Neuromechanical adaptations of foot function when hopping on a damped surface.

To preserve motion, humans must adopt actuator-like dynamics to replace energy that is dissipated during contact with damped surfaces. Our ankle plantar flexors are credited as the primary source of work generation. Our feet and their intrinsic foot muscles also appear to be an important source of generative work, but their contributions to restoring energy to the body remain unclear. Here, we test the hypothesis that our feet help to replace work dissipated by a damped surface through controlled activation of the intrinsic foot muscles. We used custom-built platforms to provide both elastic and damped surfaces and asked participants to perform a bilateral hopping protocol on each. We recorded foot motion and ground reaction forces, alongside muscle activation, using intramuscular electromyography from flexor digitorum brevis, abductor hallucis, soleus, and tibialis anterior. Hopping in the Damped condition resulted in significantly greater positive work and contact-phase muscle activation compared with the Elastic condition. The foot contributed 25% of the positive work performed about the ankle, highlighting the importance of the foot when humans adapt to different surfaces.NEW & NOTEWORTHY Adaptable foot mechanics play an important role in how we adjust to elastic surfaces. However, natural substrates are rarely perfectly elastic and dissipate energy. Here, we highlight the important role of the foot and intrinsic foot muscles in contributing to replacing dissipated work on damped surfaces and uncover an important energy-saving mechanism that may be exploited by the designers of footwear and other wearable devices.

Neuromechanical adaptations of foot function to changes in surface stiffness during hopping.

Humans choose work-minimizing movement strategies when interacting with compliant surfaces. Our ankles are credited with stiffening our lower limbs and maintaining the excursion of our body's center of mass on a range of surface stiffnesses. We may also be able to stiffen our feet through an active contribution from our plantar intrinsic muscles (PIMs) on such surfaces. However, traditional modeling of the ankle joint has masked this contribution. We compared foot and ankle mechanics and muscle activation on low, medium, and high stiffness surfaces during bilateral hopping using a traditional and anatomical ankle model. The traditional ankle model overestimated work and underestimated stiffness compared with the anatomical model. Hopping on a low stiffness surface resulted in less longitudinal arch compression with respect to the high stiffness surface. However, because midfoot torque was also reduced, midfoot stiffness remained unchanged. We observed lower activation of the PIMs, soleus, and tibialis anterior on the low and medium stiffness conditions, which paralleled the pattern we saw in the work performed by the foot and ankle. Rather than performing unnecessary work, participants altered their landing posture to harness the energy stored by the sprung surface in the low and medium conditions. These findings highlight our preference to minimize mechanical work when transitioning to compliant surfaces and highlight the importance of considering the foot as an active, multiarticular, part of the human leg.NEW & NOTEWORTHY When seeking to understand how humans adapt their movement to changes in substrate, the role of the human foot has been neglected. Using multi-segment foot modeling, we highlight the importance of adaptable foot mechanics in adjusting to surfaces of different compliance. We also show, via electromyography, that the adaptations are under active muscular control.

Boot-insole effects on comfort and plantar loading at the heel and fifth metatarsal during running and turning in soccer.

Plantar loading may influence comfort, performance and injury risk in soccer boots. This study investigated the effect of cleat configuration and insole cushioning levels on perception of comfort and in-shoe plantar pressures at the heel and fifth metatarsal head region. Nine soccer academy players (age 15.7 ± 1.6 years; height 1.80 ± 0.40 m; body mass 71.9 ± 6.1 kg) took part in the study. Two boot models (8 and 6 cleats) and two insoles (Poron and Poron/gel) provided four footwear combinations assessed using pressure insoles during running and 180° turning. Mechanical and comfort perception tests differentiated boot and insole conditions. During biomechanical testing, the Poron insole generally provided lower peak pressures than the Poron/gel insole, particularly during the braking step of the turn. The boot model did not independently influence peak pressures at the fifth metatarsal, and had minimal influence on heel loads. Specific boot-insole combinations performed differently (P < 0.05). The 8-cleat boot and the Poron insole performed best biomechanically and perceptually, but the combined condition did not. Inclusion of kinematic data and improved control of the turning technique are recommended to strengthen future research. The mechanical, perception and biomechanical results highlight the need for a multi-faceted approach in the assessment of footwear.

