The influence of stimulation frequency and ankle joint angle on the momentexerted by human dorsiflexor muscles

The purpose of this study was to investigate the force-frequency relationsh ips and the post-tetanic twitch potentiation as a function of joint angle ( i.e. muscle length) in human skeletal muscles under isometric conditions. T he dorsiflexor muscles of healthy subjects were stimulated at different ank le joint angles by means of constant frequency bursts at seven submaximal f requencies (50, 33, 25, 20, 16, 12, 8 Hz) with a duration of two seconds. P articular attention has been focused on the stability of recruitment in the range of joint angles examined. The results show that moment-frequency cur ves of human dorsiflexors change as a function of ankle angle: especially f or the lower stimulation frequency range (8, 12, 16, 20 Hz), the normalized moment increases from dorsiflexion to plantar flexion (i.e. with increasin g muscle length) resulting in a leftward shift of the normalized moment-fre quency curves. Post-tetanic twitch potentiation is shown to be ankle joint dependent as well. (C) 2001 Elsevier Science Ltd. All rights reserved.