Muscle-Specific Sensitivity to Voluntary Physical Activity and Detraining.

Area of Science:

• Exercise physiology
• Skeletal muscle biology
• Adaptation and plasticity

Background:

• Aerobic exercise induces skeletal muscle adaptations like myosin heavy chain (MHC) shifts, increased capillary networks, and mitochondrial biogenesis.
• The impact of detraining on these whole-muscle adaptations, particularly across different muscle types, is not well understood.
• Muscle phenotype, training volume, and intensity likely influence adaptation magnitude and reversibility.

Purpose of the Study:

• To investigate the reversibility of exercise-induced adaptations in fast (plantaris) and slow (soleus) muscles following detraining.
• To compare the rate at which adaptations in MHC isoforms, capillarity, and mitochondrial markers return to baseline levels in plantaris versus soleus muscles.
• To determine if muscle-specific responses to detraining are influenced by prior training volume.

Main Methods:

• Comparison of soleus and plantaris muscles from rats subjected to different aerobic training and detraining protocols.
• Analysis of myosin heavy chain (MHC) isoforms to assess fast-to-slow shifts.
• Quantification of capillary density and measurement of mitochondrial oxidative markers.

Main Results:

• Detraining abolished fast-to-slow MHC gains in the plantaris muscle, while soleus muscle showed some influence of training volume on MHC return to basal levels.
• Capillary enhancement was observed in the plantaris across all trained/detrained groups but only in the fully trained soleus group.
• Mitochondrial markers for aerobic capacity increased in the trained plantaris but not in the soleus muscle.

Conclusions:

• The fast plantaris muscle is more sensitive to both aerobic exercise and subsequent detraining than the slow soleus muscle.
• The soleus muscle requires a higher aerobic challenge (intensity/duration) to elicit significant adaptations.
• Muscular aerobic fitness gains from voluntary running, and their loss during detraining, are primarily observed in fast muscle phenotypes.