Significance of peripheral afferent input to the alpha-motoneurone pool for enhancement of tremor during an isometric fatiguing contraction

The objective of this study was to investigate the contribution of peripher al afferent input to the enhancement of isometric tremor during a sustained submaximal isometric contraction. It was hypothesised that during muscle f atigue, when excitatory drive is high, peripheral afferent input may augmen t oscillations in the stretch reflex are and result in bursting motor-unit activity and increased tremor. Nine healthy subjects maintained isometric p lantar flexions at 30% of their maximum voluntary contraction until the lim it of endurance, under three test conditions. Two paradigms were used to re duce afferent input to the triceps surae alpha-motoneurone pool: (1) contin ued vibration of the Achilles tendon, and (2) ischaemic partial block of th e tibial nerve. These were compared to a control experiment, in which there was no intervention. By recording H-reflexes from the gastrocnemius and so leus muscles, it was possible to assess the effectiveness of reducing the a fferent input. When H-reflex suppression had stabilised, the fatiguing cont raction was commenced and tremor was computed from the continuously recorde d torque signal. Superimposed maximum twitches were elicited as indirect me asures of excitatory drive. The increase in tremor root mean square through out the fatiguing contraction was significantly less for both the vibration and ischaemic conditions. Furthermore, tremor mean power frequency decreas ed significantly with endurance time in the control experiment, while no si gnificant change was seen in the other two experimental conditions. It is c oncluded that the enhancement of isometric tremor seen during a fatiguing s ubmaximal isometric contraction is facilitated by peripheral afferent input to the alpha-motoneurone pool.