The reduction in human motoneurone responsiveness during muscle fatigue is not prevented by increased muscle spindle discharge

Abstract

Non-technical summary: The responsiveness of motoneurones within the spinal cord to non-invasive stimulation is markedly reduced during a fatiguing maximal effort if tested when voluntary drive from the brain is transiently interrupted. We tested a major possible cause of this effect by vibrating the tendon of the contracting muscle to increase excitatory input to the motoneurones. Application of tendon vibration had a negligible effect on the fatigue-induced reduction of motoneurone responsiveness. Hence, we believe the reduction in responsiveness is caused by changes to the intrinsic properties of motoneurones due to their repetitive activity during the sustained maximal effort. Motoneurone excitability is rapidly and profoundly reduced during a sustained maximal voluntary contraction (MVC) when tested in the transient silent period which follows transcranial magnetic stimulation (TMS) of the motor cortex. One possible cause of this reduction in excitability is a fatigue-induced withdrawal of excitatory input to motoneurones from muscle spindle afferents. We aimed to test if muscle spindle input produced by tendon vibration would ameliorate suppression of the cervicomedullary motor-evoked potential (CMEP) in the silent period during a sustained MVC. Seven subjects performed a 2 min MVC of the elbow flexors. Stimulation of the corticospinal tract at the level of the mastoids was preceded 100 ms earlier by TMS. These stimulus pairs were delivered every 10 s during the 2 min MVC. Stimulus pairs at 30, 50, 70, 90 and 110 s were delivered while vibration (~80 Hz) was applied to the distal tendon of biceps. On a separate day, the protocol was repeated with both stimuli delivered to the motor cortex. The CMEP in the silent period decreased rapidly with fatigue (to ~9% of control) and was not affected by tendon vibration (P= 0.766). The motor-evoked potential in the silent period also declined rapidly (to ~5% of control) and was similarly unaffected by tendon vibration (P= 0.075). These data suggest motoneurone disfacilitation due to a fatigue-related decrease of muscle spindle discharge does not contribute significantly to the profound suppression of motoneurone excitability during the silent period. Therefore, a change to intrinsic motoneurone properties caused by repetitive discharge is most probably responsible. © 2011 The Authors. Journal compilation © 2011 The Physiological Society.