Exercise induced increases in muscle fiber number.

Area of Science:

• Exercise Physiology
• Muscle Biology
• Skeletal Muscle Adaptations

Background:

• Muscle enlargement (hypertrophy) is a known adaptation to resistance training.
• The effect of such training on the total number of muscle fibers (hyperplasia) remains a subject of investigation.
• Previous studies suggest that prolonged exercise may increase muscle fiber count.

Purpose of the Study:

• To investigate the impact of long-term, operantly conditioned weight-lifting exercise on muscle fiber number.
• To determine if induced muscular enlargement through resistance training leads to an increase in the total number of muscle fibers.
• To support or refute previous findings on exercise-induced muscle fiber hyperplasia.

Main Methods:

• Six cats were operantly conditioned to perform wrist flexion against increasing resistance for approximately 101 weeks.
• The right limb's flexor carpi radialis muscle served as the exercised group, while the left limb served as the control.
• Post-training, muscle weight and fiber counts of the exercised and control muscles were compared.

Main Results:

• Exercised muscles showed an 11% greater weight compared to control muscles (P < 0.01).
• A 9% increase in muscle fiber number was observed in the exercised muscles (P < 0.02).
• Trained cats achieved an average of 57% of their body weight in "one-arm" lifts.

Conclusions:

• Prolonged, intensive weight-lifting exercise significantly increases the total number of muscle fibers in the trained muscle.
• This study provides further evidence supporting muscle fiber hyperplasia as an adaptation to resistance training.
• The findings confirm that muscle fiber number can increase with sustained, high-resistance exercise protocols.