Mark H. Trimble

Principles underlying the organization of movement initiation from quiet stance

The purpose of this study was to determine common principles underlying the programming of movement initiation from quiet stance. Subjects were asked to initiate gait, step over a ruler, or to step over a 10 cm high obstacle at a self-paced speed and as fast as possible. The independent variables were initiation condition (gait initiation, stepping over a ruler or obstacle) and initiation speed (self-paced and as fast as possible). The dependent measures for the stance limb only were the latency between postural soleus (S(1)) EMG inhibition and tibialis anterior (TA) EMG onset, the duration of both TA and soleus (S(2)) activity following TA, duration and slope, impulse, and peak forces of the anterior-posterior (Fx) ground reaction force. Selected timing events were also monitored. Analysis of variance was used to determine main and interaction effects. The following results were obtained. (1) The interval from the inhibition of S(1) postural activity to the onset of TA remained invariant between all conditions. (2) The duration of TA increased and S(2) decreased with an increase in speed of initiation. There was no difference in TA and S(2) duration between the initiation conditions. (3) Time to heel-off remained invariant for all conditions. (4) Prior to heel-off all force variables increased with initiation speed but were similar between initiation conditions. After heel-off force variables were different between speeds and conditions being greater for fast speed and stepping over the obstacle. Two conclusions may be drawn from this study. First, the results indicate that gait initiation consists of two, highly coordinated motor programs. Heel-off of the stance limb is the division between these two programs. Second, our findings also suggest that gait initiation and stepping are governed by the same motor programs.

The relationship between clinical measurements of lower extremity posture and tibial translation

OBJECTIVE The purpose of this investigation was to determine if postures of the lower extremity were related to the amount of anterior tibial translation. DESIGN Regression model of lower extremity postural measures used to predict the amount of tibial translation for a sample of convenience. BACKGROUND Retrospective studies have indicated a link between certain lower extremity postures and prediction of anterior cruciate ligament injury status. What is not clear is whether these lower extremity postures cause, or occur as a result of anterior cruciate ligament injury. METHODS Genu recurvatum, the thigh-foot angle, and navicular drop measures were obtained from the right leg of 43 uninjured college-aged subjects. Tibial translation was determined using a KT-1000 arthrometer. Gender related differences were determined with independent t-tests. Step-wise regression was performed to assess the relationship between postural measures and subject gender, and tibial translation. RESULTS Very good to excellent inter- and intra-rater reliability was found for the postural measures. There was greater tibial translation (P=0.01) in females. The final regression model indicated a moderate relationship between sex of the subject, navicular drop and tibial translation (r=0.551). Genu recurvatum and thigh-foot angle were not significant predictors of tibial translation. CONCLUSIONS; We concluded that foot pronation has a greater impact on the amount of tibial translation than genu recurvatum and torsion of the lower leg in anterior cruciate intact subjects. RELEVANCE Static clinical measures of lower extremity posture are commonly taken as predictors of outcome (both performance and injury). There may be little association between these static measures and dynamic performance.

Neurophysiological assessment of the feasibility and safety of neural tissue transplantation in patients with syringomyelia.

The feasibility and safety of a procedure involving fetal spinal cord tissue transplantation in patients with syringomyelia was assessed using a neurophysiological protocol designed to quantitate peripheral nerve function, spinal cord reflex excitability, and spinal cord conduction pathways essential for somatosensory evoked potentials. We report here data obtained before and for 18 months following the transplantation procedure performed on the first two patients in this study. The neurophysiological assessment protocols included measures of cortical and spinal cord evoked potentials, H-reflex excitability, and peripheral nerve conduction. Prior to the procedure, both patients had significant deficits on some of the neurophysiological measures, for example, lower extremity cortical evoked potentials. However, robust measures of intact pathways, such as upper extremity cortical evoked potentials, were also observed preoperatively in both patients. Thus, it was anticipated that conduction in these intact pathways could be at risk either from complications from the transplantation procedure and/or from continued expansion of the syrinx. Following the transplantation procedure, no negative changes were observed in any of the neurophysiological measures in either patient. In addition, patient 1 showed a decrease in the rate potentiation of tibial H-reflexes on the right side and an increase in the response probability of left tibial H-reflexes. The results of this postoperative longitudinal assessment provide a first-level demonstration of the safety of the intraspinal neural tissue transplantation procedure. However, the consideration of safety is currently limited to the grafting procedure itself, since the long-term fates of the donor tissue in these two patients remain to be shown more definitively.

