Identification of nonlinear model of ankle joint dynamics during electrical stimulation of soleus

The mechanical impedance of the ankle joint was estimated in two conditions: during submaximal surface electrical stimulation of the soleus muscle, and with no stimulation applied. Both neurologically intact (n=5) and spinal cord injured subjects (n=4) were used. The mechanical impedance was measured by applying angular step and constant velocity (13–100° s⁻¹) perturbations at 10° to the ankle and measuring the resulting changes in torque. A five-element lumped model consisting of an inertial element, a parallel elastic element, and an elastic element in series with a viscous element and a pure tension generator produced a good fit for predicting the compliance characteristics of the ankle for both the relaxed and stimulated conditions. The elastic elements were piecewise linear with different values for the dorsiflexion and plantarflexion directions. The viscous element was velocity-dependent and it decreased in value as the velocity increased. The average torque error between the measured and model's response during soleus stimulation was 10·56% for the dorsiflexed and 11·93% for the plantarflexed perturbations. However, the average error was skewed by several subjects who had excessive error, due to volitional intervention or flexor withdrawal reflex. The average model error for the perturbations without stimulation was 7·12% for dorsiflexed and 5·58% for plantarflexed perturbations.     

The intralimb coordination of the flexor reflex response is altered in chronic human spinal cord injury

The current study compared the intralimb coordination of flexor reflex responses in spinal intact and complete chronic spinal cord injured (SCI) individuals. Noxious electrocutaneous stimulation was applied at the apex of the medial arch of the foot (50 mA, 500 Hz, 1 ms pulse width, 20 ms) in 21 complete chronic SCI and 19 spinal intact volunteers and the flexor reflex response was quantified by measuring the isometric joint torques at the ankle, knee and hip. The results showed that SCI individuals had significantly smaller peak knee and hip joint flexion torques, often exhibited a net knee extension torque, and produced a much smaller hip joint flexion torque during the flexor reflex response in contrast to the spinal intact individuals. The latency of the reflex response, measured from the tibialis anterior electromyogram, was comparable in both test populations. These findings indicate that the intralimb coordination of the flexor reflex response of chronic complete SCI individuals is altered, possibly reflecting a functional reorganization of the flexion pathways of the spinal cord.     

Intrinsic and reflex stiffness in normal and spastic, spinal cord injured subjects

Mechanical changes underlying spastic hypertonia were explored using a parallel cascade system identification technique to evaluate the relative contributions of intrinsic and reflex mechanisms to dynamic ankle stiffness in healthy subjects (controls) and spastic, spinal cord injured (SCI) patients. We examined the modulation of the gain and dynamics of these components with ankle angle for both passive and active conditions. Four main findings emerged. First, intrinsic and reflex stiffness dynamics were qualitatively similar in SCI patients and controls. Intrinsic stiffness dynamics were well modeled by a linear second-order model relating intrinsic torque to joint position, while reflex stiffness dynamics were accurately described by a linear, third-order system relating half-wave rectified velocity to reflex torque. Differences between the two groups were evident in the values of four parameters, the elastic and viscous parameters for intrinsic stiffness and the gain and first-order cut-off frequency for reflex stiffness. Second, reflex stiffness was substantially increased in SCI patients, where it generated as much as 40% of the total torque variance, compared with controls, where reflex contributions never exceeded 7%. Third, differences between SCI patients and controls depended strongly on joint position, becoming larger as the ankle was dorsiflexed. At full plantarflexion, there was no difference between SCI and control subjects; in the mid-range, reflex stiffness was abnormally high in SCI patients; at full dorsiflexion, both reflex and intrinsic stiffness were larger than normal. Fourth, differences between SCI and control subjects were smaller during the active than the passive condition, because intrinsic stiffness increased more in controls than SCI subjects; nevertheless, reflex gain remained abnormally high in SCI patients. These results elucidate the nature and origins of the mechanical abnormalities associated with hypertonia and provide a better understanding of its functional and clinical implications.     

