Richard B. Stein

Difference in the amplitude of the human soleus H reflex during walking and running.

1. The Hoffman reflex, or H reflex, was strongly modulated in the human soleus muscle during both walking (4 km/h) and running (8 km/h). It was relatively low at the time of heel contact, increased progressively during the stance phase, and reached its maximum amplitude late in the stance phase. During ankle dorsiflexion the H reflex was absent. 2. During running the peak e.m.g. level of the soleus was on average 2.4 times higher than during walking but the maximum amplitude of the H reflex was never larger than during walking. In fact, the H reflex was on average significantly (P less than 0.05 for one‐tailed t test) smaller during running than during walking. Furthermore, the slope of the least‐squares line fitted to the relation between the H reflex amplitude and the background e.m.g. was always steeper for the walking data than for the running data. 3. The difference in the H reflex in the two tasks is evidence that the size of the H reflex is not simply a passive consequence of the alpha‐motoneurone excitation level, as indicated by the e.m.g., but is also influenced by other central neural mechanisms. We suggest that presynaptic inhibition is the most likely mechanism accounting for the change in the slope. 4. The modulation of the reflexes during walking and running can be interpreted in terms of the idea of automatic gain compensation. The decreased gain during running may be appropriate to reduce saturation of motor output and potential instability of the stretch reflex feed‐back loop.

What muscle variable(s) does the nervous system control in limb movements

To control force accurately under a wide range of behavioral conditions, the central nervous system would either require a detailed, continuously updated representation of the state of each muscle (and the load against which each is acting) or else force feedback with sufficient gain to cope with variations in the properties of the muscles and loads. The evidence for force feedback with adequate gain or for an appropriate central representation is not sufficient to conclude that force is the major controlled variable in normal limb movements. Mortons hypothesis, that length is controlled by a follow-up servo, has a number of difficulties related to the delays, gains, variability, and specificity in feedback pathways comprising potential servo loops. However, experimental evidence is consistent with these pathways providing servo assistance for some movements produced by coactivation of α- and static γ-motoneurons. Dynamic γ-motoneurons may provide an additional input for adaptive control of different types of movements. The idea that feedback is used to compensate for changes in muscle stiffness has received experimental support under static postural conditions. However, reflexes tend to increase rather than decrease the range of variation in muscle stiffness during some cyclic movements. Theoretical problems associated with the regulation of stiffness are also discussed. The possibilities of separate control systems for velocity or viscosity are considered, but the evidence is either negative or lacking. I conclude that different physical variables can be controlled depending on the type of limb movement required. The concept of stiffness regulation is also useful under some conditions, but should probably be extended to the regulation of the visco-elastic properties (i.e., the mechanical impedance) of a muscle or joint.

Identification of intrinsic and reflex contributions to human ankle stiffness dynamics

The authors have examined dynamic stiffness at the human ankle using position perturbations which were designed to provide a wide-bandwidth input with low average velocity. A parallel-cascade, nonlinear system identification technique was used to separate overall stiffness into intrinsic and reflex components. Intrinsic stiffness was described by a linear, second-order system similar to that demonstrated previously. Reflex stiffness dynamics were more complex, comprising a delay, a unidirectional rate-sensitive element and then lowpass dynamics. Reflex mechanisms were found to be most important at frequencies of 5-10 Hz. The gain and dynamics of reflex stiffness varied strongly with the parameters of the perturbation, the gain decreasing as the mean velocity of the perturbation increased. Under some conditions, torques generated by reflex mechanisms were of the same magnitude as those from intrinsic mechanisms. It is concluded that reflex stiffness can be large enough to be important functionally, but that its effects will depend strongly upon the particular conditions.

The modulation of human reflexes during functional motor tasks

Abstract Spinal reflexes are often viewed as stereotyped motor patterns with limited scope for modification. This presumed invariance is contrasted to the greater adaptive possibilities found in longer latency reflexes or voluntary reactions. However, recent evidence suggests that even short-latency, largely monosynaptic reflexes show a high degree of modulation during simple human motor activities such as walking and standing, and that the pattern of modulation can be specifically altered for the different functional requirements of each activity.

Long-term stimulation and recording with a penetrating microelectrode array in cat sciatic nerve

We studied the consequences of long-term implantation of a penetrating microelectrode array in peripheral nerve over the time course of 4-6 mo. Electrode arrays without lead wires were implanted to test the ability of different containment systems to protect the array and nerve during contractions of surrounding muscles. Treadmill walking was monitored and the animals showed no functional deficits as a result of implantation. In a different set of experiments, electrodes with lead wires were implanted for up to 7 mo and the animals were tested at 2-4 week intervals at which time stimulation thresholds and recorded sensory activity were monitored for every electrode. It was shown that surgical technique highly affected the long-term stimulation results. Results between measurement sessions were compared, and in the best case, the stimulation properties stabilized in 80% of the electrodes over the course of the experiment (162 days). The recorded sensory signals, however, were not stable over time. A histological analysis performed on all implanted tissues indicated that the morphology and fiber density of the nerve around the electrodes were normal.

