High Threshold Motor Units Are Eventually Recruited During Low Force Efforts to Volitional Fatigue

Abstract

There is disagreement in the literature regarding the magnitude of relative loads necessary to optimize muscle hypertrophy. Most recommend loads of 60-80% of 1 RM, while others demonstrate similar increases with loads as low as 30% 1 RM, as long as volitional fatigue is reached. PURPOSE: To use a deterministic model of individual motor unit and whole muscle fatigue to evaluate MU recruitment during low and high relative force efforts to volitional fatigue. METHODS: Our deterministic model predicts the forces and force capacities of 120 motor units, and the whole muscle, for the full range of excitations. We estimated: relative force, excitation threshold, initial firing rate, maximum firing rate and excitation adaptation of each MU. During sustained, constant force contractions, we calculated the required excitation level to meet the force demand, and monitored the instantaneous levels of each variable listed above, to determine force and force capacity changes in each active MU and the whole muscle over 0.1 s intervals. The model provided very good estimates of endurance times for intensities ranging from 15% to 90% MVC, compared to published values. RESULTS: The model predicted endurance times of 509.0 s and 14.4 s at 20% and 80% MVC, respectively, at which time all MUs were recruited for both intensities. The 20% MVC condition resulted in larger relative decreases in strength (i.e. ‘fatigue’) for all 120 MUs. At the endurance time for the 20% condition, fatigue was 6.4%, 81.8%, 7.4% and 54.3% for the lowest MU (#1), middle MU (#60), and highest threshold MU (#120) and average MU, respectively. In contrast, at the endurance time for the 80% condition, fatigue was 0.2%, 2.3%, 3.8% and 4.3% for MUs 1, 60, and 120, and average MU, respectively. Excitation adaptations resulted in decreases in firing rates for all motor units, relative to initial maximum rested values. CONCLUSION: The 20% MVC endurance trials resulted in the eventual recruitment of all MUs and > 10-fold greater average fatigue for the MU population compared to the 80% MVC condition. Therefore, low intensity contractions, sustained to volitional fatigue, may provide more challenge to the motor unit population, and a more profound overall stimulus for muscle hypertrophy, compared to high intensity contractions.