It's the age-old body-building question: Many reps of light weights, or fewer reps of heavier weights? Both camps have their adherents, as well they might, since researchers have found evidence that lifting more repetitions of lighter weight can build muscle mass just as well as fewer reps of heavier weight. But when it comes to strength training, there's no question that heavier weights yield greater gains.

Why is that? Why does strength increase with heavier weights, even when muscle mass does not?

Scientists believe they may finally have the answer, and it's out of left field: Training with heavier weights conditions our nervous system to transmit electrical signals from the brain to muscles, increasing the force those muscles can produce to a greater extent than does training with lighter weights.

In short: Between two men of comparable muscle mass, the one who trained using heavier weights will be stronger because of his nerves, and not something inherent in his own musculature.

The study, out of Nebraska University, randomly assigned 26 men to train for six weeks on a leg-extension machine loaded with either 80 or 30 percent of the maximum weight they could lift. The participants worked out three times a week, lifting until they could lift no more.

The scientists also supplied an electric current to the nerve that stimulates the muscles used in leg extensions. By comparing the force of a participant's "hardest" unassisted kick with the maximum force they can generate when aided by electric current, scientists can determine how much of that capacity a person has reached - a measure known as voluntary activation.

The researchers noted that the low-weight group increased 0.15 percent over a three-week span. The high-weight saw a rise of 2.35 percent.

What's the science? Muscles contract when they receive electrical signals from the brain. Those signals travel from the cortex to the spinal tract, speeding through the spine while jumping to other motor neurons that then excite muscle fibers. The Nebraska team discerned that the nervous system excites those neurons them more frequently when it experiences high-load training. It is likely that this increased excitation accounts for the greater strength gains despite comparable growth in muscle mass.

What does this mean for the serious bodybuilder? Less time in the gym! High-load training has proven itself a more efficient strength-training regimen. Note, however, that many cardio routines now incorporate low-weight lifting, underlining the fact that heavy lifting may be the key to building strength, but may not necessarily be the best exercise overall.

The study has been published in Frontiers of Physiology.