Changes in sagittal plane kinematics with treadmill familiarization to barefoot running.

Interest in barefoot running and research on barefoot running are growing. However a methodological issue surrounding investigations is how familiar the participants are with running barefoot. The aim of the study was to assess the amount of time required for habitually shod runners to become familiar with barefoot treadmill running. Twelve female recreational runners, who were experienced treadmill users, ran barefoot on a treadmill for three bouts, each bout consisting of 10 minutes at a self-selected speed with 5 minute rest periods. Sagittal plane kinematics of the hip, knee, ankle, and foot during stance were recorded during the first and last minute of each 10-minute bout. Strong reliability (ICC > .8) was shown in most variables after 20 minutes of running. In addition, there was a general trend for the smallest standard error of mean to occur during the same period. Furthermore, there were no significant differences in any of the biomechanical variables after 20 minutes of running. Together, this suggests that familiarization was achieved between 11 and 20 minutes of running barefoot on a treadmill. Familiarization was characterized by less plantar flexion and greater knee flexion at touchdown. These results indicate that adequate familiarization should be given in future studies before gait assessment of barefoot treadmill running.

Relationship between metabolic cost and muscular coactivation across running speeds.

OBJECTIVES: Muscular coactivation can help stabilise a joint, but contrasting results in previous gait studies highlight that it is not clear whether this is metabolically beneficial. The aim was to assess the relationship between the metabolic cost of running and muscular coactivation across different running speeds, in addition to assessing the reliability and precision of lower limb muscular coactivation. DESIGN: Eleven female recreational runners visited the laboratory on two separate occasions. On both occasions subjects ran at three speeds (9.1, 11 and 12 km h(-1)) for six minutes each. METHODS: Oxygen consumption and electromyographic data were simultaneously recorded during the final two minutes of each speed. Temporal coactivations of lower limb muscles during the stance phase were calculated. Five muscles were assessed: rectus femoris, vastus lateralis, biceps femoris, tibialis anterior and gastrocnemius lateralis. RESULTS: Nonparametric correlations revealed at least one significant, positive association between lower limb muscular coactivation and the metabolic cost of running for each speed. The length of tibialis anterior activation and muscular coactivation of the biceps femoris-tibialis anterior and gastrocnemius lateralis-tibialis anterior decreased with speed. CONCLUSIONS: These results show that longer coactivations of the proximal (rectus femoris-biceps femoris and vastus lateralis-biceps femoris) and leg extensor (rectus femoris-gastrocnemius lateralis) muscles were related to a greater metabolic cost of running, which could be detrimental to performance. The decrease in coactivation in the flexor and distal muscles at faster speeds occurs due to the shorter duration of tibialis anterior activation as speed increases, yet stability may be maintained.

Mechanisms for improved running economy in beginner runners.

UNLABELLED: Controversy surrounds whether running mechanics make good predictors of running economy (RE) with little known about the development of an economical running gait. PURPOSE: The aim of this study was to identify if mechanical or physiological variables changed during 10 wk of running in beginners and whether these changes could account for any change in RE. METHODS: A 10-wk running program (10 wkRP) was completed by 10 female beginner runners. A bilateral three-dimensional kinematic and kinetic analysis, in addition to RE and lower body flexibility measurements, was performed before and after the 10 wkRP. The Balke-Ware graded walking exercise test was performed before and after the 10 wkRP to determine VO2max. RESULTS: Seven kinematic and kinetic variables significantly changed from before to after training, in addition to a significant decrease in calf flexibility (27.3° ± 6.3° vs 23.9° ± 5.6°, P < 0.05). A significant improvement was seen in RE (224 ± 24 vs 205 ± 27 mL · kg(-1) · km(-1), P < 0.05) and treadmill time to exhaustion (16.4 ± 3.2 vs 17.3 ± 2.8 min, P < 0.05); however, VO2max remained unchanged from before to after training (34.7 ± 5.1 vs 34.3 ± 5.6 mL · kg(-1) · min(-1)). Stepwise regression analysis showed three kinematic variables to explain 94.3% of the variance in change in RE. They were a less extended knee at toe off (P = 0.004), peak dorsiflexion occurring later in stance (P = 0.001), and a slower eversion velocity at touchdown (P = 0.042). The magnitude of change for each variable was 1.5%, 4.7%, and 34.1%, respectively. CONCLUSIONS: These results show that beginner runners naturally developed their running gait as they became more economical runners.

