Ankle Push-off Research Articles

Mechanical and energetic consequences of reduced ankle plantar-flexion in human walking.

The human ankle produces a large burst of 'push-off' mechanical power late in the stance phase of walking, reduction of which leads to considerably poorer energy economy. It is, however, uncertain whether the energetic penalty results from poorer efficiency when the other leg joints substitute for the ankle's push-off work, or from a higher overall demand for work due to some fundamental feature of push-off. Here, we show that greater metabolic energy expenditure is indeed explained by a greater demand for work. This is predicted by a simple model of walking on pendulum-like legs, because proper push-off reduces collision losses from the leading leg. We tested this by experimentally restricting ankle push-off bilaterally in healthy adults (N=8) walking on a treadmill at 1.4 m s(-1), using ankle-foot orthoses with steel cables limiting motion. These produced up to ∼50% reduction in ankle push-off power and work, resulting in up to ∼50% greater net metabolic power expenditure to walk at the same speed. For each 1 J reduction in ankle work, we observed 0.6 J more dissipative collision work by the other leg, 1.3 J more positive work from the leg joints overall, and 3.94 J more metabolic energy expended. Loss of ankle push-off required more positive work elsewhere to maintain walking speed; this additional work was performed by the knee, apparently at reasonably high efficiency. Ankle push-off may contribute to walking economy by reducing dissipative collision losses and thus overall work demand.

Prosthetic ankle push-off work reduces metabolic rate but not collision work in non-amputee walking.

Individuals with unilateral below-knee amputation expend more energy than non-amputees during walking and exhibit reduced push-off work and increased hip work in the affected limb. Simple dynamic models of walking suggest a possible solution, predicting that increasing prosthetic ankle push-off should decrease leading limb collision, thereby reducing overall energy requirements. We conducted a rigorous experimental test of this idea wherein ankle-foot prosthesis push-off work was incrementally varied in isolation from one-half to two-times normal levels while subjects with simulated amputation walked on a treadmill at 1.25 m·s−1. Increased prosthesis push-off significantly reduced metabolic energy expenditure, with a 14% reduction at maximum prosthesis work. In contrast to model predictions, however, collision losses were unchanged, while hip work during swing initiation was decreased. This suggests that powered ankle push-off reduces walking effort primarily through other mechanisms, such as assisting leg swing, which would be better understood using more complete neuromuscular models.

Impulsive ankle push-off powers leg swing in human walking

Impulsive ankle push-off powers leg swing in human walking

Control of a compass gait walker based on energy regulation using ankle push-off and foot placement

SUMMARYIn this paper, we present a theoretical study on the control of a compass gait walker using energy regulation between steps. We use a return map to relate the mid-stance robot kinetic energy between steps with two control inputs, namely, foot placement and ankle push-off. We show that by regulating robot kinetic energy between steps using the two control inputs, we are able to (1) generate a wide range of walking speeds and stride lengths, including average human walking; (2) cancel the effect of external disturbance fully in a single step (dead-beat control); and (3) switch from one periodic gait to another in a single step. We hope that insights from this control methodology can help develop robust controllers for practical bipedal robots.

