective markers were attached on bony landmarks of C7 and the right and left acromion, superior iliac spines, knee epicondyles, ankle malleoli, calcaneus, 1st and 5th metatarsal heads.We also fixed a rigid plate with four non-collinear markers on the lateral side of the thigh and shank.Marker trajectories and ground reaction force (GRF) in 3D were measured using a motion capture system with eight infrared cameras (Miqus M3, Qualisys AB, Sweden) and a ground-embedded force plate (Kistler Instrumente AG, Switzerland) at sampling frequency of 200 Hz and 2 kHz, respectively.Both footwear conditions (the boot and the shoe) were tested on all participants.A static trial in a neutral standing posture and multiple self-paced walking trials on a 16-m walkway were recorded for every condition.The footwear conditions were tested in a counterbalanced order and participants had adequate time to warm up and rest before and during the trials.

Data analysis
We analyzed 3-5 successful trials for each footwear condition (on average 8 trials for each participant).Heel strike events were detected using the foot vertical velocity algorithm 13 and double-checked by visual inspection for all trials (since there was only one force plate).Hip joint center markers were created according to the modified Harrington regression equation with pelvis width only 14 .Knee and ankle joint centers were the average of medial and lateral markers of epicondyles and malleoli, respectively.Using the joint center markers for scaling thigh and shank segments were found to improve the estimation of the segments length compared to the corresponding MRI measures 15 .
A full-body constrained model 16 with a six-degrees-of-freedom (DoF) joint between pelvis and ground, three rotational DoF lumbar and hip joints, one DoF knee joint with parameterized rotational and translational coordinates, and one DoF ankle and subtalar joints was employed in this study.Additionally, axes of rotation for the knee, ankle, and subtalar joints followed their physiological movement.The original model was adapted and simplified to fit our purpose by converting head, arms, and torso to a single rigid body, re-defining the metatarsophalangeal as a weld joint, changing the rotation sequence of pelvis to rotation, obliquity, and tilt 17 , removing the constraints over joints RoM, removing patella and its kinematics constraints, and removing all muscles, actuators, and wrapping geometries.
We scaled the generic model to fit each participant's anthropometry for each condition, based on joint centers and anatomical markers, followed by marker registration using the Scale tool in Opensim v.4.4 18 .The tool was executed twice in order to fine-tune the segments dimensions and markers positions.Additionally, the mass of the foot segment in the scaled model of the boot condition was adjusted to account for the footwear mass difference.Joint angles were computed from the experimental markers using the Kalman smoothing algorithm for inverse kinematics which was shown to be a robust algorithm in the presence of soft tissue artifacts 19 .Kinematics and GRF data were smoothed using a zero-phase low-pass Butterworth filter with a 14 Hz cut-off frequency.The Inverse Dynamics tool computed joint moments in the joint coordinate system as a clinically meaningful interpretation of net muscle activation 20 .We computed the knee adduction moment using the Joint Reaction tool as the external moment applied on the tibia and expressed in the tibia coordinate system 21 .Joint power was calculated as the product of joint moment (Nm) and joint angular velocity (rad/s) 22 .The net joint work was calculated as the time integration of the joint power, for both positive (concentric) and negative (eccentric) power phases, and for each muscle group 23 .www.nature.com/scientificreports/ The time series of GRF in stance phase and joint angle, moment, and power in the entire gait cycle were timenormalized (0-100%) and averaged across each participant's trials.Also, nondimentional quantities were computed and scaled to the normal participant 24 using the average mass and leg length across all individuals (Eq.(1)): where S is a scale factor computed for each participant according to their mass m i and leg length l i , and the cor- responding measures of normal mass m norm and leg length l norm .Finally, the ensemble averages of joint kinematics and kinetics were low-pass filtered at 7 Hz.