Determination of the frequency response of isometric soleus muscle in the cat using random nerve stimulation

1. The frequency response of isometric soleus muscle was determined efficiently by analysis of the unfused tension generated during short periods of random stimulation of the divided ventral roots, in anaesthetized cats.

Contribution of peripheral afferents to the activation of the soleus muscle during walking in humans

SummarySmall, rapid stretches were applied to the soleus muscle during the stance phase of walking by lifting the forefoot with a pneumatic device. Stretch responses were induced in the soleus muscle by the disturbance. The amplitude and time course of the responses from the soleus muscle were a function of both the kinematics of the disturbance and the time in the step cycle when the disturbance was applied. The step cycle was divided into 16 equal time parts, and data obtained within each of these parts were averaged together. The electromyographic (EMG) response of the soleus muscle showed a time course that was similar to the time course of the angular velocity induced by the disturbance at the ankle. Three linear equations were used to predict the EMG response from the soleus muscle as a function of the angular kinematics of the disturbance: 1) velocity, 2) velocity and displacement, 3) velocity, displacement and acceleration. Introduction of a pure delay between the EMG and the kinematics substantially improved the predictions. Most of the variance (70%) in the EMG response could be accounted for by the velocity of the disturbance alone with an optimal delay (average 38 ms). Inclusion of a displacement term significantly increased the variance accounted for (85%), but further addition of an acceleration term did not. Since the velocity of the disturbance accounted for most of the variance, the reflex gain was estimated from the velocity coefficient. This coefficient increased in a ramp-like fashion through the early part of the stance phase, qualitatively similar to the increase in the H-reflex. Based on these identified gains, this reflex pathway was estimated to contribute substantially (30% to 60%) to the activation of the soleus muscle particularly during the early part of the stance phase.

Presynaptic inhibition in humans.

Presynaptic inhibition plays an important role in controlling sensory processing of information in humans, as in other animals. However, because of experimental constraints the methods for measuring presynaptic inhibition are necessarily more indirect in humans. The most common method uses the modulation of the H-reflex by vibratory or electrical inputs. However, these stimuli can produce postsynaptic as well as presynaptic changes so it is important to use very short periods of stimulation and measure changes at a latency where presynaptic changes predominate. In addition, the stimuli should be superimposed upon a steady background of EMG activity, preferably in a single motor unit, to maintain the postsynaptic state at a constant level. Recent studies indicate that presynaptic inhibition is used as part of the program for voluntary movement and that it can be rapidly and dramatically adapted to the task being carried out. This task-dependent modulation is produced by pattern generators within the central nervous system as well as sensory feedback from the periphery, but the relative importance of the two remains uncertain. Clinical disorders, such as spasticity, affect the ability of humans to modulate presynaptic inhibition, and contribute to the deficits observed. Improved methods for treating the symptoms pharmacologically and electrically can improve function in these patients.

Principles underlying new methods for chronic neural recording.

Chronic recording is possible from nerve fibers which have grown through holes in an insulating medium (regeneration electrodes) or which are enclosed by an insulating sheath (cuff electrodes). Use of three electrodes in a balanced configuration permits good rejection of electromyographic (EMG) signals and other sources of electrical interference (fluorescent lights, 60 Hz signals from the mains, etc.). Equations are derived and tested for predicting the amplitude and form of the signals expected for a given cuff length and diameter. These equations can be used to design electrode units optimally for a given application. Finally, the use of transformers permits the neural signals to be carefully matched to the recording apparatus and further optimizes the neural signal-to-noise and signal-to-EMG ratios. Use of these methods in several physiological and clinical applications, as well as potential abuses, are discussed.

H-reflex modulation during walking in spastic paretic subjects.

Hoffman (H) reflexes were elicited from the soleus muscle during treadmill walking in 21 spastic paretic patients. The soleus and tibialis anterior muscles were reciprocally activated during walking in most patients, much like that observed in healthy individuals. The pattern of H-reflex modulation varied considerably between patients, from being relatively normal in some patients to a complete absence of modulation in others. The most common pattern observed was a lack of H-reflex modulation through the stance phase and slight depression of the reflex in the swing phase, considerably less modulation than that of normal subjects under comparable walking conditions. The high reflex amplitudes during periods of the step cycle such as early stance seems to be related to the stretch-induced large electromyogram bursts in the soleus in some subjects. The abnormally active reflexes appear to contribute to the clonus encountered during walking in these patients. In three patients who were able to walk for extended periods, the effect of stimulus intensity was examined. Two of these patients showed a greater degree of reflex modulation at lower stimulus intensities, suggesting that the lack of modulation observed at higher stimulus intensities is a result of saturation of the reflex loop. In six other patients, however, no reflex modulation could be demonstrated even at very low stimulus intensities.