Mechanical properties of a spring-hinged floor reaction orthosis

Introduction: For children with cerebral palsy (CP) whose gait pattern is characterized by excessive knee flexion in midstance, a Floor Reaction Orthosis (FRO) is commonly prescribed. FROs are very stiff andaimto counteract excessivekneeflexion inmidstance, by shifting the ground reaction force anteriorly. Although an FRO is effective in this respect [1], it impedes plantar flexion in preswing, therebyobstructingpush-off power. A spring-like FROcouldpotentially be more beneficial, since it can store energy at the beginning of the stance phase, being returned in preswing, thereby creating ankle push-off power. Literature shows that energy cost of walking with a typical spring-like Ankle-Foot Orthosis can be minimized by selecting an optimal stiffness [2]. Recently, a new type of hinge with adjustable springswas designed, which can be integrated into an FRO. However, exact knowledge of themechanical properties of the hinge is lacking. This study aims to quantify the mechanical properties of this new hinge build within an FRO. Patients/materials and methods: One test FRO with an integrated 14mm Neuro Swing® ankle joint (Fior & Gentz, Germany) was constructed.Weusedfive springswithin thehinge,withdifferent stiffnesses. The spring’s mechanical properties were measured with BRUCE [3]. Each spring was fully compressed and released slowly (i.e. a few seconds for the whole range of motion (ROM)) three times, while the hinge angle and the exerted net moment were continuously measured [3]. ROM (deg) (i.e. elastic range), stiffness (Nm/deg) (i.e. slope of the linear fit of the relation between angle and net joint moment in the ROM), threshold (Nm) (i.e. exerted moment at the start of ROM), and hysteresis [%ESTOR] (i.e. storage (ESTOR) minus release (ERLS) of energy) were averaged for each spring. ESTOR was defined as the surface underneath the compression phase (upper line) and ERLS as the surface underneath the release phase (lower line). Results Discussion and conclusions: Our results indicate that the five springs seem to be useful in optimizing orthoses with respect to energy cost of walking in children with CP. Hysteresis, probably mainly caused by friction of the spring in the shaft of the hinge, is present in all springs, resulting in a loss of energy that potentially could be used for ankle push-off. Nevertheless, FROs with integrated springs might still be more beneficial compared to conventional FROs, in which no energy can be stored at all. However, it remains unknown whether the stiffness of the springs is sufficient to counteract knee flexion in stance. This will be subject of further research to investigate the effects of the mechanical properties of the springs on gait in CP.

The dynamic optimization approach to locomotion dynamics: human-like gaits from a minimally-constrained biped model

This work presents a unifying framework to study energy-efficient optimal gaits for a bipedal model without elastic elements. The model includes a torso, flat feet, and telescoping legs, equipped with rotational hip and ankle joints. Two general types of gaits are studied: with and without a flight phase. The support surface can be level ground, sloped, or staircase. The algorithm achieves the optimum within the admissible space by using a minimal set of realistic physical constraints, and avoiding a priori assumptions on kinetic and kinematic parameters such as extended or instantaneous double-support, collisional or collisionless foot-ground contact, step length, step period, etc. The gait optimization for this simple model predicts many features of human locomotion including the optimality of pendular walking and impulsive running at slow and fast progression speeds, ankle push-off prior to touch-down, swing leg retraction, landing on a near vertical leg in gaits with flight phase, and burst hip torques at both ends of the swing phase.

A method to differentiate the causes of stiff-knee gait in stroke patients

Stiff-knee gait (SKG) is a common abnormal gait pattern in patients after stroke characterized by insufficient knee flexion (KF) during swing. Overactivity of the rectus femoris (RF) is considered the primary cause of SKG. Inadequate push-off has been indicated as an additional cause in the recent literature, as KF depends on knee flexion velocity in preswing (KFV).We used the peak of vertical acceleration of the malleolus (PMVA) as a kinematic-based indirect measure of push-off and studied its relationship with KF and KFV in a sample of 20 healthy subjects walking fast (v=95±5%heights−1), at self-selected speed (v=74±5%heights−1), slow (v=54±6%heights−1) and very slow (v=38±5%heights−1) and in a sample of 52 stroke patients with SKG (age 60±11, v=20±11%heights−1).In healthy subjects PMVA occurred before knee flexion acceleration (p<0.001) and hip flexion acceleration (p<0.001). KF appeared as a bottom-up mechanism driven by the ankle push-off. From a regression analysis, the PMVA-KFV cause–effect relationship resulted strictly linear, with R2=0.967, KFV=0+7.1×PMVA, P<0.0001.Data from SKG patients were compared to this normal cause–effect model. For 44/52 patients the reduced KFV was combined with lack of push-off. Data from 8/52 patients only were statistically outside the 95%CI of the model, thus requiring for a braking mechanism to explain KFV reduction.In stroke adults of our sample the push-off impairment (85% of cases) and not the inappropriate knee extension moment produced by the thigh muscles was the primary cause of SKG. This result could explain the low average efficacy (<10°) of focal and surgical treatments at the quadriceps.The presented model could be used to differentiate the primary cause of SKG between inadequate push-off and braking activity of the thigh muscles, thus increasing the effectiveness of the selected treatment.

