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Massaging injured muscles may boost healing by clearing immune cells that interfere with tissue regrowth, a study in mice suggests.

The approach may help people with muscle injuries from blood clots or physical damage, says at Harvard University.

Many people use massage to relieve painful muscles after exercise or for minor strains, but it is unknown if this has long-term beneficial effects. Massage is generally thought to improve blood flow and help disperse harmful waste products, yet the exact mechanisms are unclear.

Seo鈥檚 team made a robotic massager that can gently pummel a mouse鈥檚 leg at right angles to the muscle while the limb is cradled in a soft clamp. The device 鈥� similar to a used by human fitness buffs 鈥� was used on animals whose leg muscles had been injured by having the blood flow temporarily cut off and then a toxin injected.

Mice that were given two weeks of twice-daily 鈥渕echanotherapy鈥� could exert about a 50 per cent greater force with their injured leg than those left untreated.

Samples taken from the muscle tissue over the two weeks showed that massage was helping to get rid of immune cells called neutrophils. These normally home in on injured tissues and help to kill bacteria.

The neutrophils seem to interfere with the normal process of muscle regeneration, where immature muscle cells develop into functioning muscle fibres. The researchers found this by growing neutrophils and muscle cells together in a dish.

鈥淭hese neutrophils help the muscle cells to grow at the beginning, but if they stay too long, they prevent muscle cells from making new muscle fibres to replace the damaged ones,鈥� says Seo.