Sydney 31 October -5 November 1999 |
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Effects of low-frequency electrical stimulation on force-time characteristics of the tibialis anterior in spinal cord injured males S. Zhou*1, S.D.R. Harridge2, A. Hartkopp3
& F. Biering-Sørensen4 2Department of Physiology, Royal Free & University College Medical School, London, United Kingdom 3Copenhagen Muscle Research Centre, Denmark 4Centre for Spinal Cord Injured, Rigshospitalet, Denmark It has been reported that prolonged low-frequency electrical stimulation can alter muscle contractile properties in spinal cord injured individuals whose tibialis anterior (TA) previously showed the characteristics of fast twitch muscle (Harridge et al. 1998). The purpose of this study was to examine the effects of such training on the time related contractile properties of TA. Nine male subjects aged 39± 9 years and had sustained a spinal cord injury 11± 5 years participated in the study. The TA of the right leg was stimulated percutaneously at a frequency of 10 Hz, with a duty cycle of 5 s on / 5 s off, and at a current that elicited ~70% of maximal force at 10 Hz. Subjects trained under supervision, from 2 to 6 hours per day, 5 times per week for 4 weeks (Harridge et al. 1998). The contraction time and electromechanical delay (EMD) in single twitch, tetanic contractions at 10, 20, 50 & 100 Hz, as well as pre and post a 5 min fatigue test at 20 Hz, were examined pre and post the 4 weeks of training. The training resulted in a trend of elongation in the contraction time of the twitch, increase in tension of the twitch and tetanic contractions (n.s.), and improved resistance to fatigue (p<0.01). There was also a trend that EMD became elongated in tetanic contractions, especially in response to low frequency stimuli, and was less affected by the fatigue test after the training (n.s.). It has been reported that EMD is significantly correlated to peak tension and rate of force development (Zhou et al. 1995), and to muscle fibre type composition (Viitasalo & Komi 1981). The present findings appeared to support that the chronic low-frequency training could affect the force-time characteristics of the muscle. These changes in the excitation-contraction properties may precede any significant variations in the myosin heavy chain expression in these subjects.
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