Minimum load threshold in resistance training: insights into muscle metabolism, excitation, and fatigue across the repetition continuum.

Background: High external loads are typically prioritised in resistance training (RT), but low-load RT (<50% 1 repetition maximum (RM)) has gained attention for its practical and physiological advantages. While mechanical tension and metabolic perturbation are key drivers of hypertrophic adaptations, the mechanisms underlying effective thresholds for low-load RT remain unclear. Purpose: To investigate the physiological responses, fatigue, and recovery during concentric-only RT performed to failure across a spectrum of % 1RM, with particular focus on the potential relevance of the critical load (CL) concept as a physiological boundary. Methods: A total of 12 participants (six women, moderate RT experience) performed exhaustive unilateral leg extension at 10%, 30%, 50%, 70%, 90% 1RM. Muscle deoxyhaemoglobin and muscle excitation were measured respectively with near-infrared spectroscopy and electromyography (EMG). Heart rate, blood lactate, and rate of perceived exertion were measured as indicative of whole-body responses. Maximal voluntary contractions (MVC) were executed before and up to 30 min after each protocol. CL was calculated based on the load used and the time required to reach muscle failure. Responses between different % 1RM were compared using 1-way and 2-way repeated measures analyses of variance, followed by Results: The 30%, 50%, 70%, 90% 1RM protocols induced muscle failure and similar levels of local and whole-body metabolic perturbation, while the 10% did not lead to failure and induced lower metabolic perturbation. Muscle excitation upon exhaustion increased with increasing external loads and did not lead to common EMG levels between % 1RM. A total of 30% and 50% 1RM protocols caused significant MVC reduction Conclusion: The characterisations of the acute physiological responses to RT across light to heavy loads performed in this study provide potential insight for determining the minimal load threshold in RT and suggest a potential proximity between the CL and the load associated with blood flow occlusion during contraction.