Muscle force estimation during gait using Angle-EMG-Force relationship.

Measuring the muscle force during gait can provide crucial knowledge for clarifying the walking mechanism and preventing injuries. However, non-invasive muscle force measurement is a major challenge in biomechanics. Previous research has investigated the relationship between the amplitude of electromyography (EMG) and muscle force. By examining the EMG-force relationship of each muscle, the generated muscle force can be measured on the basis of the EMG amplitude during gait. This study aimed to investigate the angle-EMG-force relationship of lower limb muscles and estimate the muscle force during gait. The EMG and muscle force were measured in a static muscle force measurement task, and the angle-EMG-force relationship was analyzed based on these data. The results indicate that the muscle force can be estimated using the angle-EMG-force relationship during gait.Clinical Relevance-This study contributes to a more correct analysis of the muscle force during gait.

New function and passive mechanism of transfemoral prosthetic knee for running safely.

Transfemoral amputees who run are required to learn how to control their prosthetic leg motion to avoid falling with unintended prosthetic knee flexion because the function of the existing prosthetic knee for running is likened to a simple hinge joint during stance. However, the risk of falling and injury is a barrier to participation in sports and exercise. We have addressed this concern by developing a passive mechanism for a prosthetic knee; however, the mechanism that locks knee flexion with ground reaction forces (GRFs) could not completely avoid unintended prosthetic knee flexion. The present study aims to reconsider the function of the prosthetic knee for running and propose a new prosthetic knee mechanism. Time is an alternative way to control the lock/unlock of flexion without GRFs; therefore, the new prosthetic knee mechanism should limit flexion after a certain period from the moment that the prosthesis leaves the ground. We developed a rough prototype to confirm the function of the new prosthetic knee and conducted an evaluation experiment. The subject who was attached to the simulated thigh socket and prototype of the new prosthetic knee performed level walking. The results indicated that the new mechanism allowed flexion only during the first half of the swing phase, meaning the subject was able to walk without falling. According to the literature, swing time is approximately constant among different speeds. The new mechanism would appropriately function under actual running conditions. Clinical Relevance-This proposes a new passive prosthetic knee mechanism for above-knee amputees to run safely.

Kinematic characteristics during gait in frail older women identified by principal component analysis.

Frailty is associated with gait variability in several quantitative parameters, including high stride time variability. However, the associations between joint kinematics during walking and increased gait variability with frailty remain unclear. In the current study, principal component analysis was used to identify the key joint kinematics characteristics of gait related to frailty. We analyzed whole kinematic waveforms during the entire gait cycle obtained from the pelvis and lower limb joint angle in 30 older women (frail/prefrail: 15 participants; non-frail: 15 participants). Principal component analysis was conducted using a 60 × 1224 input matrix constructed from participants' time-normalized pelvic and lower-limb-joint angles along three axes (each leg of 30 participants, 51 time points, four angles, three axes, and two variables). Statistical analyses revealed that only principal component vectors 6 and 9 were related to frailty. Recombining the joint kinematics corresponding to these principal component vectors revealed that frail older women tended to exhibit greater variability of knee- and ankle-joint angles in the sagittal plane while walking compared with non-frail older women. We concluded that greater variability of knee- and ankle-joint angles in the sagittal plane are joint kinematic characteristics of gait related to frailty.

A comparison of computation methods for leg stiffness during hopping.

Despite the presence of several different calculations of leg stiffness during hopping, little is known about how the methodologies produce differences in the leg stiffness. The purpose of this study was to directly compare Kleg during hopping as calculated from three previously published computation methods. Ten male subjects hopped in place on two legs, at four frequencies (2.2, 2.6, 3.0, and 3.4 Hz). In this article, leg stiffness was calculated from the natural frequency of oscillation (method A), the ratio of maximal ground reaction force (GRF) to peak center of mass displacement at the middle of the stance phase (method B), and an approximation based on sine-wave GRF modeling (method C). We found that leg stiffness in all methods increased with an increase in hopping frequency, but Kleg values using methods A and B were significantly higher than when using method C at all hopping frequencies. Therefore, care should be taken when comparing leg stiffness obtained by method C with those calculated by other methods.

Effects of functional range of knee extension for transfemoral prosthesis on stair ascent motion.

We previously proposed a passive mechanism as the link knee joint unit (LKJ) for a transfemoral prosthesis for stair ascent. The prototype allowed the experimental subjects to ascend stairs without the use of a handrail. In the present study, we modified the LKJ unit and developed further two designs of the LKJ unit. One has full knee extension function during the prosthetic stance phase (condition 1). The other design mechanically trades off the functional range of knee extension against stability of the LKJ unit (condition 3). In the stair ascent experiment with six able-bodied subjects, all subjects succeeded in ascending stairs with the three LKJ conditions without the use of a handrail. No difference was found in joint angels and joint moments of the intact and prosthetic legs among all LKJ conditions. However, subjective assessment for ease of LKJ extension during stair ascent showed that the participants felt easier to extend the LKJ unit in the condition 1 and 2 than the condition 3. It is suggested that the condition 1 or 2 is appropriate for prosthesis users who can ascend stairs with the LKJ unit. For prosthesis users who are not familiar with the LKJ unit, the condition 3 would be useful to learn how to use it.

