Author(s): COLOSIO, A., D’HOE, B., BOURGOIS, J., BOONE, J., Institution: UNIVERSITY OF GHENT, Country: BELGIUM, Abstract-ID: 1908

INTRODUCTION:
Low-load resistance training (RT, loads <50 % the 1 repetition maximum (RM)) performed to muscle failure is an emerging form of RT for populations that cannot train with high external loads due to practical or health constraints. However, uncertainty is still present regarding the minimum load threshold to perform this form of training, the acute muscle and whole-body responses to RT performed at different %1RM, and the training stimulus and fatigue accumulated when exercising with different external loads. Accordingly, this study was aimed at gaining insight into these aspects of exercise prescription across different %1RM.
METHODS:
12 participants (6 women, RT experience >2 months, 21.8±1.8 years, 70.2±10.8 kg, 177.0±9.7 cm) performed a 1RM test and 5 exhaustive protocols of unilateral leg extension (at 10, 30, 50, 70, 90 %1RM). During the leg extension protocols, local muscle metabolic perturbation and muscle excitation were measured at the vastus lateralis (respectively with near-infrared spectroscopy (NIRS) and electromyography (EMG)), vastus medialis and rectus femoris (EMG only). Heart rate, blood lactate accumulation and rate of perceived exertion were measured as indicative of whole-body responses, while electrically stimulated maximal voluntary contractions (MVC) were executed before and up to 30 min after each protocol to determine fatigue accumulation and recovery. Responses between different %1RM were compared using 1-way repeated measures and Friedman ANOVAs, followed by post-hoc analyses. Spearman’s correlations were calculated between fatigue accumulation and the main physiological variables.
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 (in 30 min) and presented lower levels of metabolic perturbation. Muscle excitation upon exhaustion increased with increasing external loads and did not lead to common EMG signals between %1RM. All protocols induced fatigue and significant drops in MVC. There was no evident difference in the changes in voluntary activation between %1RM. Moderate correlations were found in fatigue accumulation and parameters of metabolic perturbation, while fatigue accumulation and recovery were moderately correlated with the total kg lifted. Finally, post-hoc modelling of participants critical load (CL, the boundary between sustainable and unstainable exercise), detected this important metabolic threshold at 31.7 ± 11.9 %1RM.
CONCLUSION:
Our findings highlight a common level of metabolic perturbation with loads of ≥30 %1RM, contrasting with lower loads. %1RM seem an important determinant of maximal muscle excitation levels upon muscle failure, and we potentially identified the minimum RT load threshold in coincidence with the exercises CL. Furthermore, we observed a relationship between fatigue, volume of training and metabolic perturbation which might have important implications when designing low-load RT.