Slide 8 of 13
The image above shows a microscopic image of skeletal muscle damage. A muscle fiber is divided into functional units known as sarcomeres which are defined as the distance between 2 sets of Z - lines. Under normal conditions the Z-lines from adjacent muscle fibers are alligned. In this image there is a high degree of Z - line disruption. This disruption hampers the function of the muscle and has been implicated as one of the potential sources of muscle soreness.
Exercise that violates the principle of progression can lead to this type of skeletal muscle damage. While the body eventually repairs this microscopic damage it is better to avoid the soreness and restriction by adding new exercises slowly or by progressing the intensity, volume or duration of activity in a reasonable manner.