Post-Activation Potentiation and Potentiated Motor Unit Activation Patterns in Boys and Men

Abstract

Post-activation potentiation (PAP) describes the enhancement of twitch torque following a conditioning contraction in skeletal muscle. In adults, PAP is greater in muscles with a higher percentage of type-II fibres and PAP-related augmented contractility is accompanied by a decrease in motor unit (MU) firing rates (MUFRs). Children may have lower PAP due to lower type-II muscle fibre composition and lower activation of their higher-threshold (type-II) MUs compared to adults. Changes in potentiated MU activation have not been examined in children. Therefore, the purpose of this study was to determine whether there are child-adult differences in PAP and MU activation patterns of the potentiated knee extensors. Twenty-three boys (10.5±1.3 years) and 20 men (23.1±3.3 years) completed familiarization and experimental sessions. In the experimental session, knee extensor maximal volitional contraction (MVC) torque (Biodex System 3) was first determined. Maximal isometric evoked-twitch torque (Digitimer stimulator model DS7AH) and MU activation patterns during submaximal contractions (20% and 70% MVC) were then recorded before and after a conditioning contraction (5s MVC). PAP was calculated as the percent-increase in evoked-twitch torque after the conditioning contractions. MU activation patterns were examined during submaximal contractions before and after a 5s MVC, using Trigno Galileo surface electrodes (Delsys Inc) and decomposed into individual MU action potential (MUAP) trains (NeuroMap, Delsys Inc). PAP was higher in men than in boys (98.3±37.1% vs. 68.8±18.3%, respectively; p=0.002). In both the 20% MVC and 70% MVC contractions, boys and men displayed a reduction in MUFRs for a given MU size (MUAPamp), specifically in the higher-threshold MUs. This reduction was greater in the boys than the men in the 70% MVC contractions, and may be due to their greater fatigue resistance, as well as to a lower impact of potentiation at higher intensities.