Impact of different mechanical and metabolic stimuli on the temporal dynamics of muscle strength adaptation

A fundamental task in exercise physiology is to determine and ultimately improve the adaptations that take place in the human body, an integrated network of various physiological systems e.g., muscle, tendon and bone. Investigating the temporal dynamics (time-course) of adaptations in these diverse systems may help us gain new knowledge about the functioning of the neuro-motor system in healthy and pathological conditions. The aim of this review was to explore the temporal dynamics of muscular strength adaptations in studies implementing a resistance training intervention. In addition, we categorised these studies under mechanical or metabolic stimuli to identify whether certain stimuli cause faster muscle strength gains. Searches were performed using PubMed and Google Scholar databases. The review comprised 708 participants from 57 training groups within 40 studies that met the inclusion criteria. The results revealed that the mean time point of first significant increase in muscle strength of all studies was 4.3 weeks and the corresponding increase was on average about 17%. A plateau in muscle strength increase (~25%) was found to occur between weeks 8 and 12. Categorisation into stimuli groups revealed that performing training in a hypoxic environment is likely to produce a leftward shift (~25% increase at ~2.8 weeks) in the dose-response relationship compared to blood flow restriction and supplementation. However, stimuli that cause faster muscle strength gains may also induce imbalanced adaptation between the muscle and the surrounding biological structures potentially triggering a degradation in some parts of the network (i.e., leading to an increased risk of injury).