Length dependence of active force production in skeletal muscle

The sliding filament and cross-bridge theories of muscle contraction provid e discrete predictions of the tetanic force-length relationship of skeletal muscle that have been tested experimentally. The active force generated by a maximally activated single fiber (with sarcomere length control) is maxi mal when the filament overlap is optimized and is proportionally decreased when overlap is diminished. The force-length relationship is a static prope rty of skeletal muscle and, therefore, it does not predict the consequences of dynamic contractions. Changes in sarcomere length during muscle contrac tion result in modulation of the active force that is not necessarily predi cted by the crossbridge theory. The results of in vivo studies of the force -length relation ship suggest that muscles that operate on the ascending li mb of the force-length relationship typically function in stretch-shortenin g cycle contractions, and muscles that operate on the descending limb typic ally function in shorten-stretch cycle contractions. The joint moments prod uced by a muscle depend on the moment arm and the sarcomere length of the m uscle. Moment arm magnitude also affects the excursion (length change) of a muscle for a given change in joint angle, and the number of sarcomeres arr anged in series within a muscle fiber determines the sarcomere length chang e associated with a given excursion.