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Tonic-to-phasic shift of lumbo-pelvic muscle activity during 8 weeks of bed rest and 6-months follow up

Daniel L. Belav,^1,2,3 Carolyn A. Richardson,² Stephen J. Wilson,¹ Dieter Felsenberg,³ and Jörn Rittweger⁴

¹School of Information Technology and Electrical Engineering, ²School of Health and Rehabilitation Sciences, The University of Queensland, Brisbane, Australia; ³Zentrum für Muskel-und Knochenforschung, Charité Campus Benjamin Franklin, Berlin, Germany; and ⁴Institute for Biophysical and Clinical Research into Human Movement, Manchester Metropolitan University, Alsager, Cheshire, United Kingdom

Submitted 2 August 2006 ; accepted in final form 8 February 2007

Prior motor control studies in unloading have shown a tonic-to-phasic shift in muscle activation, particularly in the short extensors. Tonic muscle activity is considered critical for normal musculoskeletal function. The shift from tonic-to-phasic muscle activity has not been systematically studied in humans in unloading nor at the lumbo-pelvic (LP) region. Ten healthy young male subjects underwent 8 wk of bed rest with 6-mo follow up as part of the "Berlin Bed-Rest Study." A repetitive knee movement model performed in the prone position is used to stimulate tonic holding LP muscle activity, as measured by superficial EMG. Tonic and phasic activation patterns were quantified by relative height of burst vs. baseline electromyographic linear-envelope signal components. Statistical analysis shows a shift toward greater phasic activity during bed rest and follow up (P < 0.001) with a significant interaction across muscles (P < 0.001) specifically affecting the short lumbar extensors. These changes appear unrelated to skill acquisition over time (P all 0.196). This change of a shift from tonic LP muscle activation to phasic is in line with prior research on the effects of reduced weight bearing on motor control.

electromyography; unloading; motor control; lumbar spine; Berlin Bed-Rest Study