Weight Training Beneficial for All, Affects Aged Muscles Differently

The Journal of Strength and Conditioning Research,

Differences in muscle responses to exercise in older versus younger men reflect differences on the hormonal, molecular, and gene-expression level, reports a study in the January issue of official research journal of the National Strength and Conditioning Association.

The differences include genetic up-regulation of a key enzyme related to muscle breakdown in older men. The results help in understanding the "molecular control points" for aging-related muscle atrophy—and may lead to new approaches to mitigating the adverse effects of muscle wasting in older adults. The senior author was Chad M. Kerskick, PhD, CSCS, ATC, NSCA-CPT, of University of Oklahoma, Norman.

Older men show different responses to resistance exercise

The researchers compared responses to resistance exercise (weightlifting) in a group of older and younger men—average ages 21 and 68 years. Before and after the men performed a series of exercises (squat, leg press, and leg extension exercises), blood samples were obtained to measure key hormones involved in muscle responses to exercise.

In addition, samples of muscle tissue (biopsies) were obtained to measure gene expression of two specific enzymes—called atrogin-1 and MuRF-1—involved with the process of skeletal muscle breakdown. Recent studies have suggested that these enzymes reflect age-related differences in muscle metabolism and muscle response to exercise.

As expected, the younger men had higher levels of the hormones cortisol and insulin-like growth factor-1 (IGF-1), before and after exercise. In both age groups, cortisol levels increased significantly five minutes after exercise. "Cortisol increases when the body is stressed and speeds up muscle breakdown whereas IGF-1 increases are associated with increased growth of cells and tissues in the human body," Dr. Kerskick explains.

In addition, 24 hours after exercise, the level of IGF-1 had increased in older men. Recent studies have suggested that IGF-1 may reduce degradation of skeletal muscle protein.

Before exercise, the older men had higher expression of the MuRF-1 gene, which has been linked to age-related muscle atrophy (wasting). There was no age-related difference in expression of atrogin-1. Changes in MuRF-1 and atrogin-1 after exercise did not differ significantly between the younger and older men. However, men with higher expression of MuRF-1 had lower levels of IGF-1.

Muscle mass and strength decrease with aging, which is thought to be related to reductions in the levels of anabolic hormones and growth factors. Muscle growth response to exercise (hypertrophy) is also decreased with aging. Recent studies have tried to identify the molecular-level changes in muscle protein degradation contributing to these aging-related differences in exercise response.

The new results help to clarify some of the differences between younger and older muscle. The increased expression of MuRF-1 in older men could be a precursor to muscle atrophy, or may possibly represent an adaptive change to help maintain muscle mass.

In addition to advancing scientific understanding of the effects of aging, the results may have practical applications—if they can inform the development of exercise regimens to maximize muscle response to exercise in older people.

Dr. Kerskick and co-authors write, "Additional research into the molecular control points for muscle atrophy is important and can help elucidate modifications to resistance exercise training to optimize results and ultimately helping fitness professionals and clinicians better understand muscle physiology with exercise and advancing age."

Source: Wolters Kluwer Health: Lippincott Williams & Wilkins