The biochemical basis of muscle function during stress imposed by isokinetic exercise has mainly focused on measuring muscle proteins (CK and myoglobin) and lactate present in the blood plasma, whereas the range of low-molecular weight metabolites during muscular stress has received limited attention. This study investigated the effect of concentric isokinetic exercise of the knee extensor and flexor muscles (functional to standing, walking and running), at 80% and 40%MVC intensities of equal workloads, on saliva, urine and blood serum matrices. A hypothesis-free approach was employed to mathematically determine, by chemometrics, which untargeted metabolites measured by 1H NMR spectroscopy are altered in response to muscular exercise of this nature.
The results demonstrate that the saliva matrix is highly susceptible to 1H NMR spectral interference from salivette exposure (a routine method of saliva collection in sports/exercise) with 1H resonances arising throughout the spectrum, exceeding the affected spectral region of δ 3-4 documented in the literature. Short-chain organic acids (lactate, acetate, n-butyrate and formate) and N-acetyl sugars were the strongest discriminators of saliva sampled pre- and post-exercise. However, the origin of these metabolites is equivocal since bacterial metabolism and salivary gland secretion via exercise stimulation of the autonomic nervous system both contribute towards salivary composition. Urinary citrate, glycine and hippurate exhibited a decrease following isokinetic exercise, whereas trimethylamine N-oxide increased. According to the literature, this pattern has previously been associated with kidney stress on the renal papilla and tubules from the filtration of myoglobin protein from the blood which accumulates as a result of leakage from the muscle membrane during stress or injury. Thus, these urinary biomolecular markers may reflect renal filtration of muscle proteins present in the blood following exercise and may be indirectly representative of isokinetic exercise-induced muscular stress. The serum matrix was less sensitive to exercise-induced change than urine in this isokinetic intervention study, since the homeostasis of blood is maintained by the renal system.
Further work is needed to cross-validate conventional assays used to measure biomarkers of muscular stress with the metabolomics platform to confirm that urinary trimethylamine N-oxide, citrate, glycine and hippurate are indirect biomolecular markers of muscular exercise. In sports medicine, NMR-urinalysis may provide diagnostic and prognostic information on injured and recovering athletes to determine whether or not they are fit to play, and predict risk of injury based on their current state of health.