Adaptive changes in work capacity, skeletal muscle capillarization and enzyme levels during training and detraining

Area of Science:

• Exercise Physiology
• Muscle Biology
• Sports Science

Background:

• Endurance training enhances oxygen transport and utilization in skeletal muscle.
• Muscle capillarization and oxidative enzyme activity are key adaptations to exercise.
• The reversibility of these adaptations during detraining requires further investigation.

Purpose of the Study:

• To investigate the effects of unilateral and bilateral leg training on maximal oxygen uptake (VO2 max).
• To examine the impact of training and subsequent detraining on muscle capillarization and oxidative enzyme activity.
• To determine the time course of adaptation and de-adaptation in skeletal muscle.

Main Methods:

• Six male subjects underwent 8 weeks of unilateral leg cycling training.
• Muscle biopsies were analyzed for capillary density (capillaries per mm², capillaries per fiber, capillaries around fiber type), fiber area, and enzyme activities.
• VO2 max was measured before, during, and after training and an 8-week detraining period.

Main Results:

• Training increased VO2 max by 14.6% (two-leg) and 23.1% (one-leg).
• Muscle biopsies showed a ~20% increase in capillaries per fiber and capillaries around fiber type, with minor fiber area increase.
• Detraining led to a decrease in capillaries per fiber and capillaries around fiber type, while capillaries per mm² remained elevated.

Conclusions:

• Training-induced improvements in oxidative capacity and muscle capillarization are largely reversible within 8 weeks of detraining.
• Capillaries per fiber and capillaries around fiber type adaptations are sensitive to detraining.
• Sustained benefits in capillaries per mm² and fiber area per capillary suggest potential long-term improvements in diffusion distance.