The effect of treadmill gait training on low-frequency depression of the soleus H-reflex: comparison of a spinal cord injured man to normal subjects.

H-Reflex recruitment curves were obtained at 0.1 and 1 Hz in the right soleus of an incomplete SCI man before and after training and on 12 neurologically normal individuals. Low frequency depression (LFD) was calculated by the formula: 1 - (H-wave amplitude at 1 Hz/0.1 Hz) x 100. Training consisted of treadmill walking at the speed matching his overground fast walking. The subject trained for 30 min every other day for 10 days under supervision and then continued three times a week for 4 months at a health club. Maximum H/M ratio of the right soleus (78%) was greater than that of the normals (67%) and did not change following training (79%). The mean LFD of the SCI subject was 24% prior to training compared to 42% for the normal subjects. Following training, LFD increased to 35%. In addition, the reflex threshold appears to have increased following training. This was accompanied by 47 and 45% increases in the subjects self selected and fast gait velocities, respectively. We conclude that training adaptations enabled the SCI subject to increase his gait velocity due to an improved ability to gate peripheral afferent feedback during gait.

Modulation of triceps surae H-reflexes as a function of the reflex activation history during standing and stepping

The facilitatory effectiveness of spindle afferent feedback is controlled by modulation of segmental reflex excitability such that the level of muscle activation is appropriate for the task. Phase-dependent modes of reflex modulation have been well-characterized. We hypothesized that segmental reflex excitability of the triceps surae was also modulated in a manner associated with the activation history of the spindle afferents and the segmental reflex pathway during isometric contractions, standing and stepping. In the first experiment. pairs of soleus (S) H-reflexes were evoked 80 ms apart with equal strength stimuli at rest and while subjects isometrically contracted their S against loads of 10%. 20%. and 50% of their maximum voluntary efforts. The percent depression of the second H-reflex relative to the first was used as a measure of the effect of reflex activation history. At rest, the second H-reflexes were depressed an average of 73% relative to the first. The degree of depression was progressively reduced as the plantarflexion torque increased. In the second experiment, paired H-reflexes were obtained from the S and medial (MG) and lateral gastrocnemii (LG) muscles while subjects were standing and during the stance phase of step initiation. The degree of depression of the second H-reflex during standing ( > 78%) was similar in magnitude to that produced at rest in Experiment I. At the end of the stance phase of stepping. depression of the second H-reflex of all three muscles was reduced to less than 25%. We conclude that the segmental reflex excitability is modulated as a function of the reflex activation history during these tasks.

Acute effects of locomotor training on overground walking speed and H-reflex modulation in individuals with incomplete spinal cord injury.

Abstract Objective: The purpose of this study was to assess the effect of a single bout of a locomotor-training paradigm on overground walking speed and H-reflex modulation of individuals with incomplete spinal cord injury (SCI). Methods: Self-selected and maximum walking speeds and soleus H-reflexes (H/M ratios) during standing and stance and swing phases of walking (self-selected velocity) were obtained from 4 individuals with American Spinal Injury Association impairment classification D. Data were collected immediately before and after a single bout of locomotor training with body weight support on a treadmill. The pretraining H/M ratios of the SCI subjects were also compared with values from 4 able-bodied subjects who did not receive the intervention. Maximum H/M ratios while standing and during midstance and midswing phases of overground walking were considerably greater in the SCI subjects than in the control subjects. Results: After the single bout of training, self-selected and maximum overground walking speeds of the subjects with SCI increased by 26% and 25%, respectively. Furthermore, H-reflexes were significantly more depressed in the SCI subjects during overground walking (28% less during stance, 34% less during swing). Conclusions: Although preliminary, these findings indicate that a single bout of locomotor training produced immediate increases in walking velocity and acute neurophysiologic changes in individuals with incomplete SCI.

