Aging protein saps muscle strength.

Grandma and grandpa rarely win a weightlifting contest with their grandchildren. As people get old, they begin losing significant amounts of muscle mass, a deterioration scientists call sarcopenia (SN: 8/10/96, p. 90). Even the muscle that remains in the elderly isn't usually as strong as it once was.

At last month's Integrative Biology of Exercise meeting in Portland, Maine, Dawn A. Lowe of the University of Minnesota in Minneapolis and her colleagues offered a clue to why aged muscle is weaker: In older muscle, a key protein, myosin, is shirking its job.

Muscle contraction is, in essence, molecular motion. Armies of myosin proteins simultaneously pull together countless filaments made of the protein actin. The head of the myosin protein mediates this contraction by changing from a state in which it weakly binds actin to one in which it holds firmly.

While comparing the muscles of young and elderly male rats, Lowe and her colleagues observed that aged muscle had fewer myosin heads in the strong-binding state during a contraction. Young muscle fibers had about 30 percent of the myosin in that state, while the elderly muscle had only about 22 percent.

This may partly explain why aged rat-muscle fibers exert only about 82 percent the force of young muscle. The next step, says Lowe, is to determine why myosin becomes less effective with age.