Potential roles of force cues in human stance control

Human stance is inherently unstable. A small deviation from upright body orientation is enough to yield a gravitational component in the ankle joint torque, which tends to accelerate the body further away from upright (‘gravitational torque’; magnitude is related to body-space lean angle). Therefore, to maintain a given body lean position, a corresponding compensatory torque must be generated. It is well known that subjects use kinematic sensory information on body-space lean from the vestibular system for this purpose. Less is known about kinetic cues from force/torque receptors. Previous work indicated that they are involved in compensating external contact forces such as a pull or push having impact on the body. In this study, we hypothesized that they play, in addition, a role when the vestibular estimate of the gravitational torque becomes erroneous. Reasons may be sudden changes in body mass, for instance by a load, or an impairment of the vestibular system. To test this hypothesis, we mimicked load effects on the gravitational torque in normal subjects and in patients with chronic bilateral vestibular loss (VL) with eyes closed. We added/subtracted extra torque to the gravitational torque by applying an external contact force (via cable winches and a body harness). The extra torque was referenced to body-space lean, using different proportionality factors. We investigated how it affected body-space lean responses that we evoked using sinusoidal tilts of the support surface (motion platform) with different amplitudes and frequencies (normals ±1°, ±2°, and ±4° at 0.05, 0.1, 0.2, and 0.4 Hz; patients ±1° and ±2° at 0.05 and 0.1 Hz). We found that added/subtracted extra torque scales the lean response in a systematic way, leading to increase/decrease in lean excursion. Expressing the responses in terms of gain and phase curves, we compared the experimental findings to predictions obtained from a recently published sensory feedback model. For the trials in which the extra torque tended to endanger stance control, predictions in normals were better when the model included force cues than without these cues. This supports our notion that force cues provide an automatic ‘gravitational load compensation’ upon changes in body mass in normals. The findings in the patients support our notion that the presumed force cue mechanism provides furthermore vestibular loss compensation. Patients showed a body-space stabilization that cannot be explained by ankle angle proprioception, but must involve graviception, most likely by force cues. Our findings suggest that force cues contribute considerably to the redundancy and robustness of the human stance control system.     

Morphology and Morphometry of Human Chronic Spinal Cord Injury Using Diffusion Tensor Imaging and Fuzzy Logic

Diffusion tensor imaging (DTI) was performed on regions rostral to the injury site in four human subjects with chronic spinal cord injury (SCI) and equivalent regions in four neurologically intact subjects. Apparent diffusion coefficients were measured and compared between subjects. A fuzzy logic tissue classification algorithm was used to segment gray and white matter regions for morphometric analysis, including comparisons of cross-sectional areas of gray and white matter along with frontal and sagittal diameters. Results indicated a general decrease in both longitudinal and transverse diffusivity in the upper cervical segments of subjects with chronic SCI. Further, a decrease in the cross-sectional area of the entire spinal cord was observed in subjects with SCI, consistent with severe atrophy of the spinal cord. These observations have implications in tracking the progression of SCI from the acute to the chronic stages. We conclude that DTI with fuzzy logic tissue classification has potential for monitoring morphological changes in the spinal cord in people with SCI.     

Tension regulation during lengthening and shortening actions of the human soleus muscle

In the present study we investigated tension regulation in the human soleus (SOL) muscle during controlled lengthening and shortening actions. Eleven subjects performed plantar flexor efforts on an ankle torque motor through 30° of ankle displacement (75°–105° internal ankle angle) at lengthening and shortening velocities of 5, 15 and 30° · s⁻¹. To isolate the SOL from the remainder of the triceps surae, the subject's knee was flexed to 60° during all trials. Voluntary plantar flexor efforts were performed under two test conditions: (1) maximal voluntary activation (MVA) of the SOL, and (2) constant submaximal voluntary activation (SVA) of the SOL. SVA trials were performed with direct visual feedback of the SOL electromyogram (EMG) at a level resulting in a torque output of 30% of isometric maximum. Angle-specific (90° ankle angle) torque and EMG of the SOL, medial gastrocnemius (MG) and tibialis anterior (TA) were recorded. In seven subjects from the initial group, the test protocol was repeated under submaximal percutaneous electrical activation (SEA) of SOL (to 30% isometric maximal effort). Lengthening torques were significantly greater than shortening torques in all test conditions. Lengthening torques in MVA and SVA were independent of velocity and remained at the isometric level, whereas SEA torques were greater than isometric torques and increased at higher lengthening velocities. Shortening torques were lower than the isometric level for all conditions. However, whereas SVA and SEA torques decreased at higher velocities of shortening, MVA torques were independent of velocity. These results indicate velocity- and activation-type-specific tension regulation in the human SOL muscle. Accepted: 11 October 1999     

