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 frequen cy (MF) increases with increasing force output, possibly reflecting th e greater size and conduction velocity of the later-recruited (fast tw itch) fibres. Muscles, such as the erector spinae, in which fast twitc h fibres are smaller than slow twitch, may display an atypical relatio nship between force output and median frequency. The present study sou ght to investigate this possibility. Ten healthy men held forces rangi ng from 20-80% maximal voluntary contraction (MVC) of the back extenso rs for 4-6 s, at muscle lengths corresponding to 30, 60 and 90% of the lumbar spine's range of flexion (ROF), RIF was determined from surfac e electromyograms recorded from thoracic and lumbar regions of the ere ctor spinae. In each region, MF was significantly higher at 30% ROF (s hort 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 incre ased with force up to 40-50% MVC and then levelled off, whereas in the lumbar region RIF was relatively stable up to 30-40% MVC and then dec lined with increasing force. The results suggest that the mean fibre s ize of the later recruited motor units is, in the thoracic region, lar ger, and in the lumbar region, smaller, than that of the earlier-recru ited motor units. Copyright (C) 1996 Elsevier Science Ltd.