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Department of Clinical Neurophysiology, Prince Henry Hospital, Sydney, Australia. l.wilson@unsw.edu.au

1. There are discrepancies in the literature about the reproducibility of forces at which human muscle spindle afferents accelerate their discharge during isometric voluntary contractions. The aim of this study was to determine for single muscle spindle afferents both the reproducibility of the 'acceleration threshold' and the factors contributing to variability of 'acceleration threshold'. 2. Microneurographic recordings were made from muscle spindle afferents innervating tibialis anterior while subjects performed isometric ankle dorsiflexions. Subjects matched the force of their contractions with a visually displayed 'ramp-and-hold' template. Template parameters were determined by the force of maximal isometric ankle dorsiflexion (MVC), and expressed as per cent MVC. The required 'ramp' rate and 'hold' force was adjusted between trials (range, 0.5-5% MVCs-1 and 0.5-20% MVC, respectively). The duration of the hold phase was 4 s and, following each contraction, stretch was applied transversely to the tendon to minimize the influence of any 'after-effects' on spindle afferent responses in subsequent contractions. 3. For each contraction, the force at which the rate of muscle spindle discharge increased was defined as the 'acceleration threshold'. Of twenty-six muscle spindle afferents innervating tibialis anterior, all but two increased their discharge in the test contractions. In 90% of contractions, acceleration thresholds were less than 3.2% MVC (range, 0.01-11.9% MVC). 4. Individual muscle spindle afferents increased their discharge at similar but not identical forces in repeated contractions. There was a positive correlation between the rate of contraction and the acceleration threshold (P < 0.001), but the strength of the target contraction had no effect on the threshold, and there was no trend for thresholds to change over time. 5. The results suggest, first, that most muscle spindle endings in the human pretibial muscles receive a significant increase in fusimotor drive during relatively weak isometric efforts and secondly, that when fusimotor after-effects are controlled, much of the residual variability in 'acceleration threshold' for any one spindle in repeated contractions is due to extrafusal factors, particularly variability in contraction rate.

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