Soleus H-reflex excitability changes in response to sinusoidal hip stretches in the injured human spinal cord

Imposed static hip stretches substantially modulate the soleus H-reflex in people with an intact or injured spinal cord while stretch of the hip flexors affect the walking pattern in lower vertebrates and humans. The aim of this study was to assess the effects of dynamic hip stretches on the soleus H-reflex in supine spinal cord injured (SCI) subjects. Sinusoidal movements were imposed on the right hip joint at 0.2 Hz by a Biodex system. H-reflexes from the soleus muscle were recorded as the leg moved in flexion or extension. Stimuli were sent only once in every hip movement cycle that each lasted 5 s. Torque responses were recorded at the hip, knee, and ankle joints. A hip phase-dependent soleus H-reflex modulation was present in all subjects. The reflex was facilitated during hip extension and suppressed during hip flexion. There were no significant differences in pre- or post-stimulus soleus background activity between the two conditions. Oscillatory responses were present as the hip was maximally flexed. Sinusoidal hip stretches modulated the soleus H-reflex in a manner similar to that previously observed following static hip stretches. The amount of reflex facilitation depended on the angle of hip extension. Further research is needed on the afferent control of spinal reflex pathways in health and disease in order to better understand the neural control of movement in humans. This will aid in the development of rehabilitation strategies to restore motor function in these patients.     

Soleus H-reflex modulation during body weight support treadmill walking in spinal cord intact and injured subjects

The soleus H-reflex modulation pattern was investigated in ten spinal cord intact subjects during treadmill walking at varying levels of body weight support (BWS), and nine spinal cord injured (SCI) subjects at a BWS level that promoted the best stepping pattern. The soleus H-reflex was elicited by tibial nerve stimulation with a single 1-ms pulse at an intensity that the M-waves ranged from 4 to 8% of the maximal M-wave (M_max). During treadmill walking, the H-reflex was elicited every four steps, and stimuli were randomly dispersed across the gait cycle which was divided into 16 equal bins. EMGs were recorded with surface electrodes from major left and right hip, knee, and ankle muscles. M-waves and H-reflexes at each bin were normalized to the M_max elicited at 60–100 ms after the test reflex stimulus. For every subject, the integrated EMG area of each muscle was established and plotted as a function of the step cycle phase. The H-reflex gain was determined as the slope of the relationship between H-reflex and soleus EMG amplitudes at 60 ms before H-reflex elicitation for each bin. In spinal cord intact subjects, the phase-dependent H-reflex modulation, reflex gain, and EMG modulation pattern were constant across all BWS (0, 25, and 50) levels, while tibialis anterior muscle activity increased with less body loading. In three out of nine SCI subjects, a phase-dependent H-reflex modulation pattern was evident during treadmill walking at BWS that ranged from 35 to 60%. In the remaining SCI subjects, the most striking difference was an absent H-reflex depression during the swing phase. The reflex gain was similar for both subject groups, but the y-intercept was increased in SCI subjects. We conclude that the mechanisms underlying cyclic H-reflex modulation during walking are preserved in some individuals after SCI.     

Command control for functional electrical stimulation hand grasp systems using miniature accelerometers and gyroscopes

Recent commercially available miniature sensors have the potential to improve the functions of functional electrical stimulation (FES) systems in terms of control, reliability and robustness. A new control approach using a miniature gyroscope and an accelerometer was studied. These sensors were used to detect the linear acceleration and angular velocity of residual voluntary movements on upper limbs and were small and easy to put on. Five healthy subjects and three cervical spinal cord injured subjects were recruited to evaluate this controller. Sensors were placed on four locations: the shoulder, upper arm, wrist and hand. A quick forward-and-backward movement was employed to produce a distinctive waveform that was different from general movements. A detection algorithm was developed to generate a command signal by identifying this distinctive waveform through the detection of peaks and valleys in the sensor's signals. This command signal was used to control different FES hand grasp patterns. With a specificity of 0.9, the sensors had a success rate of 85–100% on healthy subjects and 82–97% on spinal cord injured subjects. In terms of sensor placement, the gyroscope was better as a control source than the accelerometer for wrist and hand positions, but the reverse was true for the shoulder.     

