The effects of repeated isotonic contractions on the H-reflex

The efficacy of exercising a spastic limb is not known and is very difficult to study due to problems with quantifying spasticity. In recent years, investigators have been noting a correlation between changes in the Hoffmann reflex (H-reflex) and the spastic state. The purpose of this experiment was to determine the effects of repeated submaximal contractions of specific loads relative to one's maximal force production on the H-reflex. Twelve healthy subjects were tested for their one-repetition maximal contraction of their gastrocsoleus complex. On another day, at least two days later, subjects participated in the experimental protocol which consisted of measuring the H-reflex before and after ten repetitions of plantarflexion at 0%, 25%, 50%, and 75% of their one-repetition maximum. The H-reflex was elicited by stimulating the tibial nerve in the popliteal fossa using a Viking EMG stimulator (Nicolet, Minneapolis, MN). Recording electrodes were placed 3 cm apart over the soleus muscle along the mid-dorsal line of the right leg. The subjects were prone on a table with their right foot on a footplate. Although all post-exercise measures of the H-reflex were decreased from their baseline by 50% or more, a repeated measures ANOVA revealed no significant difference between the four workloads (F (3,33) = 1.056, p>.05). The data indicate that repeated isotonic contractions of the soleus muscle depress the H-reflex, and consequently motoneuron excitability, regardless of the intensity of the contractions. The data also imply that supraspinal centers have a greater influence on presynaptic inhibition than peripheral input from la afferent fibers as seen by the lack of a difference between workloads.