Signaling of Ankle Joint Position by Receptors in Different Muscles

Plots were made of multiunit activity versus ankle joint position for receptors in each of the 12 muscles crossing the cat ankle joint, except peroneus tertius, by recording from populations of afferent fibers in muscle nerves. The discharge was measured 15 or 30 sec after terminating the movements that altered the position of the joint. These recordings were dominated by large-spike activity that would be expected to originate mainly from primary spindle endings. Seven of the 12 muscles also cross other joints. Their responses at a given ankle joint position were so altered by changes in the position of the knee or toe joints that they could not reliably signal the position of the ankle joint. As judged from multiunit recording, receptors in each of the five muscles specific to the ankle joint were influenced by more than one axis of ankle joint displacement.

Single-unit recording from dorsal root filaments was used to determine whether primary or secondary spindle receptors in soleus and tibialis anterior could selectively signal one axis of ankle joint rotation. Individual soleus receptors were tested both on the flexion extension axis and with a combined adduction–eversion movement.

For 38 of the 70 soleus receptors examined (54%), firm adduction–eversion produced a level of activity greater than that caused by 10° of flexion, and for 77% the level of activity was greater than that caused by 5° of flexion. For 168 of the 184 tibialis anterior receptors studied (91%), firm abduction inversion produced a level of activity greater than that caused by 10° of extension. Thus few receptors were found that responded exclusively to one axis of rotation.

One way in which the position of the ankle joint could be specified in the face of multiaxial receptor activity is by examining the receptor discharge from more than one muscle. A suggestion for how the nervous system might do this is given in the discussion.     

Cortical and spinal control of ankle joint muscles before and during gait initiation

The purpose of this study was to investigate control of the ankle joint muscles before and during gait initiation. Seven healthy humans, aged 20-30 years old, participated in this study. Motor-evoked potentials (MEPs) were recorded from the soleus and the tibialis anterior muscles, and H-reflexes were evoked from the soleus muscle in the stance leg of gait initiation. The soleus H-reflexes were depressed throughout all the periods before and during gait initiation. The soleus MEP amplitudes were decreased in some periods before gait initiation, but were increased in other periods before and during gait initiation. The MEP amplitudes in the tibialis anterior muscle were increased before the onset of the EMG activity, and this increase persisted through gait initiation. The findings indicate that the ankle joint flexor is under intensive cortico-spinal control before and during gait initiation. Both the cortical and spinal pathways are involved in preparing and controlling the activity of the ankle joint extensor for gait initiation.     

Human leg model predicts ankle muscle-tendon morphology, state, roles and energetics in walking

A common feature in biological neuromuscular systems is the redundancy in joint actuation. Understanding how these redundancies are resolved in typical joint movements has been a long-standing problem in biomechanics, neuroscience and prosthetics. Many empirical studies have uncovered neural, mechanical and energetic aspects of how humans resolve these degrees of freedom to actuate leg joints for common tasks like walking. However, a unifying theoretical framework that explains the many independent empirical observations and predicts individual muscle and tendon contributions to joint actuation is yet to be established. Here we develop a computational framework to address how the ankle joint actuation problem is resolved by the neuromuscular system in walking. Our framework is founded upon the proposal that a consideration of both neural control and leg muscle-tendon morphology is critical to obtain predictive, mechanistic insight into individual muscle and tendon contributions to joint actuation. We examine kinetic, kinematic and electromyographic data from healthy walking subjects to find that human leg muscle-tendon morphology and neural activations enable a metabolically optimal realization of biological ankle mechanics in walking. This optimal realization (a) corresponds to independent empirical observations of operation and performance of the soleus and gastrocnemius muscles, (b) gives rise to an efficient load-sharing amongst ankle muscle-tendon units and (c) causes soleus and gastrocnemius muscle fibers to take on distinct mechanical roles of force generation and power production at the end of stance phase in walking. The framework outlined here suggests that the dynamical interplay between leg structure and neural control may be key to the high walking economy of humans, and has implications as a means to obtain insight into empirically inaccessible features of individual muscle and tendons in biomechanical tasks.     

