Training by low-frequency stimulation of tibialis anterior in spinal cord-injured men

Stephen DR Harridge, Copenhagen Muscle Research Centre
Jesper L. Andersen, Copenhagen Muscle Research Centre
Andreas Hartkopp, Copenhagen Muscle Research Centre
Shi Zhou, Southern Cross University
Fin Biering-Sorensen, Copenhagen Muscle Research Centre
Claudia Sandri, Centre of Muscle Biology and Physiopathology, Padova
Michael Kjaer, Copenhagen Muscle Research Centre

Abstract

The tibialis anterior muscle of nine paraplegic men was chronically stimulated (2-6 h per day; at 10 Hz, 5 s on, 5 s off) under isometric loading conditions for 5 days per week for 4 weeks. After 4 weeks of training, muscle fatigue resistance in an electrically evoked test had increased by an average of 75% (P < .01, n = 9), but there were no changes in the relative composition of the three myosin heavy chain (MHC) isoforms. Five of the subjects continued training for an additional 5 weeks (2 h per day, 3 days per week). Although there was a tendency for twitch time to peak torque to increase after this additional period, no change occurred in relative MHC isoform content. However, in situ hybridization analysis revealed that even after 2 weeks of stimulation, there was evidence of upregulation of the mRNA for the MHC-I isoform and downregulation of the MHC-IIX isoform, a development that continued in weeks 4 and 9. This study provides evidence, at the level of gene transcription, that a fast-to-slow change in MHC isoform composition may be possible in human muscle when its usage is significantly increased.