Statistical analysis
The D' Agostino-Pearson K 2 test of normality was used to check the distribution of the data prior to statistical analysis.Discrete variables (walking velocity and joint work) as well as time-series data (GRF, Joint kinematics and kinetics) were compared between footwear conditions by paired-samples t-test.In the case of the nonparametric test 25 , we used 100 k permutations to achieve stable results numerically.Also, the results of both parametric and nonparametric tests agreed qualitatively.All statistical tests were two-sided with a significance level of P < 0.05 and conducted using the spm1d package 26 .Additionally, to see the clinical relevance of the differences between footwear conditions, effect size was calculated using Cohen's d statistics, interpreted as 0.2 = small, 0.5 = medium, 0.8 = large, and 1.2 = very large 27 and reported only for the significant differences.A custom-made script in Python v.3.8 was employed for all data and statistical processings.

Results
The results indicated no significant difference in walking velocity between conditions (shoe: 1.38 ± 0.12 m/s, boot: 1.36 ± 0.12 m/s, t(19) = 0.83, P = 0.41).The average ensemble graphs of GRF in three directions are presented in Fig. 2.There was only a significant difference between the footwear conditions in the mediolateral GRF in 1.8-3.5% of stance phase (P = 0.01, d = 0.73).

Discussion
This study aimed at investigating the influence of boots on lower limb joints kinematics and kinetics compared to casual shoes.To be concise, we only include studies in which the task of level walking has been investigated, because other tasks have different mechanics and are not related to our topic.Walking velocity was not statistically different between walking with boots and walking with casual shoes, so that it wouldn't affect our gait variables.No significant differences in any percentage of stance phase were observed in any direction of GRF, except a small area at early stance in the mediolateral GRF which was significantly lower in the boot condition, whereas previous studies reported that walking with hard boots is associated with greater vertical GRF 4,[28][29][30] .Although in our study the second peak of the vertical GRF in the boot condition was slightly greater than the www.nature.com/scientificreports/shoe condition, this difference was not statistically significant.This could be probably due to the differences in footwear designs and materials among studies.The most intuitive result in joint kinematics was the decreased ankle RoM in the entire gait cycle.Given that the boot has more heel-to-toe-drop compared to the shoe, the ankle joint was more plantarflexed at the heel strike as well as the rest of the stance phase as expected.These findings are consistent with the previous studies 4,5,11,12,31 .According to Hamill and Bensen (1996), wearing boots that restrict ankle dorsiflexion can impair agility performance by slowing down the reaction time 4 .The ankle dorsiflexor moment, power, and work in both eccentric and concentric contraction phases were significantly higher in the boot condition in the early stance phase of gait.This suggests that the ankle dorsiflexor muscles, particularly the Tibialis Anterior, are overactivated and overloaded, which is consistent with the EMG study of Schulze et al. 8 and the muscle simulation study of Wright et al. 32 .This could increase the risk of disorders such as tibialis anterior muscle overuse injury, shin splint, and chronic exertional compartment syndrome 8,33 .Although the ankle plantarflexor works in both eccentric and concentric contraction phases of walking with boots were slightly less than those in the shoe condition, these differences were not statistically significant.Contrary to Böhm and Hösl 5 , and Kersting et al. 12 , the net ankle joint work was not different between the footwear conditions.
Unexpectedly, the subtalar joint kinematics was not affected by footwear condition, except a nonsignificant reduction in the range of pronation motion.That's probably due to our definition of foot segment and the corresponding marker set (the application of forefoot markers to track the motion of the rearfoot segment and the subtalar joint).A one-segment foot model with forefoot markers as tracking markers likely fails to measure the subtalar joint kinematics and hence kinetics accurately 34 .We suggest implementing a multi-segment foot model or attaching more markers on the rearfoot for future studies.However, the boots significantly decreased the subtalar supinator moment and power generation in the stance phase, as well as the subtalar supinator concentric and eccentric works (Fig. 7), suggesting lower loads applied on the soft tissue in the medial side of the ankle.This may have clinical implications for disorders related to ligament and tendons such as tibialis posterior tendon dysfunction.More pathology-specific studies on the clinical population are required to better understand the clinical implications of boots.The knee joint kinematics and kinetics (moment and power) in the stance phase were barely affected by the footwear conditions; this finding is consistent with Cikajlo and Matjacić 11 and Böhm and Hösl 5 .Contrary to Böhm and Hösl and Kersting et al. who reported that the knee eccentric work in the stance phase was significantly increased during walking with stiff boots 5,12 , in our results, although the knee eccentric work in the entire gait cycle was slightly greater in the boot condition, this difference was not statistically significant.
The results demonstrated that there was no significant difference in the ankle plantarflexor moment and power generation at the time of push-off phase of gait between the boot and shoe conditions; this finding is consistent with Böhm and Hösl 5 , and in contrast with Cikajlo and Matjacić 11 .Further analysis on joint angular velocity (Fig. 6) also showed that, in accordance with Nesterovica-Petrikova et al. 31 , ankle plantarflexion velocity was notably reduced at the time of ankle push-off in the boot condition.On the other hand, there was significantly more hip flexor moment and power generation in the pre-swing and initial swing phases in the boot condition.These findings indicate that the moment generated by ankle plantarflexor muscles, which was equal in magnitude to the shoe condition, was not sufficient to produce sufficient joint velocity/acceleration, and thus, greater contribution of the hip joint to the propulsive power occurred 35 .In other words, hip joint pull-off strategy would compensate for the insufficient ankle joint push-off, and assist swing initiation and forward acceleration of the leg.The boot condition also increased the knee extensor moment and power absorption in the initial swing phase of gait.This could indicate that the more net propulsive power generated by ankle push-off and hip pulloff strategies