A number of studies have demonstrated that disuse of a muscle group results in significant muscle weakness. Until recently, however, it wasn't clear if chronic strength training was protective against strength loss. Deschenes and colleagues recently published some very important work in the American Journal of Medicine and Rehabilitation. Eleven men with a history of strength training (average = 5+ years) and 11 control participants completed the study. Both groups had their knee joint immobilized for 7 consecutive days and used crutches to get around. Muscle strength and mass, as well as several other neuromuscular variables, were measured before and after the 7 day period. Interestingly, the authors found that both groups got significantly weaker as a result of their disuse. Muscle activation was higher for the trained participants at both pre and posttesting, but other variables were not affected by the intervention. It was therefore concluded that a history of chronic strength training did not prevent muscle weakness.
It is important to point out that the trained group was still roughly 50% stronger than the untrained participants after the 7 day period. At the end of the study, the strength of the trained group was substantially higher than the pretest values for the control group. Thus, the participants' strength training background was still beneficial in the sense that the trained group had a much greater capacity for change. Put another way, when a muscle group is weak and an unexpected situation requiring disuse of a joint occurs, there isn't much further down to go!