Ankle flexor muscles in the cat: Length-active tension and muscle unit properties as related to locomotion

The mechanical properties of the whole muscle and fast-twitch muscle units of the cat hindlimb pretibial flexors have been explored and related to normal locomotion. Tibialis anterior (TA) is parallel-fibered and functionally crosses a single joint, the ankle, whereas extensor digitorum longus (EDL) is pinnate and spans the ankle, knee, metatarsophalangeal and interphalangeal joints. The active tetanic tension of TA remains near its peak value over a range of muscle lengths associated with normal ankle movement. In contrast, the length-tension curve of EDL is sharply peaked. However, normal corollary action of the knee, ankle and metatarsophalangeal joints during stepping minimizes EDL's excursion and maintains it at or near a length optimal for peak tension development. EDL is capable of producing synchronous but sterotyped digit and ankle movements while TA provides for independent ankle flexion at all relevant joint angles. The mechanical properties of 84 TA and 98 EDL fast-twitch muscle units were studied by measuring twitch contraction time (≤45 msec), peak tetanic tension, response to repetitive stimulation, and contractile fatigue resistance during electrical stimulation of single alpha axons, functionally isolated from ventral root filaments. These mechanical properties were essentially similar for both muscles with the exception of mean peak tetanic tension which was 30% lower for TA units (14 gm-wt) than for EDL units (20 gm-wt). A high proportion of units in both muscles demonstrated fatigue resistance which is reflective of the repetitive, phasic demand upon these muscles during locomotion.     

Adaptation in synergistic muscles to soleus and plantaris muscle removal in the rat hindlimb.

Although the soleus muscle comprises only 6% of the ankle plantar flexor mass in the rat, a major role in stance and walking has been ascribed to it. The purpose of this study was to determine if removal of the soleus muscle would result in adaptations in the remaining gastrocnemius and plantaris muscles due to the new demands for force production imposed on them during stance or walking. A second purpose was to determine whether the mass or the fiber type of the muscle(s) removed was a more important determinant of compensatory adaptations. Male Sprague-Dawley rats underwent bilateral removal of soleus muscle, plantaris muscle, or both muscles. For comparison, compensatory hypertrophy was induced in soleus and plantaris muscles by gastrocnemius muscle ablation. After forty days, synergist muscles remaining intact were removed. Mass, and oxidative, glycolytic, and contractile enzyme activities were determined. Despite its role in stance and slow walking, removal of the soleus muscle did not elicit a measurable alteration in muscle mass, or in citrate synthase, lactate dehydrogenase, or myofibrillar ATPase activity in gastrocnemius or plantaris muscles. Similarly, removal of the plantaris muscle, or soleus and plantaris muscles, had no effect on the gastrocnemius muscle, suggesting that this muscle was able to easily meet the new demands placed on it. These results suggest that amount of muscle mass removed, rather than fiber type, is the most important stimulus for compensatory hypertrophy. They also suggest that slow-twitch motor units in the gastrocnemius muscle play an important role during stance and locomotion in the intact animal.     

Contribution of muscle short-range stiffness to initial changes in joint kinetics and kinematics during perturbations to standing balance: A simulation study

Simulating realistic musculoskeletal dynamics is critical to understanding neural control of muscle activity evoked in sensorimotor feedback responses that have inherent neural transmission delays. Thus, the initial mechanical response of muscles to perturbations in the absence of any change in muscle activity determines which corrective neural responses are required to stabilize body posture. Muscle short-range stiffness, a history-dependent property of muscle that causes a rapid and transient rise in muscle force upon stretch, likely affects musculoskeletal dynamics in the initial mechanical response to perturbations. Here we identified the contributions of short-range stiffness to joint torques and angles in the initial mechanical response to support surface translations using dynamic simulation. We developed a dynamic model of muscle short-range stiffness to augment a Hill-type muscle model. Our simulations show that short-range stiffness can provide stability against external perturbations during the neuromechanical response delay. Assuming constant muscle activation during the initial mechanical response, including muscle short-range stiffness was necessary to account for the rapid rise in experimental sagittal plane knee and hip joint torques that occurs simultaneously with very small changes in joint angles and reduced root mean square errors between simulated and experimental torques by 56% and 47%, respectively. Moreover, forward simulations lacking short-range stiffness produced unreasonably large joint angle changes during the initial response. Using muscle models accounting for short-range stiffness along with other aspects of history-dependent muscle dynamics may be important to advance our ability to simulate inherently unstable human movements based on principles of neural control and biomechanics.     

