Influence Of Autoregulatory Progressive Resistance Exercise On Motor Unit Firing Patterns

An autoregulatory progressive resistance exercise (APRE) training regimen adjusts intensity and volume according to an individual's daily performance and levels of fatigue. This training style may lead to distinct effects on neuromuscular adaptations due to the consistent demand of optimized motor unit recruitment during high-intensity exercises. PURPOSE: The purpose of this study was to examine the effects of a 4-week APRE program on motor unit firing patterns in resistance-trained men. METHODS: Seven resistance trained males (age: 21 ± 2.3 yrs; height: 176.5 ± 6.8 cm; weight: 70.2 ± 17.8 kg) participated in a 4-week APRE program. All participants completed pre and post testing sessions comprised of 3 knee extension maximal voluntary contractions (MVC), a trapezoidal ramp tracing at 85% of their peak force, and a knee extension at 85% of their peak torque. Surface electromyography was used to record muscle activation of the vastus lateralis (VL). Signals were later decomposed into their constituent motor unit action potential (MUAP) trains and assessed for relative behavioral properties. For subsequent analysis of firing behaviors, the slopes and y-intercepts were calculated between motor unit action potential size (MUAPsize) and mean firing rate (MFR). Paired samples t-tests were used to compare the pre and post measurements for the slopes and y-intercepts of MFR vs MUAPsize) at 85% of their MVC. RESULTS: There was a significant decrease in the MFR vs MUAPsize slope from pre to post (p = 0.044; -41.37 ± 6.32 to -57.26 ± 11.65; mean ± SE). Additionally, there was a significant increase in the MFR vs MUAPsize intercept from pre to post (p = 0.013; 9.29 ± 0.68 to 10.41 ± 0.51; mean ± SE). CONCLUSIONS: These results suggest that high intensity APRE training elicits motor unit firing adaptations in previously resistance trained individuals. The decrease in MFR vs. MUAPsize slopes suggests that the motor unit pool consisted of smaller motor units following training; this may indicate increased synchronization or enhanced rate coding of lower-threshold motor units in order to achieve the same relative force target.