Passive Ankle-Foot Prosthesis Prototype with Extended Push-Off

Current commercially available prosthetic feet have succeeded in decreasing the metabolic cost and increasing the speed of walking compared to walking with conventional, mostly solid prosthetic feet. However, there is still a large discrepancy when compared with a non-disabled gait, and the walking pattern remains strongly disturbed. During the stance phase of the leg, these prostheses store and return energy using a spring element. This spring returns to its neutral position, which generates a push-off, but the foot extends much less than with a non-disabled gait. The walking pattern may improve with a more extended push-off. In this paper, we present a passive ankle-foot prosthesis that aims to deliver an extended ankle push-off using a specific planetary gearbox arrangement and locking mechanisms in order to release the energy in the spring over the full natural stretching of the ankle. In recent years, both powered and passive prosthetic devices have been developed. The prosthetic foot presented in this paper is a passive system, such that it has the possibility to be made lighter and more robust than, for example, one driven by an electric motor. Preliminary walking experiments were conducted with a transfemoral amputee.

Locomotor adaptation to a soleus EMG-controlled antagonistic exoskeleton

Locomotor adaptation in humans is not well understood. To provide insight into the neural reorganization that occurs following a significant disruption to one's learned neuromuscular map relating a given motor command to its resulting muscular action, we tied the mechanical action of a robotic exoskeleton to the electromyography (EMG) profile of the soleus muscle during walking. The powered exoskeleton produced an ankle dorsiflexion torque proportional to soleus muscle recruitment thus limiting the soleus' plantar flexion torque capability. We hypothesized that neurologically intact subjects would alter muscle activation patterns in response to the antagonistic exoskeleton by decreasing soleus recruitment. Subjects practiced walking with the exoskeleton for two 30-min sessions. The initial response to the perturbation was to "fight" the resistive exoskeleton by increasing soleus activation. By the end of training, subjects had significantly reduced soleus recruitment resulting in a gait pattern with almost no ankle push-off. In addition, there was a trend for subjects to reduce gastrocnemius recruitment in proportion to the soleus even though only the soleus EMG was used to control the exoskeleton. The results from this study demonstrate the ability of the nervous system to recalibrate locomotor output in response to substantial changes in the mechanical output of the soleus muscle and associated sensory feedback. This study provides further evidence that the human locomotor system of intact individuals is highly flexible and able to adapt to achieve effective locomotion in response to a broad range of neuromuscular perturbations.

Impulsive ankle push-off powers leg swing in human walking

Rapid unloading and a peak in power output of the ankle joint have been widely observed during push-off in human walking. Model-based studies hypothesize that this push-off causes redirection of the body center of mass just before touch-down of the leading leg. Other research suggests that work done by the ankle extensors provides kinetic energy for the initiation of swing. Also, muscle work is suggested to power a catapult-like action in late stance of human walking. However, there is a lack of knowledge about the biomechanical process leading to this widely observed high power output of the ankle extensors. In our study, we use kinematic and dynamic data of human walking collected at speeds between 0.5 and 2.5 m s(-1) for a comprehensive analysis of push-off mechanics. We identify two distinct phases, which divide the push-off: first, starting with positive ankle power output, an alleviation phase, where the trailing leg is alleviated from supporting the body mass, and second, a launching phase, where stored energy in the ankle joint is released. Our results show a release of just a small part of the energy stored in the ankle joint during the alleviation phase. A larger impulse for the trailing leg than for the remaining body is observed during the launching phase. Here, the buckling knee joint inhibits transfer of power from the ankle to the remaining body. It appears that swing initiation profits from an impulsive ankle push-off resulting from a catapult without escapement.

