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POPPING a pill could one day give people the benefits of exercise without them moving a muscle. So say scientists who have shown that activating a certain enzyme may make your muscles behave as though you鈥檝e been working out.

So far the experiments have only been done in mice. But if the same applies to people, the result could improve the lives of chronically ill patients who need exercise but can鈥檛 tolerate exertion. More contentiously, it might also help athletes improve their endurance.

R. Sanders Williams of Duke University in Durham, North Carolina, and his colleagues made the discovery when they genetically engineered mice to make high levels of a continuously active form of an enzyme called calcium/calmodulin-dependent protein kinase (CaMK) in their muscles. Even though the animals had not exercised, in many ways their muscles behaved as if they had.

For example, many of their muscle fibres converted to the 鈥渟low-twitch鈥� kind, which can sustain light loads for a long time. Their muscles were better at resisting fatigue, and the muscle cells sprouted large numbers of mitochondria, the tiny energy factories of cells鈥攁nother hallmark of sustained exercise.

Why these changes happen is a mystery, but according to exercise biology expert David Hood from York University in Toronto, the new work suggests an answer. 鈥淓very time a muscle contracts, calcium is released and elevated in the cell,鈥� says Hood. It may be that calcium, possibly along with other molecules, triggers the medley of exercise responses by activating CaMK.

This opens the doors to possible new drugs, says Williams. Drugs that inhibit CaMK already exist, so finding one that activates it to mimic exercise might not be difficult.