Biomechanical response to changes in natural turf during running and turning.

Integrated biomechanical and engineering assessments were used to determine how humans responded to variations in turf during running and turning. Ground reaction force (AMTI, 960 Hz) and kinematic data (Vicon Peak Motus, 120 Hz) were collected from eight participants during running (3.83 m/s) and turning (10 trials per condition) on three natural turf surfaces in the laboratory. Surface hardness (Clegg hammer) and shear strength (cruciform shear vane) were measured before and after participant testing. Peak loading rate during running was significantly higher (p < .05) on the least hard surface (sandy; 101.48 BW/s ± 23.3) compared with clay (84.67 BW/s ± 22.9). There were no significant differences in running kinematics. Compared with the "medium" condition, fifth MTP impact velocities during turning were significantly (RM-ANOVA, p < .05) lower on clay (resultant: 2.30 m/s [± 0.68] compared with 2.64 m/s [± 0.70]), which was significantly (p < .05) harder "after" and had the greatest shear strength both "before" and "after" participant testing. This unique finding suggests that further study of foot impact velocities are important to increase understanding of overuse injury mechanisms.

Knee joint stiffness during walking in knee osteoarthritis.

OBJECTIVE: To investigate the construct validity of walking knee stiffness as a measure to differentiate between individuals with and without knee osteoarthritis (OA) and the construct validity of walking knee stiffness as related to self-reported knee stiffness. The contributors to walking stiffness and its relationship with loading rate and adduction moment are also investigated. METHODS: Thirty-seven individuals with knee OA and 11 asymptomatic controls participated. Knee stiffness was calculated during walking as the change in knee flexion-extension moment divided by the change in knee flexion angle. Forward-stepwise regression models and Pearson's correlation coefficients were used to evaluate the relationships between variables. RESULTS: Knee stiffness in walking was significantly greater in the OA group (mean +/- SD 10.1 +/- 4.4 Nm/ degrees /kg x 100) compared with the controls (mean +/- SD 5.6 +/- 1.5 Nm/degrees/kg x 100) (P < 0.001). Knee excursion range explained 39% of the variance in walking knee stiffness (B = -0.736, P < 0.001) and knee extensor moment a further 7% (B = 6.974, P = 0.045). In the OA group, walking knee stiffness was not associated with self-reported stiffness (r = 0.029; P = 0.863). For the OA group, greater self-reported stiffness was associated with lower peak knee adduction moment (B = -0.354, P < 0.001). CONCLUSION: The construct validity of walking knee stiffness is supported. The poor correlation between walking stiffness and self-reported stiffness suggests the 2 measures evaluate different aspects of knee stiffness. Since a measure of walking stiffness is likely to provide valuable information, future research evaluating its clinical significance is merited.

Natural turf surfaces: the case for continued research.