Passive stiffness of hindlimb muscles in anurans with distinct locomotor specializations

Anurans (frogs and toads) have been shown to have relatively compliant skeletal muscles. Using a meta-analysis of published data we have found that muscle stiffness is negatively correlated with joint range of motion when examined across mammalian, anuran and bird species. Given this trend across a broad phylogenetic sample, we examined whether the relationship held true within anurans. We identified four species that differ in preferred locomotor mode and hence joint range of motion (Lithobates catesbeianus, Rhinella marina, Xenopus laevis and Kassina senegalensis) and hypothesized that smaller in vivo angles (more flexed) at the knee and ankle joint would be associated with more compliant extensor muscles. We measured passive muscle tension during cyclical stretching (20%) around L₀ (sarcomere lengths of 2.2 μm) in fiber bundles extracted from cruralis and plantaris muscles. We found no relationship between muscle stiffness and range of motion for either muscle–joint complex. There were no differences in the passive properties of the cruralis muscle among the four species, but the plantaris muscles of the Xenopus and Kassina were significantly stiffer than those of the other two species. Our results suggest that in anurans the stiffness of muscle fibers is a relatively minor contributor to stiffness at the level of joints and that variation in other anatomical properties including muscle–tendon architecture and joint mechanics as well as active control likely contribute more significantly to range of motion during locomotion.     

A comparison of electrical activity in the triceps surae at maximum isometric contraction with the knee and ankle at various angles

The purpose of this study was to test the endurance of the soleus muscle, and to examine the joint position at which it is most active, while simultaneously suppressing the activity of the gastrocnemius. Ten young males performed maximum isometric contraction of the triceps surae for 100 s, and the endurance and plantar flexion torque of this muscle were measured at various angles of the knee and ankle joints. The electromyogram was measured simultaneously and subsequently converted into integrated electromyogram (IEMG) values. With the knee flexed at 130 degrees, the rate of change in IEMG values for the soleus (0.454% x s(-1)) with the ankle in a neutral position was significantly higher than that for the medial and lateral gastrocnemius. Both with the ankle dorsiflexed at 10 degrees and in the neutral position, the rate of change in IEMG for the soleus was significantly higher with the knee flexed at 90 degrees and 130 degrees than with the knee fully extended. With the knee flexed at 90 degrees and 130 degrees, the IEMG activity of the soleus during the initial (5-10 s) and final 5 s tended to be higher than those for the medial and lateral gastrocnemius, regardless of the ankle joint position. We conclude that the position in which the soleus acts most selectively during a sustained maximum isometric contraction of the triceps surae is with the ankle in a neutral position and the knee flexed at 130 degrees.     

Are skeletal muscles independent actuators? Force transmission from soleus muscle in the cat

It is unclear if skeletal muscles act mechanically as independent actuators. The purpose of the present study was to investigate force transmission from soleus (SO) muscle for physiological lengths as well as relative positions in the intact cat hindlimb. We hypothesized that force transmission from SO fibers will be affected by length changes of its two-joint synergists. Ankle plantar flexor moment on excitation of the SO was measured for various knee angles (70-140 degrees ). This involved substantial length changes of gastrocnemius and plantaris muscles. Ankle angle was kept constant (80 degrees -90 degrees ). However, SO ankle moment was not significantly affected by changes in knee angle; neither were half-relaxation time and the maximal rate of relaxation (P > 0.05). Following tenotomy, SO ankle moment decreased substantially (55 +/- 16%) but did not reach zero, indicating force transmission via connective tissues to the Achilles tendon (i.e., epimuscular myofascial force transmission). During contraction SO muscle shortened to a much greater extent than in the intact case (16.0 +/- 0.6 vs. 1.0 +/- 0.1 mm), which resulted in a major position shift relative to its synergists. If the SO was moved back to its position corresponding to the intact condition, SO ankle moment approached zero, and most muscle force was exerted at the distal SO tendon. Our results also suggested that in vivo the lumped intact tissues linking SO to its synergists are slack or are operating on the toe region of the stress-strain curve. Thus, within the experimental conditions of the present study, the intact cat soleus muscle appears to act mechanically as an independent actuator.     

