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

The 10% 1RM protocol failed to induce muscle failure and produced lower metabolic perturbation compared to higher loads.

• Participants performed exhaustive unilateral leg extension at 10%, 30%, 50%, 70%, and 90% 1RM
• Only the 10% 1RM condition did not lead to muscle failure within the protocol
• The 10% 1RM induced lower local and whole-body metabolic perturbation compared to all other loads
• The 30%, 50%, 70%, and 90% 1RM protocols all induced muscle failure

The critical load (CL) was detected at approximately 31.7% 1RM.

• CL was calculated based on the load used and the time required to reach muscle failure
• CL was identified at 31.7 ± 11.9% 1RM across the 12 participants
• The authors suggest a potential proximity between the CL and the load associated with blood flow occlusion during contraction
• CL represents a physiological boundary separating sustainable from unsustainable exercise intensities

Muscle excitation (EMG) upon exhaustion increased with increasing external loads and did not converge to common EMG levels across % 1RM conditions.

• Electromyography was used to measure muscle excitation throughout each protocol
• Higher % 1RM loads were associated with greater EMG levels at the point of exhaustion
• EMG responses at exhaustion did not reach a common endpoint across the different % 1RM conditions
• This finding challenges the notion that all loads performed to failure produce equivalent neuromuscular states at exhaustion

The 30% and 50% 1RM protocols caused significant maximal voluntary contraction (MVC) reductions compared to baseline, indicating neuromuscular fatigue.

• MVCs were assessed before and up to 30 minutes after each protocol
• Significant MVC reductions versus baseline were observed after 30% and 50% 1RM protocols
• Fatigue was moderately correlated with metabolic markers across conditions
• The 70% and 90% conditions also caused fatigue but the specific statistical contrast versus baseline was highlighted for 30% and 50%

The 30%, 50%, 70%, and 90% 1RM protocols induced similar levels of local and whole-body metabolic perturbation when performed to failure.

• Muscle deoxyhaemoglobin was measured with near-infrared spectroscopy as an indicator of local muscle metabolism
• Blood lactate, heart rate, and rate of perceived exertion were measured as indicators of whole-body responses
• Similar metabolic perturbation across 30–90% 1RM suggests that performing sets to failure equalizes metabolic stress across a wide range of loads
• The 10% 1RM condition was the exception, producing lower metabolic perturbation

Fatigue accumulation was moderately correlated with metabolic markers across the repetition continuum.

• Repeated-measure correlations were calculated between fatigue accumulation and the main physiological variables
• The correlation between fatigue (MVC reduction) and metabolic markers was described as moderate
• This suggests metabolic perturbation is a contributing but not exclusive driver of fatigue during resistance training to failure
• The study included 12 participants (six women) with moderate resistance training experience

A minimum load threshold for resistance training may exist near the critical load, which approximates the load associated with blood flow occlusion during contraction.

• The CL concept, borrowed from endurance exercise physiology, was applied to resistance training as a physiological boundary
• The detected CL of ~31.7% 1RM is proposed as a potential minimum effective load threshold for RT
• The authors suggest proximity between CL and the load at which intramuscular pressure occludes blood flow during contraction
• Loads below the CL (e.g., 10% 1RM) did not produce muscle failure or sufficient metabolic perturbation, potentially limiting hypertrophic stimuli