The complex nature of positive and negative work often practiced in human activity is apparent in the current literature on the topic. Much theoretical effort has been expended on examination of the positive and negative work relationships during exercise, particularly as numerous differences between the effect of a concentric and eccentric muscular contraction exist. These differences range from the force able to be exerted and energy cost of the effort in each type of muscle action, physiological response to the difference in efficiency between the two types of work, the ratings of perceived exertion elicited, and the role of each type of contraction in the precipitation of a muscle injury and the relative contribution of each to a training effect. This review examines the complex nature of positive and negative work and our current understanding of these complex issues.