The sites of neural adaptation induced by resistance training in humans

Abstract

Although it has long been supposed that resistance training causes adaptive changes in the CNS, the sites and nature of these adaptations have not previously been identified. In order to determine whether the neural adaptations to resistance training occur to a greater extent at cortical or subcortical sites in the CNS, we compared the effects of resistance training on the electromyographic (EMG) responses to transcranial magnetic (TMS) and electrical (TES) stimulation. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseous muscle of 16 individuals before and after 4 weeks of resistance training for the index finger abductors (n = 8), or training involving finger abduction-adduction without external resistance (n = 8). TMS was delivered at rest at intensities from 5 % below the passive threshold to the maximal output of the stimulator. TMS and TES were also delivered at the active threshold intensity while the participants exerted torques ranging from 5 to 60 % of their maximum voluntary contraction (MVC) torque. The average latency of MEPs elicited by TES was significantly shorter than that of TMS MEPs (TES latency = 21.5 +/- 1.4 ms; TMS latency = 23.4 +/- 1.4 ms; P < 0.05), which indicates that the site of activation differed between the two forms of stimulation. Training resulted in a significant increase in MVC torque for the resistance-training group, but not the control group. There were no statistically significant changes in the corticospinal properties measured at rest for either group. For the active trials involving both TMS and TES, however, the slope of the relationship between MEP size and the torque exerted was significantly lower after training for the resistance-training group (P < 0.05). Thus, for a specific level of muscle activity, the magnitude of the EMG responses to both forms of transcranial stimulation were smaller following resistance training. These results suggest that resistance training changes the functional properties of spinal cord circuitry in humans, but does not substantially affect the organisation of the motor cortex.

Figure 1. The experimental set-up and plot of motor evoked potential (MEP) size versus stimulus intensity for a single individual

A, the device used to restrain the hand during the experiments and to apply loads during resistance training. The finger made contact with the horizontal section of the L-shaped main shaft. The vertical section of the shaft was fixed in series with pulley 1, which rotated in the horizontal plane. An inextensible wire was fixed between pulley 1 and pulley 2. Pulley 2 was oriented in the vertical plane. Weights were hung from an additional wire that was attached to pulley 2. During training, a potentiometer was aligned in series with pulley 1. For static trials in the experiments, the wire between pulleys 1 and 2 was disconnected and a torque transducer was aligned in series with pulley 1. B, an example of a sigmoid fit to an MEP size versus stimulus intensity plot for an individual participant. The peak slope of the function is its tangent at S₅₀. MEP_max, maximal MEP amplitude; M_max, maximal M-wave amplitude. Adapted with permission from Elsevier Science from Carroll et al. (2001b).

Figure 2. Responses to transcranial magnetic (TMS) and electrical (TES) stimulation at various levels of voluntary contraction

A, mean MEPs recorded in response to TMS and TES at each of the levels of voluntary contraction for an individual participant. The small arrows illustrate the stimulus artefacts. Note that the onset latency, specified by the dashed line, is approximately 2 ms shorter for the TES responses. B, linear regressions of MEP amplitude on the absolute torque exerted from 45 to 5 ms prior to the stimulus for an individual participant in the resistance-training group before (pre) and after training (post) for the two methods of stimulation.

Figure 3. Mean ratios of MEP amplitude divided by the absolute torque versus target torque

Mean ratios of MEP amplitude divided by the absolute torque exerted from 45 to 5 ms prior to the stimulus at each target torque level before and after training for the two methods of stimulation and the two training groups. UT, unresisted-training group; RT, resistance-training group. * Significant difference pre-training versus post-training (P < 0.05).

Figure 4. Effect of training on mean MEP amplitude at each target torque level

Mean MEP amplitude at each target torque level before and after training for the two methods of stimulation and the two training groups. * Significant difference pre-training versus post-training (P < 0.05).