Corticospinal adaptations to strength training and its associations to rate of force development

Abstract

Introduction. Neuromuscular determinants underlying rate of force development during rapid muscle contractions may be more relevant compared to maximal strength in many athletic events characterized by power and speed, as well as in clinical relations with older adult and patient populations. Earlier studies have described various neuromuscular determinants associated with rapid muscle contractions, and while rate of force development is affected by both intrinsic neural and contractile properties the neural determinants seem to be particularly important at the onset and up to 75 milliseconds into rapid contractions. Variety of strength training modalities have been demonstrated to improve both the neural properties and rate of force development, and many associations between the two are evident. However, majority of strength training studies have addressed the associations considering the whole corticospinal tract but not in smaller segments. Therefore, the objective of this study was to investigate potential differences separately at cortical and spinal level.

Methods. Previously untrained (n=14, 28±5 years) participants completed a 7-week strength training intervention performed in conventional modality for whole body. The participants were measured for maximal voluntary contraction (MVC), and rate of force development (RFD) in three overlapping time frames (0–50, 0–75 and 0–100 ms) in isometric knee extension. Electrical stimulation of the peripheral femoral nerve was used for interpolated twitch technique (ITT) and maximal M-wave (M-max) of the rectus femoris (RF). Single-pulse electrical stimulation of the lumbar spine (LS) and transcranial magnetic stimulation (TMS) were used to measure changes in lumbar evoked potential (LEP) and motor evoked potential (MEP) peak-to-peak amplitude, as well as in spinal and corticospinal silent period (SP). All measures were taken over control period, at baseline prior to intervention, 3.5-weeks into the intervention and post to the intervention.

Results. No group level change in MVC or RFD in any time frame were observed following the strength training. Group level change in corticospinal and spinal excitability measured by MEP and LEP amplitudes remained unchanged across the intervention. Similarly, group level change in corticospinal and spinal inhibition measured by SP resulted in non-significant difference compared to baseline. Multiple regression analysis across all force and neurophysiological measures resulted in non-significant correlations.

Conclusions. No associations between rapid muscle contractions and corticospinal adaptations at either level could be demonstrated by the results of this study. Conventional whole body strength training may be suboptimal to improve rate of force development measured by unilateral isometric contractions. More importantly both corticospinal adaptations and early phase rate of force development present high intra- and inter-individual variation, thus various methodological considerations potentially compromising their assessment should be acknowledged.