Introduction: Strength gains in the first weeks of strength training are assumed to be due to neural adaptation, i.e. adaptive changes within the nervous system in response to strength training (Sale, 1988). Direct evidence of supraspinal adaptations has been provided by Electroencephalogram (EEG) signal processing of the movement-related cortical potentials (MRCP) and the P300 peak. Attenuation of MRCP amplitudes secondary to training was found, reflecting improved force generating capacity of individual motor units (Falvo et al., 2010; Jochumsen et al., 2017). Besides, P300 amplitude increases were found to take place following motor training, reflecting a progressive reduction in attentional demands imposed by the motor task (Hu et al., 2015; Rietschel et al., 2014; Zhao et al., 2014). This adaptation process was referred to as neural adaptations, however, might be the result of motor skill acquisition. Since research showed that motor training without skill acquisition is insufficient to induce changes in corticospinal excitability or P300 amplitudes (Jensen et al., 2005; Rietschel et al., 2014), this EEG study compared supraspinal adaptations to a training period of a constrained versus free exercise, to investigate if neural adaptations could be regarded as motor learning.
Method: A small sample of university students (4 males, 3 females) performed two strength training conditions (bicep curls) each consistent of six heavy resistance training sessions (70-80% RM) over a 2-week period (total experimental duration is 4 weeks). One condition, the free condition, consisted of a free movement requiring much coordination, while the other condition, the constrained condition, consisted of a constrained movement limiting the demand for coordination. EEG recordings and one repetition maximum tests were conducted at day 1, 14 and 28. Primary outcome measures of the study were the MRCP (CNV, N200), and P300 amplitudes (of EEG electrodes Cz, C1, C2). The secondary outcome measure was the gain in strength.
Results & Discussion: In the constrained condition, with solely one movement trajectory to be practiced, coordination demands were limited, which possibly resulted in motor learning of other motor control parameters, since an increase in supraspinal activation at movement onset (increased N200) was measured. The cognitive demand (P300) remained unchanged. The free condition movement required more coordination (many possible movement trajectories) and a secondary task increased complexity, resulting in a longer motor learning process and less strength gain compared to the constrained condition. Results indicate an initial phase of motor learning in this condition, but different learning from the constrained condition, as they suggest higher usage of cognitive neural resources, reflected by P300 amplitude decreases. However, no measurable skill acquisition seems to have been elicited by the free condition, since there was no decrease in motor unit recruitment, as the N200 amplitudes remained unchanged. CNV amplitudes remained unchanged following both conditions.
Conclusion: The present study provides evidence that the early phase of strength training of a free versus constrained movement induce different supraspinal adaptations. It revealed that the adaptation process in response to strength training, up to now discussed as neural adaptations, is in fact a motor learning process.