Voluntary drive-dependent changes in vastus lateralis motor unit firing rates during a sustained isometric contraction at 50% of maximum knee extension force

Abstract

The purpose of the present study was to relate the expected inter-subject variability in voluntary drive of the knee extensor muscles during a sustained isometric contraction to the changes in firing rates of single motor units. Voluntary activation, as established with superimposed electrical stimulation was high (range: 91-99%, n=8) during a short maximal contraction, but was lower (range: 69-100%) in most subjects at the point of force failure during a sustained (49.1+/-10.1 s) fatiguing contraction at 50% of maximum force. On a different experimental day the firing behaviour of 27 single motor units was recorded with wire electrodes in the vastus lateralis muscle, 24 of which could be monitored from the time of recruitment to the point of force failure (53.6+/-9.8 s). Motor unit firing behaviour differed considerably among subjects. During the second half of the sustained, fatiguing contraction the changes in firing rate firing rate variability of early recruited units ranged from -10% to +100% and from -50% to +160% respectively among subjects. There were significant positive linear relations between voluntary activation, on the one hand, and rectified surface electromyogram (rsEMG, r=0.82), the changes in motor unit firing rate ( r=0.49) and firing rate variability ( r=0.50) towards the point of force failure on the other. The present data suggest that differences in voluntary drive that appear among subjects during fatigue may be an important determinant of motor unit firing behaviour.

Changes in force, surface and motor unit EMG during post-exercise development of low frequency fatigue in vastus lateralis muscle

“…Moreover, it was shown that LFF developed following this task. The force level was limited to 50% MFC, because at higher levels stable and SMUAPs becomes more difficult (De Ruiter et al 2004). In the present study stable and selective recordings were very important because we wanted to study the same motor units pre-exercise and for 27 min post-exercise.…”

“…The method to record and analyse SMUAPs has been described in more detail before (De Ruiter et al 2004). Custom made fine-wire electrodes were constructed from four polytereftalate butylene coated wires (0.044 mm stainless steel core, Capable b.v., the Netherlands).…”

“…Single motor unit action potentials were identified based on their amplitude and shape using custom written Matlab software (De Ruiter et al 2004). Motor unit templates were made and updated with each contraction to allow for slow and gradual changes in SMUAP amplitude and/or shape due to small and unavoidable changes in position.…”

“…During voluntary muscle contractions in vivo individual motor units usually fire at rates between 5 pps and 30 pps (Carpentier et al 2001;De Ruiter et al 2004) which is exactly within range of frequencies that LFF manifests itself when electrical stimulation is applied (Edwards et al 1977;Saugen et al 1997;Ratkevicius et al 1998). Moreover, from the work on isolated muscle preparations (Rijkelijkhuizen et al 2003) and electrically stimulated single motor axons in humans , it is known that the pulse frequency during electrical stimulation needs to be increased during LFF in order to maintain force output at the required level.…”

Knee extensor muscle oxygen consumption in relation to muscle activation

“…Knee extensor endurance is greater at extended (short muscle length) compared to Xexed (long muscle length) knee angles during sustained isometric contractions to exhaustion, even at the same relative contraction intensity (Hisaeda et al 2001;Kooistra et al 2005;Ng et al 1994;Place et al 2005). One reason for this diVerence could be a diVerence in maximal voluntary activation between knee angles (de Ruiter et al 2004;Suter and Herzog 1997). When relative contraction intensities between knee angles are compared, normalizing to the most accurate maximal value at each knee angle is important.…”