It is well documented that health and social benefits can be attained through participation in sport and exercise. Participation, particularly in sports, benefits from appropriate surface provisions that are safe, affordable and high quality preferably across the recreational to elite continuum. Investment, construction and research into artificial sports surfaces have increased to meet this provision. However, not all sports (e.g. golf, rugby and cricket) are suited to training and match-play on artificial turf without compromising some playing characteristics of the games. Therefore, full sport surface provision cannot be met without the use of natural turf surfaces, which also have an important role as green spaces in the built environment. Furthermore, a significant number of people participate in outdoor sport on natural turf pitches, although this is a declining trend as the number of synthetic turf surfaces increases. Despite natural turf being a common playing surface for popular sports such as soccer, rugby and cricket, few biomechanical studies have been performed using natural turf conditions. It is proposed that if natural turf surfaces are to help meet the provision of sports surfaces, advancement in the construction and sustainability of natural turf surface design is required. The design of a natural turf surface should also be informed by knowledge of surface-related overuse injury risk factors. This article reviews biomechanical, engineering, soil mechanics, turfgrass science, sports medicine and injury-related literature with a view to proposing a multidisciplinary approach to engineering a more sustainable natural turf sport surface. The present article concludes that an integrated approach incorporating an engineering and biomechanical analysis of the effects of variations in natural turf media on human movement and the effects of variations in human movement on natural turf is primarily required to address the longer-term development of sustainable natural turf playing surfaces. It also recommends that the use of 'natural turf' as a catch-all categorization in injury studies masks the spatial and temporal variation within and among such surfaces, which could be important.

External frontal plane loads may be associated with tibial stress fracture.

PURPOSE: The role of applied external loads in tibial stress fracture is poorly understood. The purpose of this study was to determine whether the magnitude and angle of frontal and sagittal force vectors and the magnitude of the free moment of ground reaction force (the torsional moment between the foot and the ground) during running gait differ between military recruits with and without a history of tibial stress fracture. METHODS: Ten male military recruits with tibial stress fracture history and 20 matched controls performed shod running trials over a force plate. The magnitude and the direction of the frontal and sagittal plane ground reaction force, in addition to the free moment, were compared between the groups. RESULTS: The frontal plane force vector was directed significantly more medially in the stress fracture group during midstance and late stance (P < 0.05). The magnitude of frontal and sagittal plane ground reaction forces and the free moment were not higher in the stress fracture group compared with controls. CONCLUSION: These data highlight differences in the direction with which external forces in the frontal plane are applied in military recruits with a history of tibial stress fracture. These differences may be important in the development of the injury.

Influence of orthotic devices prescribed using pressure data on lower extremity kinematics and pressures beneath the shoe during running.

BACKGROUND: Orthotic devices are frequently prescribed as a conservative treatment of lower extremity injury. The purpose of this study was to investigate the influence of orthotic devices prescribed using pressure data on lower extremity movement and loading patterns. METHODS: Twenty-two subjects ran barefoot over a pressure plate for the prescription of orthotic devices. The influence of the prescribed orthoses on lower extremity kinematics and pressure beneath the shoe was assessed by collection of data for 10 running trials with a neutral shoe and 10 with the addition of the orthotic device. For each running trial, initial and peak angles were determined for rearfoot inversion-eversion, lower leg internal rotation, ankle dorsi-plantar flexion, knee flexion and rearfoot eversion velocity. In addition, the relative pressure on the lateral side to medial side of the shoe (pressure balance) was determined by dividing the foot into areas of medial and lateral heel and five metatarsals. Peak lateral and medial heel and foot balance were determined during early stance to indicate differences in balance during this phase. FINDINGS: The orthotic devices resulted in a significant reduction in peak eversion and eversion velocity and a significant increase in the initial inversion angle (P<0.05). In addition, the peak ankle dorsi-flexion and initial dorsi-flexion angle were significantly increased (P<0.05). Consistent with the observed increase in initial inversion, the early pressure balance data revealed a significantly more lateral (less medial) concentration of pressure (P<0.05). INTERPRETATION: It is concluded that the devices used in the present study have resulted in the production of shoe inserts that successfully lower peak eversion and eversion velocity by encouraging the foot to operate in a more inverted orientation throughout the initial stance phase of running. In addition, there is evidence that orthotic effects can be detected through the use of pressure data collected from beneath the shoe.

