Abstract
Muscle spindle sensitivity is altered by specific types of earlier conditioning of the muscle due to a muscle property known as intrafusal muscle thixotropy. Short-length contraction (hold-short conditioning) of a muscle leads to increases in muscle spindle sensitivity of the muscle to stretch, whereas long-length contraction (hold-long conditioning) leads to decreases in the sensitivity. We studied the after-effect of hold-short or hold-long conditioning of the right biceps on tonic vibration reflex (TVR) of this muscle in 21 healthy men. An electrical vibrator with a frequency of 80 Hz was applied to the radial tendon of the muscle to elicit TVR. We measured the integrated surface electromyogram (EMG) to estimate the magnitude of TVR at 30, 60, 90, 120 s after conditioning. EMG values were expressed as a percentage of amplitudes from maximum voluntary contractions (MVC). EMG activity increased with vibration time increased, suggesting that the vibratory stimulus elicited TVR in the muscle. We found differences in EMG activity between conditionings. EMG activity was higher after hold-short conditioning (5.0 ± 4.7% MVC at 120 s, P = 0.001) than after its control (3.1 ± 3.5% MVC). On the other hand, EMG activity was similar after hold-long conditioning (2.4 ± 3.1% MVC) and its control (2.5 ± 2.1% MVC). In conclusion, hold-short conditioning has enhancing after-effects on TVR. [J Physiol Sci. 2007;57 Suppl:S160]