Effects of Level,Loading Rate,Injury and Repair on Biomechanical Response of Ovine Cervical Intervertebral Discs

A need exists for pre-clinical large animal models of the spine to translate biomaterials capable of repairing intervertebral disc (IVD) defects. This study characterized the effects of cervical spinal level, loading rate, injury and repair with genipin-crosslinked fibrin (FibGen) on axial and torsional mechanics in an ovine cervical spine model. Cervical IVDs C2–C7 from nine animals were tested with cyclic tension–compression (? 240 to 100 N) and cyclic torsion (±?2° and?±?4°) tests at three rates (0.1, 1 and 2 Hz) in intact, injured and repaired conditions. Intact IVDs from upper cervical levels (C2–C4) had significantly higher torque range and torsional stiffness and significantly lower axial range of motion (ROM) and tensile compliance than IVDs from lower cervical levels (C5–C7). A tenfold increase in loading rate significantly increased torque range and torsional stiffness 4–8% (depending on amplitude) (p?<?0.001). When normalized to intact, FibGen significantly restored torque range (FibGen: 0.96?±?0.14, Injury: 0.88?±?0.14, p?=?0.03) and axial ROM (FibGen: 1.00?±?0.05, Injury: 1.04?±?0.15, p?=?0.02) compared to Injury, with a values of 1 indicating full repair. Cervical spinal level must be considered for controlling biomechanical evaluations, and FibGen restored some torsional and axial biomechanical properties to intact levels.     

Circadian variation of flexor withdrawal and crossed extensor reflexes in patients with restless legs syndrome

An evening state of spinal hyperexcitability has been proposed to be a possible cause of evening increases in restless legs syndrome symptoms. Thus, the objective of the current study was to assess the circadian variation in spinal excitability in patients with restless legs syndrome based on flexor withdrawal reflex and crossed extensor reflex responses. The reflexes were elicited on 12 participants with restless legs syndrome and 12 healthy control participants in the evening (PM) and the morning (AM). Reflex response magnitudes were measured electromyographically and kinematically. Both the reflexes showed a circadian rhythm in participants with restless legs syndrome but not in control participants. Changes in ankle (median flexor withdrawal reflex PM: 16.0 ° versus AM: 2.8 °, P = 0.042; crossed extensor reflex PM: 0.8 ° versus AM: 0.2 °, P = 0.001) angle were significantly larger, and ankle angular velocity (median flexor withdrawal reflex PM: 38.8 ° s^?1 versus AM: 13.9 ° s^?1, P = 0.049; crossed extensor reflex PM: 2.4 ° s^?1 versus AM: 0.5 ° s^?1, P = 0.002) was significantly faster in the evening compared with the morning in participants with restless legs syndrome, for both reflexes. For participants with restless legs syndrome, evening change in hallux angle was significantly larger than morning responses (median PM: 5.0 ° versus AM: 1.3 °, P = 0.012). No significant differences for any of the electromyographic or kinematic variables were observed between participants with restless legs syndrome and controls. The flexor withdrawal reflex and the crossed extensor reflex show a circadian rhythm in participants with restless legs syndrome suggesting an evening increase in spinal excitability. We hypothesize the circadian variation in spinal excitability may be due to a possible nocturnal form of afferent circuitry central sensitization in the dorsal horn of the spinal cord in patients with restless legs syndrome.     

