Neurological Adaptations Account for Late Late Phase Improvements in Muscular Fitness
Every undergrad ex phys student can probably recite from memory the time-course changes in muscular fitness as they relate to neurological adaptations (early phase...0-6 months) and morphological changes in muscle tissue (late phase...beyond 6 months). In the early stages of training before increases in muscle size occur, rapid improvements in strength and power are attributed to improved neuromuscular performance with adaptations broadly grouped into improvements in muscle tissue activation and motor unit synchronization. As training age increases, with consistent resistance training, muscle cross-section area increases and contractile properties improve resulting in enhance force generating capacity. Very little data exists examining changes in performance and physiology many years into development (Late late phase...2 years or longer). Such a longitudinal study would be difficult to conduct but data we have collected in recent years working in college football may give us some insight into late late phase improvements.
Data from three years of off-season improvements in strength and power among college football juniors and seniors was evaluated. Juniors and Seniors were selected to remove early and late phase developers from the consideration. Most, if not all, Jrs and Srs would have completed at least 2 full years of consistent training at a relatively high level. Most would have completed more than that due to the greater prevalence of good S&C at the high school level in the United States. Averaging improvements in strength (1RM) and power (peak watts) in squats, bench press, push press, powercleans, and hang cleans as well as examining changes in lean body mass (Bod Pod and DEXA) allow us to roughly evaluate driving factors in muscular fitness changes in this population.
Data from 126 players was evaluated. On average, players gained 2.3 (+/- 1.7) lbs of lean body mass per year. Improvements in strength were found to be 26.1% (+/-6.3%) across one off-season and power improved 46.5% (+/-9.7%). The relatively low increase in muscle mass is not surprising as our training system primarily targets power using moderate- to moderate-heavy loads at moderate to fast speeds and fairly low training volume; only 10% of our total training volume is designed to strictly improve strength. Yet, strength improved quite substationaly (Effect size: 2.75). The observed power improvements (Effect size: 4.32) are extremely high for late late development phase athletes. Thus we see substantial improvements in muscular fitness with very little change in muscle mass. From this it is reasonable to suggest that neurological changes account for the bulk of muscular fitness improvements among these athletes.
Based on these data I suggest that both early phase and late late phase improvements in muscular fitness are driven by improvements in neurological performance, with morphological changes being the predominant driver in late phase development. I am currently collecting and examining EMG data that may demonstrate a more direct link between the two but in the meantime, we should acknowledge a greater role of the nervous system in muscular fitness performance and work to target such adaptations with highly trained athletes.
Matt Rhea, PhD