The effects of vibration-induced altered stretch reflex sensitivity on maximal motor unit firing properties.

It is well known that muscle spindles have a monosynaptic, excitatory connection with alpha motoneurons. However, the influence of muscle spindles on human motor unit behavior during maximal efforts remains untested. It has also been shown that muscle spindle function, as assessed by peripheral reflexes, can be systematically manipulated with muscle vibration. Therefore, the purpose of this study was to analyze the effects of brief and prolonged vibration on maximal motor unit firing properties. A crossover design was utilized, where each of the 24 participants performed 1-3 maximal knee extensions under 3 separate conditions: 1) control; 2) brief vibration, applied during the contraction; and 3) after prolonged vibration, applied for ~20 min prior to the contraction. Multi-channel EMG was recorded from the vastus lateralis during each contraction and decomposed into its constituent motor unit action potential trains. Surprisingly, an approximate 9% reduction in maximal voluntary strength was seen not only after prolonged vibration, but also, during brief vibration. In addition, both vibration conditions had a large, significant effect on firing rates (a decrease in the rates) and a small to moderate, non-significant effect on recruitment thresholds (a small increase in the thresholds). Therefore, vibration had a detrimental influence on both maximal voluntary strength and motor unit firing properties, which we propose is due to altered function of the stretch reflex pathway.