Effects of regular heel-raise training aimed at the soleus muscle on dynamic balance associated with arm movement in elderly women

The effects of low-intensity muscle training with heel-raises on dynamic balance associated with bilateral arm flexion were investigated in postmenopausal elderly women. Twenty-six elderly women were evenly grouped into training and control groups. Training group subjects performed 100 heel raises per day for 2 months. The training was aimed at hypertrophy of the soleus muscle, which has a relatively high proportion (ca. 90%) of slow-twitch muscle fibers and is one of the main postural muscles. Dynamic balance was measured while arm flexion was performed in response to a visual stimulus (simple-reaction condition) or at the subjects' own pace (own-timing condition). The following parameters were compared before and after the training period: plantar flexion strength, thicknesses of the gastrocnemius and soleus (by ultrasound), reaction time of the anterior deltoid in the simple-reaction condition, activation onset timing of postural muscles with respect to the deltoid, movement angles of ankle and hip joints, and postural fluctuation. In the training group only, the following training-related effects were demonstrated: (a) increase in plantar flexor strength and thickness of the soleus, (b) shortening of the deltoid reaction time, (c) earlier activation of the erector spinae in the simple-reaction condition and the soleus in the own-timing condition, and (d) increase in ankle movement in the own-timing condition and a decrease in postural fluctuation. This heel-raise training in the elderly can increase soleus thickness within the triceps surae and improve postural control modality and stability that are effectively contributed to by the leg muscle. This training consists of a low-intensity exercise that requires neither special machines nor a specific environment and can be performed safely for all old-aged groups.     

A noninvasive technique for in vivo measurement of joint torques of biarticular muscles.

The objective of this work was to develop a noninvasive method to measure the joint torques produced by biarticular muscles at two joints simultaneously. During intramuscular stimulation of the cat medial gastrocnemius (MG) muscle, torques at the ankle and knee joints were calculated from forces measured in two dimensions at the end point of the cat paw under isometric conditions. The method was verified by the known anatomical properties of cat MG muscle and the tibialis anterior (TA) muscle. The MG muscle was shown to produce a significant flexion torque at the knee, besides an extension torque at the ankle. This was in agreement with its anatomical arrangement. The TA muscle produced primarily an ankle flexion torque. The small knee torque, due to measurement errors, yielded an estimate of measurement accuracy of 3.0 +/- 2.1% (n = 52). The coupling ratio of the MG muscle, defined as T(ankle)/T(knee), varied significantly with both knee and ankle angles. The profile of MG mechanical coupling agreed qualitatively with changes in limb configuration. The method can be used to measure recruitment properties of electrically stimulated biarticular muscles, and may potentially be used to study the biomechanics of biarticular coupling.     

Load dependence of ventricular performance explained by model of calcium-myofilament interactions

Although a simple concept of load-independent behavior of the intact heart evolved from early studies of isolated, intact blood-perfused hearts, more recent studies showed that, as in isolated muscle, the mode of contraction (isovolumic vs. ejection) impacts on end-systolic elastance. The purpose of the present study was to test whether a four-state model of myofilament interactions with length-dependent rate constants could explain the complex contractile behavior of the intact, ejecting heart. Studies were performed in isolated, blood-perfused canine hearts with intracellular calcium transients measured by macroinjected aequorin. Measured calcium transients were used as the driving function for the model, and length-dependent rate constants yielding the highest concordance between measured and model-predicted midwall stress at different isovolumic volumes were determined. These length-dependent rate constants successfully predicted contractile behavior on ejecting contractions. This, along with additional model analysis, suggests that length-dependent changes in calcium binding affinity may not be an important factor contributing to load-dependent contractile performance in the intact heart under physiological conditions.     