in the boot condition requires more eccentric contraction of knee extensors (power absorption) to  help smooth the gait 35 .This is in agreement with the EMG study in which greater activation of rectus femoris was observed during walking with stiff boots 8 .This bi-articular muscle functions as the main contributor to both hip joint power generation and knee joint power absorption in pre-and early swing phases of gait 22 .Increased activation of rectus femoris, as well as other quadriceps muscles which was found in EMG studies to be greater in hard boots 9,10 , could increase the compressive force on the patella and cause pain.Accordingly, Sinclar et al. found that patellofemoral contact force, loading rate, and pressure were significantly greater when running with military boots compared to running shoes 36 .A temporal shift in the power generation (concentric) phase of hip flexors as well as the power absorption (eccentric) phase of knee extensors is evident in the graphs, highlighting the compensatory adjustments at hip and knee joints in response to the reduced ankle RoM and angular velocity as well as the insufficient ankle power generation.We speculate that several boot features such as the shaft height, shaft stiffness, sole stiffness, heel-to-toe-drop, and mass are the causes of such biomechanical alterations in the propulsion phase of gait.Further analysis on the knee joint moment revealed that external adduction moment was significantly greater in the boot condition in almost the entire stance phase of gait (Fig. 5).During normal walking, the ground reaction vector passes medially to the knee joint center, generating an adduction moment that tends to adduct the knee joint.So, the majority of the loads are applied to the medial compartment of the knee 37 that could be associated with degenerative effects on the cartilage tissue and joint space narrowing over time.There is a strong relationship between the knee adduction moment and joint loading 38 and therefore, the onset, severity, and progression of osteoarthritis in the medial compartment of the knee 39,40 .Furthermore, excessive external knee adduction moment exerts abnormal load on the soft tissues on the lateral side of the knee such as knee abductor muscles, ligaments, and joint capsule, causing rupture 21 and joint instability.It is also a predictor of ACL injury and patellofemoral pain in landing tasks 41,42 .Consequently, this may lead to pain, functional impairment and disability.
Hip joint angles in sagittal and transverse planes were not statistically different between footwear conditions.In the frontal plane, however, the hip joint adduction angle was significantly greater in the boot condition in a small area in late-stance, followed by a significantly lower abduction angle in the initial swing phase.The hip extensor moment in the loading response phase was significantly greater in boot condition, highlighting a greater need to control the forward acceleration of the trunk and hip.This was previously found to be a compensation strategy in lower limb prosthesis users 35,43 .In the terminal swing phase, the hip extensor moment was also significantly greater in the boot condition, indicating a greater need for hip extensor muscles to decelerate the forward swinging leg 35 .This might be as a result of the greater boot mass that increases the moment of inertia of the leg, and hence, inertial forces and moments used in Inverse Dynamics calculations 44 .The greater hip external rotator and abductor moments and concentric and eccentric works observed in the boot condition could imply that more pelvic stability 35 is required during walking with boots.Contrary to Böhm and Hösl and Kersting et al. who didn't observe any significant differences in the net concentric and eccentric works at the hip joint 5,12 , in our results, they were significantly greater in the boot condition.Overall, the significantly increased hip extensor, abductor, and external rotator moments and the net hip joint work in the boot condition suggest that walking with boots may increase the muscles activation, force, and hence, the hip joint contact force which might be a risk factor for developing hip joint degenerative diseases over time.Previous research has established a link between the cumulative moment of the hip joint, notably in the frontal plane, and the subsequent advancement of radiographic signs of hip osteoarthritis 45 .
Based on the findings of the present study, compensatory changes in response to the reduced ankle RoM and angular velocity occurred at not only the knee joint, but also at the hip joint and the later one potentially contributes to the propulsive power of gait required for an efficient swing phase.Such adaptations were previously seen in individuals who used ankle foot orthosis or transtibial prosthesis which compromise ankle power generation during push-off phase of gait 35,43 .To the best of the author's knowledge, this is the first study that addresses such alteration in lower limb joints biomechanics during walking with boots compared to casual shoes.Further studies are suggested in this field to confirm the present findings that overall would provide useful information for designing footwear and developing more effective interventions for boot-related injuries or conditions that require boots as part of the treatment or rehabilitation plan.Moreover, according to the present findings, it may be suggested that wearing boots could be effective for the prevention and management of disorders related to ankle ligament and tendons.More analysis of cross-correlation or coordination between joints kinematics and kinetics are suggested to better explain these results.
Our gait modeling, which implemented constrained inverse kinematics, physiological movement of the knee joint, a robust regression equation for hip joint center estimation, and Kalman Smoothing algorithm, would enhance the accuracy of our simulation compared to the conventional gait model 46 used in the previous studies 5,11 .This approach, in addition to footwear differences, may explain the conflicting results between our findings and the previous studies to some extent.This study was limited by the lack of data on muscle activation and strength.Furthermore, we could not apply any algorithms to reduce residual forces and moments accounted for dynamic inconsistencies 47 , as there was only one force plate and we were not able to measure all the external forces on the body in a gait cycle.In this study, only the healthy young men were included, which makes it difficult to generalize the results to women or other age groups.We suggest further analyses on muscle force and joint load to better understand the alterations and adaptations in response to walking with boots.More prospective cohort studies are also required to understand the risk factors for boot-related injuries in the long-term.