Identification of ankle plantar-flexors dynamics in response to electrical stimulation

Modeling the muscle response to functional electrical stimulation (FES) is an essential step in the design of closed-loop controlled neuroprostheses. This study was aimed at identifying the dynamic response of ankle plantar-flexors to FES during quiet standing. Thirteen healthy subjects stood in a standing frame that locked the knee and hip joints. The ankle plantar-flexors were stimulated bilaterally through surface electrodes and the generated ankle torque was measured. The pulse amplitude was sinusoidally modulated at five different frequencies. The pulse amplitude and the measured ankle torque fitted by a sine function were considered as input and output, respectively. First-order and critically-damped second-order linear models were fitted to the experimental data. Both models fitted similarly well to the experimental data. The coefficient of variation of the time constant among subjects was smaller in the case of the second-order model compared to the first-order model (18.1% vs. 79.9%, p < 0.001). We concluded that the critically-damped second-order model more consistently described the relationship between isometric ankle torque and surface FES pulse amplitude, which was applied to the ankle plantar-flexors during quiet standing.     

Frequency response characteristics of ankle plantar flexors in humans following spinal cord injury: relation to degree of spasticity

Frequency response characteristics of the ankle plantar flexors were studied in adults both with and without spinal cord injury (SCI) to determine how the muscle contractile properties change after SCI, and to see if there is a relation between the severity of spasticity and how the properties change. Ten controls and ten complete, chronic spinal cord injured subjects were tested, where the tibial nerve was stimulated electrically in a stochastic manner with the ankle fixed isometrically at various joint angles. A nonparametric linear frequency response function was derived, from which a second-order transfer function was calculated. The contractile dynamics were then characterized by the three classic second-order parameters: gain, damping ratio, and natural frequency. We found that in subjects with low degrees of spasticity (as determined by clinical evaluation), the contractile dynamics presented the largest changes, in which the speed of contraction increased significantly while there were no statistical differences in the gains between the two groups. This similarity emerged even though there was noticeable atrophy in the SCI patient group. Differences between the controls and subjects with high levels of spasticity were markedly different, in that these SCI subjects had slower contractile speeds than the controls, but significantly lower gains. Moderately spastic subjects fell somewhere in between, where the speed of muscle contraction increased modestly yet the gain was significantly smaller than that of the control subjects. These findings indicate that in subjects with chronic spinal cord injury, the severity of spasticity can significantly influence the degree of change in muscle contractile properties. It appears that high degrees of spasticity tend to preserve contractile dynamics, while in less spastic subjects, muscle contractile properties may display faster response characteristics. © 2002 Biomedical Engineering Society. PAC2002: 8719Ff, 8717Nn, 8754Dt, 8719Rr     

Riluzole decreases flexion withdrawal reflex but not voluntary ankle torque in human chronic spinal cord injury

The objectives of this study were to probe the contribution of spinal neuron persistent sodium conductances to reflex hyperexcitability in human chronic spinal cord injury. The intrinsic excitability of spinal neurons provides a novel target for medical intervention. Studies in animal models have shown that persistent inward currents, such as persistent sodium currents, profoundly influence neuronal excitability, and recovery of persistent inward currents in spinal neurons of animals with spinal cord injury routinely coincides with the appearance of spastic reflexes. Pharmacologically, this neuronal excitability can be decreased by agents that reduce persistent inward currents, such as the selective persistent sodium current inhibitor riluzole. We were able to recruit seven subjects with chronic incomplete spinal cord injury who were not concurrently taking antispasticity medications into the study. Reflex responses (flexion withdrawal and H-reflexes) and volitional strength (isometric maximum voluntary contractions) were tested at the ankle before and after placebo-controlled, double-blinded oral administration of riluzole (50 mg). Riluzole significantly decreased the peak ankle dorsiflexion torque component of the flexion withdrawal reflex. Peak maximum voluntary torque in both dorsiflexion and plantarflexion directions was not significantly changed. Average dorsiflexion torque sustained during the 5-s isometric maximum voluntary contraction, however, increased significantly. There was no effect, however, on the monosynaptic plantar and dorsiflexor H-reflex responses. Overall, these results demonstrate a contribution of persistent sodium conductances to polysynaptic reflex excitability in human chronic spinal cord injury without a significant role in maximum strength production. These results suggest that intrinsic spinal cellular excitability could be a target for managing chronic spinal cord injury hyperreflexia impairments without causing a significant loss in volitional strength.     