Immediate Effects on the Ankle and Hip Joints of Enhanced Ankle Push-off in Gait of Cerebral Palsy Subjects

〔目的〕健常成人の歩行では，足関節と股関節が運動力学的に補完的な役割を果たしトレードオフの関係性を示す．本研究の目的は，CP児・者の歩行中のけりだし強化が，足関節と股関節に即時的な変化が生じるかを検討することである．〔対象〕10ｍ以上の連続歩行が可能な右片麻痺CP児・者2名とした．〔方法〕3次元動作解析装置，床反力計および赤外線カメラを用いて通常歩行と強いけりだし歩行を計測し，けりだしにおける足関節と股関節の比(A2/H3比)を算出した．パラメータの比較はMann-Whitney検定を行った．〔結果〕強いけりだし歩行により股関節伸展モーメントの増加を認め，A2/H3比は有意な増加を示さなかった．〔結語〕CP児・者では，強いけりだし歩行遂行に足関節での活動ではなく，股関節で代償していた．

Robots in human biomechanics—a study on ankle push-off in walking

In biomechanics, explanatory template models are used to identify the basic mechanisms of human locomotion. However, model predictions often lack verification in a realistic environment. We present a method that uses template model mechanics as a blueprint for a bipedal robot and a corresponding computer simulation. The hypotheses derived from template model studies concerning the function of heel-off in walking are analysed and discrepancies between the template model and its real-world anchor are pointed out. Neither extending the ground clearance of the swinging leg nor an impact reduction at touch-down as an effect of heel lifting was supported by the experiments. To confirm the relevance of the experimental findings, a comparison of robot data to human walking data is discussed and we speculate on an alternative explanation of heel-off in human walking, i.e. that the push-off powers the following leg swing.

Functional gait asymmetry of unilateral transfemoral amputees

The aim of prosthetic devices is to mimic the function of biological systems. Numerous investigations have demonstrated significant asymmetries in unilateral amputee gait. The underlying interactions of prosthetic and intact leg are not widely discussed, so far. To get more insight into the functionality of asymmetries, we investigated temporal and kinetic parameters of walking transfemoral amputees wearing the computerized C-Leg and the non-computerized 3R80.Experiments were conducted on an instrumented treadmill at four different walking speeds (0.5, 0.8, 1.1, 1.4m/s) measuring vertical and horizontal ground reaction forces. Single support, double support and contact times, vertical and horizontal impulses as well as their asymmetry factors were calculated.Gait patterns were similar for both prosthetic knee joints, manifesting in (i) reduced stance times of the prosthetic leg, (ii) prolonged load transfer during double support from intact to prosthetic leg at lower speeds, (iii) reduced vertical and horizontal impulses of the prosthetic leg, (iv) net accelerating horizontal impulses during contact of the prosthetic leg, (v) missing impacts at touch-down of the prosthetic leg.Our results suggest that deficits of the prosthetic leg like missing active knee extension and ankle push-off are compensated by the intact leg. The altered touch-down configuration for the prosthetic leg enables it to provide forward propulsion while load bearing is largely shifted to the intact leg.

Kinematic and kinetic characteristics of Masai Barefoot Technology footwear

The Masai Barefoot Technology (MBT) shoe was developed as a walking device to improve gait stability and reduce the joint load. Kinematic changes with MBT shoes have been reported; however, kinetic characteristics with MBT shoes have not been adequately assessed. The purpose of this study was to investigate the immediate effects of using MBT footwear on the kinetic and kinematic changes in the lower extremity in healthy males. Fourteen healthy male subjects (mean age: 25.6±5.1 years) underwent three-dimensional gait analysis. Ground reaction forces (GRF) during the shock absorption phase were significantly decreased with MBT shoes compared with stable shoes. Gait with the MBT shoes showed significantly decreased knee extension angle in the early stance phase, a decreased hip extension angle, and an increased ankle dorsiflexion angle in the late stance phase. The peak value of the ankle planter moment, ankle negative power, and vertical component of the GRF significantly decreased with MBT shoes in the late stance phase compared with stable shoes. Therefore, MBT shoes could assist with shock absorption in the early stance phase and maintain the progression force while reducing joint moment and power. The results of this study suggest that MBT shoes might be effective to improve shock absorption, increase knee extensor muscle activity, and assist ankle push-off.

