Modelling Skeletal Muscle Motor Unit Recruitment Contributions to Contractile Function: Part 3 - Substrate Oxidation of Phosphagen, Lipid, and Carbohydrate Metabolism

Abstract

This study aimed to apply and expand a prior model of motor unit recruitment of the vastus lateralis muscle (VL) to explore theoretical motor unit-specific substrate oxidation. The model utilized repeated contractions of varied frequency and fraction of motor unit recruitment, and four different genetic expressions of motor unit proportions. The study applied prior modelled data of the vastus lateralis (VL) motor unit contractile power and turnover of adenosine triphosphate (ATPto) based on non-linear functions of estimated percentage contributions of different energy systems across various muscle fibre types and contraction frequencies. Using LabVIEW™ programming, the model then used the prior data of ATPto and known ATPto coefficients for substrate oxidation for the energy systems of phosphagen, glycolytic, and mitochondrial respiration from fatty acid and carbohydrate to calculate total and fibre type (motor unit) specific creatine phosphate catabolism, glycogenolysis, glycolytic glucose oxidation, fatty acid oxidation in mitochondrial respiration, glucose oxidation in mitochondrial respiration, and lactate production. Results revealed that for the phosphagen system, substrate turnover was far larger than research-based expressions of decreasing concentrations of creatine phosphate and ATP. This is to be expected for modelled research involving temporal summation of metabolism. Creatine phosphate is continually broken down and partially replenished during low intensity exercise, with such partial replenishment sustained during more intense exercise thanks to the creatine kinase shuttle. For carbohydrate oxidation, mitochondrial respiration accounted for greatest substrate oxidation in type I, and I-IIa motor units, where glycolysis accounted for most substrate oxidation in type IIa, IIab and IIb motor units. Fatty acid oxidation was larger for lower motor unit recruitment conditions, and highest for type I-IIa and IIa motor units. This result is logical based on the larger net muscle fibre recruitment for contraction conditions necessitating progression to fast twitch motor units causing higher substrate oxidation. Such findings reinforce the need for more research on fibre type specific substrate oxidation during different exercise intensities and durations.