Is passive stiffness in human muscles related to the elasticity of tendon structures?

The purpose of this study was to examine in vivo whether passive stiffness in human muscles was related to the elasticity of tendon structures and to performance during stretch-shortening cycle exercise. Passive torque of plantar flexor muscles was measured during passive stretch from 90° (anatomical position) to 65° of dorsiflexion at a constant velocity of 5°·s^–1. The slope of the linear portion of the passive torque-angle curve during stretching was defined as the passive stiffness of the muscle. The elongation of the tendon and aponeurosis of the medial gastrocnemius muscle (MG) was directly measured using ultrasonography during ramp isometric plantar flexion up to the voluntary maximum. The relationship between the estimated muscle force of MG and tendon elongation was fitted to a linear regression, the slope of which was defined as the stiffness of the tendon. In addition, the dynamic torques during maximal voluntary concentric plantar flexion with and without prior eccentric contraction were determined at a constant velocity of 120°·s^–1. There were no significant correlations between passive stiffness and either the tendon stiffness (r=0.19, P>0.05) or the relative increase in torque with prior eccentric contraction (r=–0.19, P>0.05). However, tendon stiffness was negatively correlated to the relative increase in torque output (r=–0.42, P<0.05). The present results suggested that passive stiffness was independent of the elasticity of tendon structures, and had no favourable effect on the muscle performance during stretch-shortening cycle exercise. Electronic Publication     

Relationship between in vivo muscle force at different speeds of isokinetic movements and myosin isoform expression in men and women

In an attempt to explore the relationship between force production during voluntary contractions at different speeds of isokinetic movement and the myofibrillar protein isoform expression in humans, an improved isokinetic dynamometer that corrects for gravitation, controls for acceleration and deceleration, and identifies a maximum voluntary activation was used. Muscle torque recordings were compared at the same muscle length (knee angle) and the torque was calculated as the average torque at each angle over a large knee angle range (75°–25°) to reduce the influence of small torque oscillation on the calculated torque. Muscle torque at fast (240° s⁻¹) versus slow (30° s⁻¹) speeds of movement, torque normalized to muscle cross-sectional area (specific tension), and absolute torque at fast speeds of movement were measured in 34 young healthy male and female short-, middle-, and long-distance runners. The relationship between the different measures of muscle function and the expression of myosin heavy chain (MyHC) isoforms using enzyme–histochemical and electrophoretic protein separation techniques were investigated. A significant correlation between the 240° s⁻¹ vs 30° s⁻¹ torque ratio and the relative area of the type II fibers and type II MyHC isoforms were observed in both the men (r=0.74;P<0.001) and the women (r=0.81; P<0.05). Thus, the present results confirm a significant relationship between in vivo human muscle function and the MyHC isoform expression in the contracting muscle. Electronic Publication     

Increases in muscle activity produced by vibration of the thigh muscles during locomotion in chronic human spinal cord injury

The purpose of this study was to determine whether the muscle vibration applied to the quadriceps has potential for augmenting muscle activity during gait in spinal cord injured (SCI) individuals. The effects of muscle vibration on muscle activity during robotic-assisted walking were measured in 11 subjects with spinal cord injury (SCI) that could tolerate weight-supported walking, along with five neurologically intact individuals. Electromyographic (EMG) recordings were made from the tibialis anterior (TA), medial gastrocnemius (MG), rectus femoris (RF), vastus lateralis (VL), and medial hamstrings (MH) during gait. Vibration was applied to the anterior mid-thigh using a custom vibrator oscillating at 80 Hz. Five vibratory conditions were tested per session including vibration applied during: (1) swing phase, (2) stance phase, (3) stance-swing transitions, (4) swing-stance transitions, and (5) throughout the entire gait cycle. During all vibration conditions, a significant increase in EMG activity was observed across both SCI and control groups in the RF, VL, and MH of the ipsilateral leg. In the SCI subjects, the VL demonstrated a shift toward more appropriate muscle timing when vibration was applied during stance phase and transition to stance of the gait cycle. These observations suggest that the sensory feedback from quadriceps vibration caused increased muscle excitation that resulted in phase-dependent changes in the timing of muscle activation during gait.     