Influence of a commercially available orthotic device on rearfoot eversion and vertical ground reaction force when running in military footwear.

The objective of this study was to investigate the influence of a commercially available orthotic device on rearfoot movement and peak impact force variables during running in combat assault boots. Eight military trainees performed running trials under two running conditions: boot with standard-issue insole and boot with the test orthotic. For each trial, vertical ground reaction force and frontal plane rearfoot angle data were collected. It was found that peak eversion angle was not significantly influenced by the orthotic device (p > 0.05), but that this peak occurred later in stance (p < 0.05). Peak impact force, average rate of loading, and peak rate of loading of impact force were all lower when the orthotic device was used (p < 0.05). The findings of this study highlight the potential of a commercially available orthotic to provide benefits more typically associated with molded prescription orthoses, providing a cost-effective option to the routine use of prescription orthotic devices.

Application of center-of-pressure data to indicate rearfoot inversion-eversion in shod running.

Although pressure plates are used to help in the selection of appropriate footwear for runners, evidence relating aspects of pressure data to movement is lacking. A study was conducted to investigate whether center-of-pressure (COP) data obtained for shod running could be used to indicate the amount of rearfoot eversion. It was hypothesized that subjects exhibiting high rearfoot eversion during the initial ground contact phase of running would also show a large lateral-to-medial deviation in the COP. Pressure plate and rearfoot movement data were collected for 33 subjects. The COP was characterized using the lateral-to-medial deviation of the COP during the eversion phase of ground contact. Correlation coefficients were determined for COP deviation versus rearfoot range of motion and versus peak rearfoot eversion (P < .05). In addition, subjects were grouped as high, moderate, or low pronators, and analysis of variance was used to test whether there were significant differences in COP deviation for these three groups (P < .05). The COP deviation was found to have a low correlation with rearfoot range of motion (R = 0.46; P < .05) and with peak rearfoot eversion (R = .54; P < .05). High pronators had significantly higher COP deviation than the medium- and low-pronation groups (P < .05). These findings support the use of COP deviation to detect high pronation. However, caution is advised in using the COP to indicate absolute rearfoot eversion.

The influence of different playing surfaces on the biomechanics of a tennis running forehand foot plant.

Research suggests that heightened impacts, altered joint movement patterns, and changes in friction coefficient from the use of artificial surfaces in sport increase the prevalence of overuse injuries. The purposes of this study were to (a) develop procedures to assess a tennis-specific movement, (b) characterize the ground reaction force (GRF) impact phases of the movement, and (c) assess human response during impact with changes in common playing surfaces. In relation to the third purpose it was hypothesized that surfaces with greatest mechanical cushioning would yield lower impact forces (PkFz) and rates of loading. Six shod volunteers performed 8 running forehand trials on each surface condition: baseline, carpet, acrylic, and artificial turf. Force plate (960 Hz) and kinematic data (120 Hz) were collected simultaneously for each trial. Running forehand foot plants are typically characterized by 3 peaks in vertical GRF prior to a foot-off peak. Group mean PkFz was significantly lower and peak braking force was significantly higher on the baseline surface compared with the other three test surfaces (p<0.05). No significant changes in initial kinematics were found to explain unexpected PkFz results. The baseline surface yielded a significantly higher coefficient of friction compared with the other three test surfaces (p<0.05). While the hypothesis is rejected, biomechanical analysis has revealed changes in surface type with regard to GRF variables.

Comparison of static and dynamic biomechanical measures in military recruits with and without a history of third metatarsal stress fracture.

