Abstract
In skeletal muscle, postactivation potentiation (PAP) is observed following a conditioning contraction (CC) as a large (two- to three-fold) increase in evoked twitch force and rate of force development (RFD). However, this enhancement has not been observed to occur during potentiated voluntary contractions. The purpose of this study was to determine whether the lack of voluntary potentiation may be related to the development of central (corticospinal) inhibition. Participants (n = 10, all males) completed voluntary and evoked index finger abduction contractions and transcranial magnetic stimulated motor-evoked potentials (MEP) of the motor cortex were recorded from the first dorsal interosseous (FDI). Central inhibition was assessed by measuring the silent period following the MEP. The FDI was potentiated via 10-s conditioning contractions at 60% of maximal index finger abduction strength, using both voluntary and evoked tetanic contractions. Immediately following CC and transcutaneous electrical twitches. Following both voluntary and tetanic CC, force and RFD of the twitch were similarly increased (~200% and ~160%, respectively). The silent period was elongated by ~10% following both forms of CC. These results indicate that corticospinal inhibition does occur following CC, but that it is unrelated to the voluntary activation during the CC. These results also show that following CC, the positive contractile effects at the muscle are concurrently accompanied by inhibitory effects at the corticospinal level.NEW & NOTEWORTHY We demonstrate that postactivation potentiation in human skeletal muscle is accompanied by central inhibition at the corticospinal level. However, the magnitude of central inhibition does not differ between peripherally evoked or voluntary conditioning contractions. Therefore, it is possible this central inhibition is related to muscle sensory feedback.
Highlights
Muscle contractile history influences subsequent muscle contractile properties, a feature that is frequently observed as force loss and contractile slowing following moderate- to high-intensity fatiguing tasks
A brief, strong conditioning contraction (CC) may enhance subsequent evoked contractile properties, such as force and rate of force development (RFD) by as much as 200%, a phenomenon known as postactivation potentiation [18, 26]
Investigations to determine the effect of potentiation on voluntary contractions have failed to demonstrate the remarkable enhancements shown during evoked electrically stimulated contraction and have largely shown equivocal results compared with unpotentiated contractile properties [4, 5, 10, 15, 27, 28]
Summary
Muscle contractile history influences subsequent muscle contractile properties, a feature that is frequently observed as force loss and contractile slowing following moderate- to high-intensity fatiguing tasks. The positive effects of potentiation on involuntary contraction, typically evoked at the peripheral motor axon level, have been consistently and reliably observed [2, 3, 11, 12]; these externally evoked contractions do not reflect intrinsic functional voluntary task performance. If there is an impairment preventing enhancement of the voluntary potentiated contraction, the site of this impairment could be proximal to the peripheral motor axon. This may be consistent with a fatigue or inhibitory process occurring at the motor cortical or spinal level, but adaptations at this level have not yet been studied concurrently with potentiation
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Schedule a callMore From: Journal of applied physiology (Bethesda, Md. : 1985)
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