Lengthy, very involved review of exercise induced muscle damage as a result of eccentric exercise. But this image was kind of cool.

Muscle Nerve. 2013 Sep 13. doi: 10.1002/mus.24077. [Epub ahead of print]
Lengthening our perspective: Morphological, cellular and molecular responses to eccentric exercise.
Hyldahl RD, Hubal MJ.
Department of Exercise Sciences, Brigham Young University, Provo, UT.
The response of skeletal muscle to unaccustomed eccentric exercise has been studied widely, yet it is incompletely understood. This review is intended to provide an up-to-date overview of our understanding of how skeletal muscle responds to eccentric actions, with particular emphasis on the underlying molecular and cellular mechanisms of damage and recovery. Our review begins by addressing the question of whether eccentric actions result in physical damage to muscle fibers and/or connective tissue. We next review the symptomatic manifestations of eccentric exercise (i.e. indirect damage markers, such as delayed onset muscle soreness), with emphasis on their relatively poorly understood molecular underpinnings. We then highlight factors that potentially modify the muscle damage response following eccentric exercise. Finally, we explore the utility of using eccentric training to improve muscle function in populations of healthy and aging individuals, as well as those living with neuromuscular disorders. 2013 Wiley Periodicals, Inc.
Copyright 2013 Wiley Periodicals, Inc., a Wiley company.