Novel knee joint mechanism of transfemoral prosthesis for stair ascent.

The stability of a transfemoral prosthesis when walking on flat ground has been established by recent advances in knee joint mechanisms and their control methods. It is, however, difficult for users of a transfemoral prosthesis to ascend stairs. This difficulty is mainly due to insufficient generation of extension moment around the knee joint of the prosthesis to lift the body to the next step on the staircase and prevent any unexpected flexion of the knee joint in the stance phase. Only a prosthesis with an actuator has facilitated stair ascent using a step-over-step gait (1 foot is placed per step). However, its use has issues associated with the durability, cost, maintenance, and usage environment. Therefore, the purpose of this research is to develop a novel knee joint mechanism for a prosthesis that generates an extension moment around the knee joint in the stance phase during stair ascent, without the use of any actuators. The proposed mechanism is based on the knowledge that the ground reaction force increases during the stance phase when the knee flexion occurs. Stair ascent experiments with the prosthesis showed that the proposed prosthesis can realize stair ascent without any undesirable knee flexion. In addition, the prosthesis is able to generate a positive knee joint moment power in the stance phase even without any power source.

Effects of inertial properties of transfemoral prosthesis on leg swing motion during stair ascent.

Stair ascent, especially the step-over-step gait, is a difficult motor task for people with transfemoral amputation. Our previous study demonstrated the effects of foot placement on the leg swing of able-bodied subjects. The study examined stair ascent with full-foot contact (FFC) and half-foot contact (HFC) as ambulation strategies. The results suggested that HFC causes the leg swing to have a greater inertial motion than FFC, as well as the applicability of the stair ascent strategy for transfemoral amputees with transfemoral prostheses without a motorized prosthetic knee joint. The present study investigated the effects of the inertial properties of a transfemoral prosthesis on leg motion during the stair ascent swing phase in simulation trials. The joint moment at the hip became smaller than that of an able-bodied subject. The peak values of the horizontal and vertical components of the joint reaction force were approximately the same as those of an able-bodied subject. These results suggest that a transfemoral prosthesis leg swing can be achieved with similar or smaller kinetic demand at the hip joint when half-foot contact on the stair steps is used as a stair ascent strategy. The mass had the largest effect of the inertial properties on the variability of the simulated kinetic parameters. The results of the present study may enhance prosthesis design with regard to the inertial properties and usability.

Effect of hopping frequency on bilateral differences in leg stiffness.

Understanding the degree of leg stiffness during human movement would provide important information that may be used for injury prevention. In the current study, we investigated bilateral differences in leg stiffness during one-legged hopping. Ten male participants performed one-legged hopping in place, matching metronome beats at 1.5, 2.2, and 3.0 Hz. Based on a spring-mass model, we calculated leg stiffness, which is defined as the ratio of maximal ground reaction force to maximum center of mass displacement at the middle of the stance phase, measured from vertical ground reaction force. In all hopping frequency settings, there was no significant difference in leg stiffness between legs. Although not statistically significant, asymmetry was the greatest at 1.5 Hz, followed by 2.2 and 3.0 Hz for all dependent variables. Furthermore, the number of subjects with an asymmetry greater than the 10% criterion was larger at 1.5 Hz than those at 2.2 and 3.0 Hz. These results will assist in the formulation of treatment-specific training regimes and rehabilitation programs for lower extremity injuries.

Preferred step frequency minimizes veering during natural human walking.

In the absence of visual information, humans cannot maintain a straight walking path. We examined the hypothesis that step frequency during walking affects the magnitude of veering in healthy adults. Subject walked at a preferred (1.77 ± 0.18 Hz), low (0.8 × preferred, 1.41 ± 0.15 Hz), and high (1.2× preferred, 2.13 ± 0.20 Hz) step frequency with and without a blindfold. We compared the absolute differences between estimated and measured points of crossing a target line after 16 m of forward walking at the three step frequencies. There was no significant difference in veering when subjects walked at the different frequencies without a blindfold. However, the magnitude of veering was the smallest at the preferred (mean ± SE=91.6 ± 33.6 cm) compared with the low (204.3 ± 43.0 cm) and high (112.7 ± 34.0 cm) frequency gaits with a blindfold. Thus, walking at a preferred step frequency minimizes veering, which occurs in the absence of visual information. This phenomenon may be associated with the previously reported minimization of movement variability, energy cost, and attentional demand while walking at a preferred step frequency.

Determinant of leg stiffness during hopping is frequency-dependent.