Somatosensory graviception inhibits the soleus H-reflexin standing man - an underwater experiment-

H-reflexes were elicited in the soleus muscle in subjects standing on a force platform in a water tank, under different loading conditions at the ankle joint. The joint loading was altered by changing the combination of buoys and weights attached to lower limb segments, while the total body mass was kept the same. The results revealed that as the joint load was reduced the H-reflex was significantly enhanced as compared to that under the control condition, while it decreased as the joint load was increased, despite the same background EMG activity level. It was demonstrated that the augmented load information from the lower limb joints has an inhibitory effect on the soleus H-reflex, suggesting that this might be one of underlying neural mechanisms responsible for the suppression of H-reflex during human upright standing.     

Fatigue effects on the coordinative pattern during cycling: Kinetics and kinematics evaluation

The aim of the present study was to analyze the net joint moment distribution, joint forces and kinematics during cycling to exhaustion. Right pedal forces and lower limb kinematics of ten cyclists were measured throughout a fatigue cycling test at 100% of PO_MAX. The absolute net joint moments, resultant force and kinematics were calculated for the hip, knee and ankle joint through inverse dynamics. The contribution of each joint to the total net joint moments was computed. Decreased pedaling cadence was observed followed by a decreased ankle moment contribution to the total joint moments in the end of the test. The total absolute joint moment, and the hip and knee moments has also increased with fatigue. Resultant force was increased, while kinematics has changed in the end of the test for hip, knee and ankle joints. Reduced ankle contribution to the total absolute joint moment combined with higher ankle force and changes in kinematics has indicated a different mechanical function for this joint. Kinetics and kinematics changes observed at hip and knee joint was expected due to their function as power sources. Kinematics changes would be explained as an attempt to overcome decreased contractile properties of muscles during fatigue.     

A mathematical analysis of the force-stiffness characteristics of muscles in control of a single joint system

Feldman (1966) has proposed that a muscle endowed with its spinal reflex system behaves as a non-linear spring with an adjustable resting length. In contrast, because of the length-tension properties of muscles, many researchers have modeled them as non-linear springs with adjustable stiffness. Here we test the merits of each approach: Initially, it is proven that the adjustable stiffness model predicts that isometric muscle force and stiffness are linearly related. We show that this prediction is not supported by data on the static stiffness-force characteristics of reflexive muscles, where stiffness grows non-linearly with force. Therefore, an intact muscle-reflex system does not behave as a non-linear spring with an adjustable stiffness. However, when the same muscle is devoid of its reflexes, the data shows that stiffness grows linearly with force. We aim to understand the functional advantage of the non-linear stiffness-force relationship present in the reflexive muscle. Control of an inverted pendulum with a pair of antagonist muscles is considered. Using an active-state muscle model we describe force development in an areflexive muscle. From the data on the relationship of stiffness and force in the intact muscle we derive the length-tension properties of a reflexive muscle. It is shown that a muscle under the control of its spinal reflexes resembles a non-linear spring with an adjustable resting length. This provides independent evidence in support of the Feldman hypothesis of an adjustable resting length as the control parameter of a reflexive muscle, but it disagrees with his particular formulation. In order to maintain stability of the single joint system, we prove that a necessary condition is that muscle stiffness must grow at least linearly with force at isometric conditions. This shows that co-contraction of antagonist muscles may actually destabilize the limb if the slope of this stiffness-force relationship is less than an amount specified by the change in the moment arm of the muscle as a function of joint configuration. In a reflexive muscle where stiffness grows faster than linearly with force, co-contraction will always lead to an increase in stiffness. Furthermore, with the reflexive muscles, the same level of joint stiffness can be produced by much smaller muscle forces because of the non-linear stiffness-force relationship. This allows the joint to remain stable at a fraction of the metabolic energy cost associated with maintaining stability with areflexive muscles.This work was supported in part by grant no. 1R01 NS 24926 from the NIH (Michael Arbib, PI). R.S. was supported by an IBM Graduate Fellowship in Computer Science     

Effects of dexamethasone treatment on insulin-stimulated rates of glycolysis and glycogen synthesis in isolated incubated skeletal muscles of the rat.