Comparison of Single Bout Effects of Bicycle Training Versus Locomotor Training on Paired Reflex Depression of the Soleus H-Reflex After Motor Incomplete Spinal Cord Injury

OBJECTIVE To examine paired reflex depression changes post 20-minute bout each of 2 training environments: stationary bicycle ergometer training (bicycle training) and treadmill with body weight support and manual assistance (locomotor training). DESIGN Pretest-posttest repeated-measures. SETTING Locomotor laboratory. PARTICIPANTS Motor incomplete SCI (n=12; mean, 44+/-16y); noninjured subjects (n=11; mean, 30.8+/-8.3y). INTERVENTION All subjects received each type of training on 2 separate days. MAIN OUTCOME MEASURE Paired reflex depression at different interstimulus intervals (10 s, 1 s, 500 ms, 200 ms, and 100 ms) was measured before and after both types of training. RESULTS (1) Depression was significantly less post-SCI compared with noninjured subjects at all interstimulus intervals and (2) post-SCI at 100-millisecond interstimulus interval: reflex depression significantly increased postbicycle training in all SCI subjects and in the chronic and spastic subgroups (P<.05). CONCLUSIONS Phase-dependent regulation of reflex excitability, essential to normal locomotion, coordinated by pre- and postsynaptic inhibitory processes (convergent action of descending and segmental inputs onto spinal circuits) is impaired post-SCI. Paired reflex depression provides a quantitative assay of inhibitory processes contributing to phase-dependent changes in reflex excitability. Because bicycle training normalized reflex depression, we propose that bicycling may have a potential role in walking rehabilitation, and future studies should examine the long-term effects on subclinical measures of reflex activity and its relationship to functional outcomes.

Control strategies for initiation of human gait are influenced by accuracy constraints

The accuracy of placement of swing limb heel-strike was used to determine strategies of motor control of the stance limb during gait initiation (GI). Subjects initiated gait as fast as possible with the swing limb heel-strike landing on either a small or large target. Stance limb ground reaction forces, electromyogram duration and temporal data for GI were measured. It was hypothesized that accuracy would affect movement speed and the rate of rise, or the slope, of the ground reaction forces that control GI. The slopes of the stance limb forces that coincide with swing limb toe-off remained invariant. However, the slopes of forces and peak forces related to swing limb heel-strike were significantly less for the small target. These initial data suggest that principles of upper extremity motor control may be generalized to the initiation of movement from upright stance.

Modulations of soleus H-reflex excitability during gait initiation: Central versus peripheral influences

Soleus and tibialis anterior electromyogram (EMG) and soleus H‐reflexes were recorded from the stance limb of an individual who suffered a traumatic peroneal nerve injury and of four nonimpaired individuals during gait initiation. The control subjects also initiated walking after swaying forward (sway‐gait initiation), which eliminated the initial tibialis anterior activation. During the initial period of gait initiation, H‐reflexes were depressed to 43% of standing values during normal‐gait initiation and 86% during sway‐gait initiation in the nonimpaired subjects. H‐reflexes of the nerve‐injured subject were depressed to 37%, even though no tibialis anterior EMG was observed. The findings support the view that reciprocal inhibition of the soleus during a task, which normally involves tibialis anterior activation, is due to a centrally mediated process.

Soleus H-Reflex Modulation After Motor Incomplete Spinal Cord Injury: Effects of Body Position and Walking Speed

Abstract Objective: To examine position-dependent (semireclined to standing) and walking speed-dependent soleus H-reflex modulation after motor incomplete spinal cord injury (SCI). Participants: Twenty-six patients with motor incomplete SCI (mean: 45 ± 15 years) and 16 noninjured people (mean: 38 ± 14 years). Methods: Soleus H-reflexes were evoked by tibial nerve stimulation. Patients were tested in semireclined and standing positions (experiment 1) and in midstance and midswing positions (experiment 2). Results: H-reflexes were significantly greater after SCI in all positions compared with noninjured people (P < 0.05). Position-dependent modulation from semireclined to standing (normally observed in noninjured people) was absent after SCI. In SCI patients, H-reflex modulation was not significantly different at 1.2 m/s compared with 0.6 m/s treadmill walking speed; in noninjured people, H-reflex modulation was significantly greater at 1.2 m/s compared with 0.6 m/s treadmill walking speed. There was a significant positive correlation between modified Ashworth scores, a clinical measure of spasticity and soleus H-reflex amplitudes tested in all positions. A significant negative correlation was also found between H-reflexes in standing and midstance positions and the amount of assistance patients required to walk. Conclusions: An improvement in position-dependent and walking speed-dependent reflex modulation after SCI may indicate functional recovery. Future studies will use H-reflex testing to track changes as a result of therapeutic interventions.