Identifying the major determinant of leg stiffness during hopping would be helpful in the development of more effective training methods. Despite the fact that overall leg stiffness depends on a combination of the joint stiffness, it is unclear how the major determinants of leg stiffness are influenced by hopping frequency. The purpose of this study was to identify the major determinant of leg stiffness over a wide range of hopping frequencies. Fourteen well-trained male athletes performed in a place hopping on two legs, at three frequencies (1.5, 2.2 and 3.0 Hz). We determined leg and joint stiffness of the hip, knee and ankle from kinetic and kinematic data. Multiple linear regression analysis revealed that knee stiffness could explain more of the variance of leg stiffness than could ankle or hip stiffness at 1.5 Hz hopping. Further, only ankle stiffness was significantly correlated with leg stiffness at both 2.2 and 3.0 Hz, and the standardized regression coefficient of ankle stiffness was higher than that of knee and hip stiffness. The results of the present study suggest that the major determinant of leg stiffness during hopping switches from knee stiffness to ankle stiffness when the hopping frequency is increased.

Acute effects of static stretching on leg-spring behavior during hopping.

Despite the fact that a stiffer leg spring is prerequisite for achieving a better performance during sports activities, effects of various types of warm-up on the leg stiffness is not well-known. The purpose of this study was to determine if static stretching influences the leg stiffness during two-legged hopping. Fourteen male subjects performed two-legged hopping at 2.2 Hz before and after a 3-min passive stretching of the triceps surae (dorsiflexion of 30°). Based on a spring-mass model, we calculated leg stiffness, which is defined as the ratio of maximal ground reaction force to maximum center of mass displacement at the middle of the stance phase. It was found that there was no significant difference in leg stiffness after passive static stretching. These results suggest that 3-min passive static stretching does not affect the leg-spring behavior and stiffness regulation during two-legged hopping. Finally, possible explanations for the invariant leg stiffness after the passive stretching are discussed.

Leg stiffness adjustment for a range of hopping frequencies in humans.

The purpose of the present study was to determine how humans adjust leg stiffness over a range of hopping frequencies. Ten male subjects performed in place hopping on two legs, at three frequencies (1.5, 2.2, and 3.0Hz). Leg stiffness, joint stiffness and touchdown joint angles were calculated from kinetic and/or kinematics data. Electromyographic activity (EMG) was recorded from six leg muscles. Leg stiffness increased with an increase in hopping frequency. Hip and knee stiffnesses were significantly greater at 3.0Hz than at 1.5Hz. There was no significant difference in ankle stiffness among the three hopping frequencies. Although there were significant differences in EMG activity among the three hopping frequencies, the largest was the 1.5Hz, followed by the 2.2Hz and then 3.0Hz. The subjects landed with a straighter leg (both hip and knee were extended more) with increased hopping frequency. These results suggest that over the range of hopping frequencies we evaluated, humans adjust leg stiffness by altering hip and knee stiffness. This is accomplished by extending the touchdown joint angles rather than by altering neural activity.

Continuous change in spring-mass characteristics during a 400 m sprint.

The purpose of the present study was to utilise a spring-mass model to (1) continuously measure vertical stiffness (K(vert)) and leg stiffness (K(leg)) over an entire 400 m sprint, and (2) investigate the relationship between leg spring stiffness (K(vert) and K(vert)) and the performance characteristics of mean forward running velocity (V(forwad)), mean stride frequency (f(stride)), and mean stride length (L(stride)). Eight well-trained male athletes performed a 400 m sprint with maximal effort on an outdoor field track. K(vert) was calculated from the subjects' body mass, ground contact time and flight time at each step. V(forwad), f(stride) and L(stride) were determined from video images. K(vert) and V(forwad) peaked at the 50-100 m interval, and consistently decreased from the middle to the later part of the sprint. K(leg) peaked at first 50 m interval, and remained constant from next 50 m interval to finish. As compared with peak values, K(vert) and V(forward) in the last 50 m decreased by about 40% and 25%, respectively. A significant positive linear relationship existed between the K(vert) and V(forward). While K(vert) was significantly correlated with f(stride), it had no correlation with L(stride). Further, no significant positive linear relationship was found between K(leg) and V(forward), f(stride), or L(stride). This result indicates that in order to keep V(forward) at later stage of a 400 m sprint, maintaining the higher f(stride) through retaining a higher K(vert) would be necessary.

Knee stiffness is a major determinant of leg stiffness during maximal hopping.

Understanding stiffness of the lower extremities during human movement may provide important information for developing more effective training methods during sports activities. It has been reported that leg stiffness during submaximal hopping depends primarily on ankle stiffness, but the way stiffness is regulated in maximal hopping is unknown. The goal of this study was to examine the hypothesis that knee stiffness is a major determinant of leg stiffness during the maximal hopping. Ten well-trained male athletes performed two-legged hopping in place with a maximal effort. We determined leg and joint stiffness of the hip, knee, and ankle from kinetic and kinematic data. Knee stiffness was significantly higher than ankle and hip stiffness. Further, the regression model revealed that only knee stiffness was significantly correlated with leg stiffness. The results of the present study suggest that the knee stiffness, rather than those of the ankle or hip, is the major determinant of leg stiffness during maximal hopping.