Contraction history affects the in vivo quadriceps torque-velocity relationship in humans

We hypothesized that the history of contraction would affect the in vivo quadriceps torque-velocity relationship. We examined the quadriceps torque-velocity relationship of the human knee extensors at the descending and ascending limb of the torque-position relationship by initiating the knee extension at a knee angle position of 1.39 rad (80°) or 0.87 rad (50°) over a 0.52 rad (30°) range of motion under conditions of constant or linearly increasing velocity. Maximal voluntary isometric knee extension torque (M₀) was measured at 1.87 rad, 0.87 rad, and 0.35 rad, and concentric torque was measured. The subjects carried out ten maximal knee extensions at ten distinct velocities, each velocity ranging between 0.52 rad·s^–1 to 5.24 rad·s^–1 in steps of 0.52 rad·s^–1. Peak concentric torque was measured and mean torque calculated from the respective torque-time curves. Peak or mean torque, computed from the individual torque-time curves, and velocity data were fitted to the Hill equation under the four experimental conditions and the curve parameters computed. The M₀ was similar at 0.87 rad and 1.39 rad, but it was significantly lower at 0.35 rad. In the low-velocity domain of the torque-velocity curve where a plateau normally occurs, peak torque was always lower than M₀. Peak and mean torque were significantly greater under linearly increasing velocity conditions and the 1.39 rad starting knee position. Mean torque but not peak torque data could be well fitted to the Hill equation and the two computations resulted in significantly different Hill curve parameters including the concavity ratio, peak power, and maximal angular velocity. We concluded that the history of contraction significantly modifies the in vivo torque-velocity relationship of the human quadriceps muscle. Muscle mechanics and not neural factors may have accounted for the inconsistencies in the human torque-velocity relationships reported previously. Electronic Publication     

Velocity specificity in early training of the knee extensors after anterior cruciate ligament reconstruction

Resistance-training velocity specificity is known to occur in isotonic training of uninjured subjects and in isokinetic training of injured patients. Whether velocity specificity occurs with isotonic training in injured patients has not been tested, despite the common use of this exercise mode in patients. Thirty-two patients recovering from anterior cruciate ligament reconstruction (ACLR) surgery were tested at approximately 2 and 6 weeks after surgery. The isokinetic injured/uninjured strength ratios of the knee extensors were compared for the test velocities of 60° · s⁻¹ and 210° · s⁻¹, as assessed before and after a 4-week training period. Isotonic training of the knee extensors at 60° · s⁻¹ was applied in formal sessions three times per week. The isokinetic injured/uninjured strength ratios were compared for the two test velocities, and there was no indication that training velocity specificity occurred in these patients. Possible reasons for this finding, which contrasts with previous work, are discussed. Accepted: 12 November 1999     

Characterization and control of muscle response to electrical stimulation

The maintenance of upright posture in neurologically intact human subjects is mediated by two major nervous pathways. The first, leading from the cerebral cortex through the spinal cord to motor neurons, activates muscles which produce postural movements. The second, leading from various sensory organs to higher centers, provides sensory feedback regarding the postural state. The path through the spinal cord is no longer intact in victims of spinal cord injury and loss of normal control of muscle activity results. Functional neuromuscular stimulation (FNS) has been shown as a feasible method for obtaining muscle contraction in paraplegic and has been proposed as a means for control of antero-posterior sway to make upright posture possible for these individuals. Before muscle can be controlled through the use of FNS, the response of muscle to electrical stimulation must be understood. In past studies, linear control theory has been applied to the analysis of this response and to the testing of various controllers. The aim of this study was to demonstrate some control issues in FNS using linear control theory, as it applies to electrical stimulation of muscle for stabilization of posture. The linearity of the muscle response was improved through closed-loop control using pole compensation techniques. The excess phase shift of the system due to the time delay in the muscle response, however, limits the ability to increase the open-loop gain in order to obtain improved performance. A suggestion for further study is the application of this methodology for uses in posture control.