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Aerobic exercise pre-conditions muscles for optimal returns from resistance exercise.

Skeletal muscle contains a designated population of adult stem cells called satellite cells. These cells are typically inactive, but if the muscle is injured or stressed (as in exercise), they can be recruited to participate in the regeneration of muscle fibers. As such, satellite cells play important roles in muscle maintenance, repair, and hypertrophy, the increase in muscle size that accompanies exercise. Findings from a new study suggest that engaging in aerobic exercise prior to resistance training increases satellite cell numbers and promotes muscle hypertrophy via increased muscle capillarization.

Muscle capillarization refers to the formation of capillaries in muscle tissue. Capillarization facilitates the delivery of oxygen, nutrients, and various signaling and growth factors to muscle tissues and plays critical roles in muscle maintenance and growth. Previous research indicates that muscle capillarization decreases with age.

The study involved 14 healthy, recreationally active young adults (average age, 22 years). Using a specially adapted exercise bike that challenged only one leg, participants engaged in 45 minutes of progressively difficult aerobic exercise conditioning three times a week for six weeks. Previous research has demonstrated that six to eight weeks of conditioning is sufficient to promote muscle capillarization.

Two weeks after completing the conditioning program, the participants began a 10-week resistance training program using both legs and primarily targeting the muscles of the thighs. Immediately after each resistance training session, participants received a whey protein supplement that contained leucine, a branched-chain amino acid that promotes muscle protein synthesis, which is essential for muscle gains. The investigators collected muscle tissue samples from the participants' legs before and after the interventions to assess muscle capillarization, fiber size, and satellite cell content and activity.

They found that aerobic conditioning promoted muscle capillarization in the conditioned leg, amplifying muscle hypertrophy in response to resistance training. They noted that the number of satellite cells increased in the conditioned leg relative to the non-conditioned one. In addition, they observed a significant relationship between the degree of capillarization and hypertrophy.

These findings suggest that engaging in aerobic exercise prior to resistance training promotes muscle capillarization, which in turn increases satellite cell numbers and promotes muscle protein synthesis and hypertrophy. Learn about other factors that promote muscle hypertrophy in this episode featuring Dr. Stuart Phillips.

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