Conclusion
This study found that more contribution of the hip joint to the propulsive power of gait is required during walking with boots compared to casual shoes.Reduced ankle RoM and angular velocity, as well as insufficient ankle power generation in gait lead to compensatory adjustments at hip and knee joints that may apply excessive and abnormal loads on hip and knee joints and muscles.Consequently, this may increase the likelihood of experiencing musculoskeletal injuries and osteoarthritis.It is imperative to undertake a comprehensive overhaul of the design of boots with the aim of mitigating their adverse effects.However, lower loads on the soft tissue in the medial side of the ankle as the result of wearing boots may have clinical implications for disorders related to ankle joint ligament and tendons.

Figure 1 .
Figure 1.The footwear.The shoe (above) and the boot (below).

Figure 2 .
Figure 2. Ground reaction force.Depicted in anterior-posterior, vertical, and medial-lateral directions for the shoe (dashed black) and the boot (solid blue) conditions.

Figure 3 .
Figure 3.The kinematics and kinetics of the hip joint.Depicted in sagittal, frontal and transverse planes for the shoe (dashed black) and the boot (solid blue) conditions.

Figure 4 .
Figure 4.The kinematics and kinetics of the knee, ankle, and subtalar joints.Depicted for the shoe (dashed black) and the boot (solid blue) conditions.

Figure 5 .
Figure 5. Knee adduction moment.Depicted for the shoe (dashed black) and the boot (solid blue) conditions.

Figure 6 .
Figure 6.Ankle angular velocity.Depicted for the shoe (dashed black) and the boot blue) conditions.

Figure 7 .
Figure 7. Joints work.Depicted in each muscle group for the shoe (dotted orange) and the boot (hashed blue) conditions.