Author(s): SIRAGO, G., LANFRANCHI, C., CARRARD, J., GREMEAUX, V., ZANOU, N., PLACE, N., Institution: UNIVERSITY OF LAUSANNE, Country: SWITZERLAND, Abstract-ID: 1412

INTRODUCTION:
Physical training is usually associated with beneficial adaptations such as increased mitochondrial biogenesis or protein synthesis in skeletal muscle. In situations where the balance between training load and recovery is disrupted, an overtraining syndrome (OTS) may develop. Reported overtraining symptoms include decreased performance and recovery capacity, sleep disturbances, and clear alterations at the skeletal muscle level, such as soreness and increased fatigability. The aim of the present study was to investigate skeletal muscle (mal)adaptations in response to overtraining using a translational approach.
METHODS:
Knee extensor neuromuscular function and vastus lateralis muscle biopsies were obtained from athletes with or without OTS (> 10h training per week). An in vitro model was developed in which electrical stimulation was applied to C2C12 myotubes to mimic endurance simulated training (s-T), one stimulation per day, vs. simulated overtraining (s-OT), three stimulations per day. Oxygen consumption rate was assessed using high-resolution respirometry, while confocal imaging was used to assess mitochondrial localization. Western blot was used to quantify contractile, mitochondrial, and signaling protein levels. Proteomics and metabolomics have been performed on the cellular models, and similar analyses are planned on human muscle samples with a focus on metabolism.
RESULTS:
Overtrained athletes (n=4) showed no differences in basal knee extensor neuromuscular function (e.g., maximal contraction force, maximal voluntary activation level), although the fatigability response appears to differ from healthy controls (n=6). Mitochondrial oxygen consumption rate revealed inefficient respiration, especially in the NS-pathway via complex 2 in human OTS muscle, and this was confirmed in the s-OT compared with s-T. Consistent with these respiratory findings, proteomics analysis revealed a decreased abundance of proteins involved in the mitochondrial respiratory chain in s-OT. Interestingly, mitochondria appeared aggregated in both stained human skinned fibres and s-OT myotubes, a feature previously reported in OTS athletes, suggesting possible impairments in mitochondrial quality control. Metabolomics data highlighted only lactate accumulation in s-OT, compared with significant increases in energy-related metabolites such as glyceraldehyde phosphate or alfa-ketoglutaric acid in s-T, in response to the last simulated exercise session. Only s-T showed positive adaptations, such as increased levels of contractile proteins and myotube hypertrophy. In contrast, s-OT showed myotube atrophy and reduced contractile protein content.
CONCLUSION:
Neuromuscular function may change during OTS, accompanying skeletal muscle changes such as mitochondrial respiratory defects, which we found to be a hallmark of skeletal muscle maladaptations in overtrained athletes. The in vitro model of s-OT provides a molecular platform to further investigate skeletal muscle maladaptations during overtraining.