The effects of a controlled energy storage and return prototype prosthetic foot on transtibial amputee ambulation

The lack of functional ankle musculature in lower limb amputees contributes to the reduced prosthetic ankle push-off, compensations at other joints and more energetically costly gait commonly observed in comparison to non-amputees. A variety of energy storing and return prosthetic feet have been developed to address these issues but have not been shown to sufficiently improve amputee biomechanics and energetic cost, perhaps because the timing and magnitude of energy return is not controlled. The goal of this study was to examine how a prototype microprocessor-controlled prosthetic foot designed to store some of the energy during loading and return it during push-off affects amputee gait. Unilateral transtibial amputees wore the Controlled Energy Storage and Return prosthetic foot (CESR), a conventional foot (CONV), and their previously prescribed foot (PRES) in random order. Three-dimensional gait analysis and net oxygen consumption were collected as participants walked at constant speed. The CESR foot demonstrated increased energy storage during early stance, increased prosthetic foot peak push-off power and work, increased prosthetic limb center of mass (COM) push-off work and decreased intact limb COM collision work compared to CONV and PRES. The biological contribution of the positive COM work for CESR was reduced compared to CONV and PRES. However, the net metabolic cost for CESR did not change compared to CONV and increased compared to PRES, which may partially reflect the greater weight, lack of individualized size and stiffness and relatively less familiarity for CESR and CONV. Controlled energy storage and return enhanced prosthetic push-off, but requires further design modifications to improve amputee walking economy.

Long-distance walking effects on trans-tibial amputees compensatory gait patterns and implications on prosthetic designs and training

Trans-tibial amputees are advised to walk as much as able people to achieve healthy and independent life. However, they usually have difficulties in doing so. Previous researches only included data from a few steps when studying the gait of amputees. Walking over a long distance was rarely examined. The objective of this study was to investigate the changes in spatial–temporal, kinetic and kinematic gait parameters of trans-tibial amputees after long-distance walking. Six male unilateral trans-tibial amputees performed two sessions of 30-min walking on a level treadmill at their self-selected comfortable speed. Gait analysis was undertaken over-ground: (1) before walking, (2) after the 1st walking session and (3) after the 2nd walking session. After the long-distance walking, changes in spatial–temporal gait parameters were small and insignificant. However, the sound side ankle rocker progression and push-off were significantly reduced. This was due to the fatigue of the sound side plantar flexors and was compensated by the greater effort in the prosthetic side. The prosthetic side knee joint showed significantly increased flexion and moment during loading response to facilitate the anterior rotation of the prosthetic shank. The prosthetic side hip extensors also provided more power at terminal stance to facilitate propulsion. Endurance training of the sound side plantar flexors, and improvements in the prosthetic design to assist anterior rotation of the prosthetic shank should improve long-distance walking in trans-tibial amputees.

Spring-like Ankle Foot Orthoses reduce the energy cost of walking by taking over ankle work

In patients with central neurological disorders, gait is often limited by a reduced ability to push off with the ankle. To overcome this reduced ankle push-off, energy-storing, spring-like carbon-composite Ankle Foot Orthoses (AFO) can be prescribed. It is expected that the energy returned by the AFO in late stance will support ankle push-off, and reduce the energy cost of walking.In 10 patients with multiple sclerosis and stroke the energy cost of walking, 3D kinematics, joint power, and joint work were measured during gait, with and without the AFO. The mechanical characteristics of the AFO were measured separately, and used to calculate the contribution of the AFO to the ankle kinetics.We found a significant decrease of 9.8% in energy cost of walking when walking with the AFO. With the AFO, the range of motion of the ankle was reduced by 12.3°, and the net work around the ankle was reduced by 29%. The total net work in the affected leg remained unchanged. The AFO accounted for 60% of the positive ankle work, which reduced the total amount of work performed by the leg by 11.1% when walking with the AFO.The decrease in energy cost when walking with a spring-like energy-storing AFO in central neurological patients is not induced by an augmented net ankle push-off, but by the AFO partially taking over ankle work.

