The effects of muscle length and force output on the EMG power spectrum of the erector spinae.

In many skeletal muscles the myoelectric power spectrum median frequency (MF) increases with increasing force output, possibly reflecting the greater size and conduction velocity of the later-recruited (fast twitch) fibres. Muscles, such as the erector spinae, in which fast twitch fibres are smaller than slow twitch, may display an atypical relationship between force output and median frequency. The present study sought to investigate this possibility. Ten healthy men held forces ranging from 20-80% maximal voluntary contraction (MVC) of the back extensors for 4-6 s, at muscle lengths corresponding to 30, 60 and 90% of the lumbar spine's range of flexion (ROF). MF was determined from surface electromyograms recorded from thoracic and lumbar regions of the erector spinae. In each region, MF was significantly higher at 30% ROF (short muscle length) than at 60 or 90% ROF (P < 0.005) and slightly (but not significantly) higher at 60 than 90% ROF. The muscle length effect on MF may reflect a reduction in conduction velocity of the stretched and narrowed muscle fibres. Force output had a significant effect on MF (P < 0.0004), although the shape of the relationship differed between the two levels of the erector spinae: in the thoracic region MF increased with force up to 40-50% MVC and then levelled off, whereas in the lumbar region MF was relatively stable up to 30-40% MVC and then declined with increasing force. The results suggest that the mean fibre size of the later recruited motor units is, in the thoracic region, larger, and in the lumbar region, smaller, than that of the earlier-recruited motor units.