With isotonic exercise, such as lifting or lowering weights, the muscle group becomes shorter or longer than in its normal resting position. Both positive and negative isotonic exercise result in increases in strength and muscle size. While negative work is more efficient than positive work, the force or stress on tissue such as muscle is much greater. When vigorous, this can produce tearing and breakdown of muscle fibers. The first day after the exercise period, there is degeneration of some of the muscle fibers.
Damage appears to occur at first in the membrane covering the muscle fibers.
An inflammatory phase then occurs followed by muscle fiber regeneration. Muscle strength can remain at lower than pre-exercise bout levels for as long as three days. The entire process takes about one week. While the reasons for the muscle fiber breakdown are not completely known, the most probable course is related to a temporary breakdown in calcium metabolism. It is not necessarily harmful.