Predictors of pelvic retraction in children with cerebral palsy derived from gait parameters and clinical testing

Excessive pelvic rotation in the transverse plane is common in patients with cerebral palsy. Knowing the underlying reasons is important for clinical decision making, since changes in pelvic retraction might have an effect on internally rotated gait. We hypothesized that the contralateral leg contributes considerably to pelvic rotation on the retracted side. Therefore the aim of this study is to calculate predictors for pelvic retraction using both, parameters from the retracted and from the contralateral protracted side.Thirty-two children with diplegia and 18 children with hemiplegia were examined by three-dimensional gait analysis followed by a clinical examination protocol. Stepwise multilinear regression of the response value mean pelvic retraction during stance phase was performed on 10 potential predictors of dynamic gait data and 10 corresponding predictors of clinical data of the retracted and the contralateral protracted side.Gait analysis revealed ankle push-off energy on the protracted side as the best predictors in hemiplegic patients explaining 59% of the variance in pelvic retraction. In diplegic patients external hip rotation of the protracted side was most accurate in predicting pelvic retraction (27%). Best clinical predictors for hemiplegic patients were ankle dorsiflexion on the retracted side (46%) and for diplegic patients it was the knee extension strength on the protracted side together with hip rotation on the retracted side (36%). In hemiplegic patients ankle push-off energy of the contralateral side is a significant compensation mechanism that might cause increased pelvic retraction to compensate for the weakness of the involved side. In diplegic patients prediction of pelvic retraction was only moderate and requires further investigation.

The effect of ankle foot orthosis stiffness on the energy cost of walking: A simulation study

BackgroundIn stroke and multiple sclerosis patients, gait is frequently hampered by a reduced ability to push-off with the ankle caused by weakness of the plantar-flexor muscles. To enhance ankle push-off and to decrease the high energy cost of walking, spring-like carbon-composite Ankle Foot Orthoses are frequently prescribed. However, it is unknown what Ankle Foot Orthoses stiffness should be used to obtain the most efficient gait. The aim of this simulation study was to gain insights into the effect of variation in Ankle Foot Orthosis stiffness on the amount of energy stored in the Ankle Foot Orthosis and the energy cost of walking. MethodsWe developed a two-dimensional forward-dynamic walking model with a passive spring at the ankle representing the Ankle Foot Orthosis and two constant torques at the hip for propulsion. We varied Ankle Foot Orthosis stiffness while keeping speed and step length constant. FindingsWe found an optimal stiffness, at which the energy delivered at the hip joint was minimal. Energy cost decreased with increasing energy storage in the ankle foot orthosis, but the most efficient gait did not occur with maximal energy storage. With maximum storage, push-off occurred too late to reduce the impact of the contralateral leg with the floor. Maximum return prior to foot strike was also suboptimal, as push-off occurred too early and its effects were subsequently counteracted by gravity. The optimal Ankle Foot Orthosis stiffness resulted in significant push-off timed just prior to foot strike and led to greater ankle plantar-flexion velocity just before contralateral foot strike. InterpretationOur results suggest that patient energy cost might be reduced by the proper choice of Ankle Foot Orthosis stiffness.

Immediate effects of different ankle pushoff instructions during walking exercise on hip kinematics and kinetics in individuals with total hip arthroplasty

Residual hip impairments, such as decreased hip muscle moment and power during walking, have been reported in patients with total hip arthroplasty (THA). Meanwhile, greater ankle power has also been reported in these patients. We investigated the interaction between hip and ankle joints during walking to determine the effects of different ankle pushoff instructions on hip biomechanics in patients with THA. Twenty-four women (age, 60.8±5.5 years) were randomly assigned to walking exercise groups with either decreased pushoff or increased pushoff. Patients in the decreased pushoff group and increased pushoff group were given the instructions “push less with your foot when you walk” and “push more with your foot when you walk,” respectively. Exercises lasted approximately 10–15min. A series of gait-related parameters were analyzed during pre-exercise, exercise, and post-exercise session. In the decreased ankle pushoff group, hip flexor power absorption and hip/ankle power ratio were higher during post-exercise than during pre-exercise. An increase in hip power from −9.8% to 32.1% was identified. The effect of increase in the hip power by the decreasing ankle pushoff was higher in the patients with greater ankle pushoff in their natural gaits. The patients in the increased ankle pushoff group showed decreased hip flexion angle and hip muscle moment and power after the walking exercise, although ankle pushoff was not increased. Walking exercise with decreased ankle pushoff may help improve the distribution of muscle power between hip flexors and ankle plantarflexors during walking in patients with THA.