When it comes to building muscle—or any fitness routine, for that matter—most people assume that more is more. More sets, more reps, more time in the gym must mean more gains. Right? Not necessarily.

As people age, muscles naturally lose mass and strength, a condition known as sarcopenia. The decline can make everyday activities harder and increases the risk of falls, disability and early death.

A research team has found that specific immune cells can connect with muscle fibers in a lightning-fast, neuron-like way to promote healing. These cells deliver quick pulses of calcium, triggering ...