Internal Muscle Friction

The force input of a muscle is the SAME while lifting, lowering or holding a weight provided the muscle moves at a constant speed - any speed, slow or fast.

If you lift a 100-pound weight at a constant speed, the muscle must exert a force equal to that of the weight to keep it moving at that speed. Less exertion results in a slower speed; greater exertion (muscle input) creates acceleration.

A force input of 100 pounds is required to hold 100 pounds in a static position. Less input causes the weight to descend; greater input, to ascend.

Similarly, a muscle input of 100 pounds is required to lower a 100-pound weight at a constant speed. Less input during the descent accelerates the weight; greater input slows the speed.

Therefore, if force input is exactly the same lifting, lowering and holding, why does output differ? Why is it more difficult to lift than lower? Nautilus inventor Arthur Jones speculated it was more than gravity. He called it "internal muscle friction," friction within the musle fibers as they slide back and forth, interacting with neighbors. He had no way to prove it.

Seventeen years later he completed a machine that could. Fresh muscles, he found, were 40% stronger lowering than lifting, few exceptions. Things changed with exercise. During the first submaximal repetitions of a set, lifting and lowering strength declined at a similar rate - a loss of approximately 2% per repetition. As the set increased in difficulty, lifting strength declined rapidly, at a rate that far exceeded that of lowering strength. When repetitions became maximum efforts, lowering strength began to increase for one reason -friction within the muscle had accumulated to a level that began to affect output. Not only did trainees have to lift resistance with tired muscles, they had to overcome the accumulating friction within the muscle (many thought more energy was expended with the friction than the weight). Friction hinders lifting, but helps the lowering phase - if its accumulation is adequate. During high-intensity training, it becomes adequate.

In one case study conducted by Jones, a highly-trained individual whose quadriceps (front thigh) lifting strength had been reduced by 98% (a high and painful level) on a leg extension exercise, demonstrated a corresponding 14% reduction in lowering strength - strength that dove initially, then climbed, bouyed by an accumulation of internal muscle friction.

You can always lower more than you can lift, especially when a muscle is fatigued. Find a way to continue when you can no longer lift (using negative-only, negative-emphasis or negative-accentuated work) to take muscles to a higher level of stimulation.

Friction can be an ally when it comes to muscle growth.