1. Rats were treated with dexamethasone for 4 days before measurement of the rates of lactate formation [which is an index of hexose transport; see Challiss, Lozeman, Leighton & Newsholme (1986) Biochem. J. 233, 377-381] and glycogen synthesis in response to various concentrations of insulin in isolated incubated soleus and extensor digitorum longus muscle preparations. 2. The concentration of insulin required to stimulate these processes half-maximally in soleus and extensor digitorum longus muscles isolated from control rats was about 100 muunits/ml. 3. Dexamethasone increases the concentration of insulin required to stimulate glycolysis half-maximally in soleus and extensor digitorum longus preparations to 250 and 300 muunits/ml respectively. The respective insulin concentrations necessary to stimulate glycogen synthesis half-maximally were about 430 and 370 muunits/ml for soleus and extensor digitorum longus muscle preparations isolated from steroid-treated rats. 5. Dexamethasone treatment did not change the amount of insulin bound to soleus muscle.     

Inter-joint coupling effects on muscle contributions to endpoint force and acceleration in a musculoskeletal model of the cat hindlimb

The biomechanical principles underlying the organization of muscle activation patterns during standing balance are poorly understood. The goal of this study was to understand the influence of biomechanical inter-joint coupling on endpoint forces and accelerations induced by the activation of individual muscles during postural tasks. We calculated induced endpoint forces and accelerations of 31 muscles in a 7 degree-of-freedom, three-dimensional model of the cat hindlimb. To test the effects of inter-joint coupling, we systematically immobilized the joints (excluded kinematic degrees of freedom) and evaluated how the endpoint force and acceleration directions changed for each muscle in 7 different conditions. We hypothesized that altered inter-joint coupling due to joint immobilization of remote joints would substantially change the induced directions of endpoint force and acceleration of individual muscles. Our results show that for most muscles crossing the knee or the hip, joint immobilization altered the endpoint force or acceleration direction by more than 90° in the dorsal and sagittal planes. Induced endpoint forces were typically consistent with behaviorally observed forces only when the ankle was immobilized. We then activated a proximal muscle simultaneous with an ankle torque of varying magnitude, which demonstrated that the resulting endpoint force or acceleration direction is modulated by the magnitude of the ankle torque. We argue that this simple manipulation can lend insight into the functional effects of co-activating muscles. We conclude that inter-joint coupling may be an essential biomechanical principle underlying the coordination of proximal and distal muscles to produce functional endpoint actions during motor tasks.     

Biomechanical characterization and clinical implications of artificially induced toe-walking: differences between pure soleus, pure gastrocnemius and combination of soleus and gastrocnemius contractures