BACKGROUND: For Royal Marine recruits in training, the third metatarsal is the most common site for stress fracture. Previous evidence regarding biomechanical factors contributing to metatarsal stress fracture development is conflicting, possibly due to the lack of differentiation between the metatarsals. The present retrospective study compares static anatomical characteristics and dynamic biomechanical variables for Royal Marine recruits with and without a history of third metatarsal stress fracture. METHODS: Ten Royal Marine recruits with a history of third metatarsal stress fracture were compared with control subjects with no previous stress fracture occurrence. Selected static anatomical variables were measured to describe the ankle and subtalar joints. Peak ankle dorsi-flexion and rearfoot eversion were measured during running. In addition, peak vertical and horizontal ground reaction force variables were compared for the two study groups. FINDINGS: No significant differences in static anatomical variables were identified between study groups. During running, peak rearfoot eversion was found to occur significantly earlier for the stress fracture group than for their matched controls, suggesting an increase in time spent loading the forefoot. The peak applied resultant horizontal force during the braking phase was directed significantly more laterally for the stress fracture group. In addition, the peak magnitude of resultant horizontal force applied during the propulsion phase was significantly lower for the stress fracture subjects. INTERPRETATION: The findings of this study highlight the importance of including dynamic biomechanical data when exploring variables associated with the development of third metatarsal stress fracture and indicate that successful interventions to reduce the incidence of this injury are likely to focus on forefoot function during braking and propulsion.

User trial and insulation tests to determine whether shock-absorbing insoles are suitable for use by military recruits during training.

A user trial was undertaken to determine whether a shock-absorbing insole is suitable for military use. Two thicknesses of insole (3 mm and 6 mm) were studied and were issued to 38 Royal Marine recruits to wear in their military boots for weeks 12 to 30 of training. Biomechanical measurements showed that both thicknesses of insole significantly (p < 0.05) attenuated the peak pressures generated at heel strike and during forefoot loading when new (relative to a no-insole condition) and that this was well maintained after wear. This was supported by mechanical tests conducted on the insoles. It was concluded that the insoles are sufficiently durable for military use. The main user complaint was that water retention reduced the comfort of the insoles; however, insulation tests conducted with a foot manikin indicated that switching from the current-issue Saran insoles to the trial insoles would not increase the risk of recruits sustaining nonfreezing cold injuries to their feet.

Biomechanical analysis of running in military boots with new and degraded insoles.

PURPOSE: The purpose of the present study was to investigate the influence of degradation using repeated impacts on the ability of different shock-absorbing insoles to reduce peak impact loading during running in military boots. METHODS: Four insole types were degraded mechanically to simulate typical running loads that occur during approximately 100 km of running. The influence of insole mechanical degradation on stiffness and impact-absorbing ability was assessed using standard test procedures. The ability of new and degraded insole samples to reduce peak impact loading during running was assessed by monitoring peak impact force and rate of loading. In addition, the influence of insoles on sagittal plane kinematics was quantified by measurement of hip, knee, and ankle joint flexion. RESULTS: Insole mechanical degradation resulted in an increase in mechanical stiffness and a decrease in ability to reduce mechanical impacts for all test insoles. Measurements taken during running indicated that only one insole type reduced peak impact loading when new, as indicated by a significant (P< 0.05) reduction in peak rate of loading. The ability of this insole type to reduce peak rate of loading during running was maintained after mechanical degradation. This insole was also found to significantly (P< 0.05) reduce peak ankle dorsiflexion. CONCLUSION: The present study identifies an insole type that reduces peak rate of loading during running both when new and when mechanically degraded. It is suggested that this indicates an insole that could potentially reduce the frequency of overuse injuries. Based on these results, this insole is recommended for use in the investigation of the practical use of insoles by military recruits, particularly for study of the influence on injury occurrence.

The influence of simulated wear upon the ability of insoles to reduce peak pressures during running when wearing military boots.

Mechanical degradation of three types of shock absorbing insoles equivalent to 100-130 km of running did not reduce their ability to attenuate the peak pressures generated during running when wearing military boots. Pressure measurements at the heel and forefoot were recorded with pressure measuring insoles placed in the boots of nine subjects. Two of the three insoles tested reduced the peak pressures (P<0.05) generated at the heel and forefoot relative to the no-insole (control) condition. The most effective insole reduced the peak pressures at the heel by 37% and at the forefoot by 24%.