Modelling Skeletal Muscle Motor Unit Recruitment Contributions To Contractile Function: Part 2 — Total (aerobic + anaerobic) ATP Turnover

Abstract

The purpose of this research was to expand understanding of the energetics of skeletal muscle contractions through modeling the total ATP turnover (totATPto = aerobic + anaerobic) of different muscle fibre types and motor units. Contraction conditions involved varied motor unit recruitment for a single contraction, in addition to 3 min of repeated contractions across varying contraction frequencies (0.5 to 2.0 Hz in 0.5 Hz increments) and fractional motor unit recruitment (0.5 to 0.95 in 0.05 increments). Contractile power data was obtained from prior research of the vastus lateralis and converted to totATPto from the free energy release of ATP hydrolysis (50 kJ×M-1) and the cellular efficiency of free energy transfer from ATP hydrolysis during muscle contraction (40%). A computational model was then developed for totATPto for repeated contractions spanning 0.5 to 2.5 Hz and fractional recruitment of 0.5 to 0.9 across 4 different genetic expressions of slow twitch (ST; Type I and I - IIa) to fast twitch (FT; Type IIa, IIab, IIb) proportionality. Completion of 3 min of repeated contractions for 0.4, 0.65 and 0.9 fractional recruitment for the 2 extremes of motor unit proportions (%ST - FT = 80 - 20 to 20 - 80) for contractions at 1 and 2 Hz resulted in totATPto of 30.28 vs. 55.66 vs. 110.28 mmol·L-1 (0.4,0.65,0.9;80 – 20; 1 Hz); 50.24 vs. 130.51 vs. 275.58 mmol·L-1 (0.4,0.65,0.9;20 – 80; 1 Hz); 60.57 vs. 111.31 vs. 220.55 mmol·L-1 (0.4,0.65,0.9;80 – 20; 2 Hz); 100.49 vs. 261.03 vs. 551.16 mmol·L-1 (0.4,0.65,0.9;20 – 80; 2 Hz). The 0.65 fractional recruitment data most reflect prior in-vivo whole muscle research of totATPto data and reveal the construct validity of the model. Further modelling is needed to improve our understanding of the metabolic significance of motor unit recruitment to skeletal muscle cellular energy transfer during exercise.