The purpose of this study was to characterize biomechanically three different toe-walking gait patterns, artificially induced in six neurologically intact subjects and to compare them to selected cases of pathological toe-walking. The subjects, equipped with lightweight mechanical exoskeleton with elastic ropes attached to the left leg's heel on one end and on shank and thigh on the other end in a similar anatomical locations where soleus and gastrocnemius muscles attach to skeleton, walked at speed of approximately 1m/s along the walkway under four experimental conditions: normal walking (NW), soleus contracture emulation (SOL), gastrocnemius contracture emulation (GAS) and emulation of both soleus and gastrocnemius contractures (SOLGAS). Reflective markers and force platform data were collected and ankle, knee and hip joint angles, moments and powers were calculated using inverse dynamic model for both legs. Characteristic peaks of averaged kinematic and kinetic patterns were compared among all four experimental conditions in one-way ANOVA. In the left leg SOL contracture mainly influenced the ankle angle trajectory, while GAS and SOLGAS contractures influenced the ankle and knee angle trajectories. GAS and SOLGAS contractures significantly increased ankle moment during midstance as compared to SOL contracture and NW. All three toe-walking experimental conditions exhibited significant power absorption in the ankle during loading response, which was absent in the NW condition, while during preswing significant decrease in power absorption as compared to NW was seen. In the knee joint SOL contracture diminished, GAS contracture increased while SOLGAS contracture approximately halved knee extensor moment during midstance as compared to NW. All three toe-walking experimental conditions decreased hip range of motion, hip flexor moment and power requirements during stance phase. Main difference in the right leg kinematic and kinetic patterns was seen in the knee moment trajectory, where significant increase in the knee extensor moment took place in terminal stance for GAS and SOLGAS experimental conditions as compared to SOL and NW. The kinetic trajectories under SOL and GAS experimental conditions were qualitatively compared to two selected clinical cases showing considerable similarity. This implies that distinct differences in kinetics between SOL, GAS and SOLGAS experimental conditions, as described in this paper, may be clinically relevant in determining the relative contribution of soleus and gastrocnemius muscles contractures to toe-walking in particular pathological gait.     

Movement behavior of high-heeled walking: how does the nervous system control the ankle joint during an unstable walking condition?

The human locomotor system is flexible and enables humans to move without falling even under less than optimal conditions. Walking with high-heeled shoes constitutes an unstable condition and here we ask how the nervous system controls the ankle joint in this situation? We investigated the movement behavior of high-heeled and barefooted walking in eleven female subjects. The movement variability was quantified by calculation of approximate entropy (ApEn) in the ankle joint angle and the standard deviation (SD) of the stride time intervals. Electromyography (EMG) of the soleus (SO) and tibialis anterior (TA) muscles and the soleus Hoffmann (H-) reflex were measured at 4.0 km/h on a motor driven treadmill to reveal the underlying motor strategies in each walking condition. The ApEn of the ankle joint angle was significantly higher (p<0.01) during high-heeled (0.38±0.08) than during barefooted walking (0.28±0.07). During high-heeled walking, coactivation between the SO and TA muscles increased towards heel strike and the H-reflex was significantly increased in terminal swing by 40% (p<0.01). These observations show that high-heeled walking is characterized by a more complex and less predictable pattern than barefooted walking. Increased coactivation about the ankle joint together with increased excitability of the SO H-reflex in terminal swing phase indicates that the motor strategy was changed during high-heeled walking. Although, the participants were young, healthy and accustomed to high-heeled walking the results demonstrate that that walking on high-heels needs to be controlled differently from barefooted walking. We suggest that the higher variability reflects an adjusted neural strategy of the nervous system to control the ankle joint during high-heeled walking.     

Neuromechanical adaptation to hopping with an elastic ankle-foot orthosis.

When humans hop or run on different surfaces, they adjust their effective leg stiffness to offset changes in surface stiffness. As a result, the overall stiffness of the leg-surface series combination remains independent of surface stiffness. The purpose of this study was to determine whether humans make a similar adjustment when springs are placed in parallel with the leg via a lower limb orthosis. We studied seven human subjects hopping in place on one leg while wearing an ankle-foot orthosis. We used an ankle-foot orthosis because the ankle joint is primarily responsible for leg stiffness during hopping. A spring was added to the ankle-foot orthosis so that it increased orthosis stiffness by providing plantar flexor torque during ankle dorsiflexion. We hypothesized that subjects would decrease their biological ankle stiffness when the spring was added to the orthosis, keeping total ankle stiffness constant. We collected kinematic, kinetic, and electromyographic data during hopping with and without the spring on the orthosis. We found that total ankle stiffness and leg stiffness did not change across the two orthosis conditions (ANOVA, P > 0.05). This was possible because subjects decreased their biological ankle stiffness to offset the orthosis spring stiffness (P < 0.0001). The reduction in biological ankle stiffness was accompanied by decreases in soleus, medial gastrocnemius, and lateral gastrocnemius muscle activation (P < 0.0002). These results suggest that an elastic exoskeleton might improve human running performance by reducing muscle recruitment.