Spatiotemporal surface EMG characteristics from rat triceps brachii muscleduring treadmill locomotion indicate selective recruitment of functionallydistinct muscle regions

Multichannel surface EMG recordings of a multiheaded skeletal muscle during cyclic locomotion combined with cineradiography were analysed in a chronic experiment. The resulting detailed two-dimensional activation pattern from the long and lateral triceps brachii heads of the rat during treadmill loc omotion were combined with gait characteristics and fibre typing of the mus cle. Shortly before ground contact of the forelimb, maximum muscle activity was found in the proximal part of the long head of the muscle. During the stance phase maximum activity was observed in the proximal part of the late ral head. The frequency dependent behaviour of cross-covariance functions o ver both muscle heads confirmed this selective shift in activation. In the lateral triceps brachii head of the investigated rats, exclusively type IT fibres were found. In the long head the frequency of type I fibres was the highest in the deep muscle layers, proximally more than distally, whereas t ype II fibres were dominant in more superficial muscle layers. A combinatio n of physiological and histological findings supports an anticipating mecha nism whereby fine-tuning of the vertical foot down manoeuvre is mainly achi eved by the (type I fibre dominated) proximal deep compartment of the biart icular long triceps brachii head and force generation is predominantly exec uted by the monoarticular lateral triceps brachii head.