During normal daily activities, muscles are required to lengthen as frequently as they shorten to produce movement. Lengthening muscle actions are associated with high forces and low energy consumption, but can often result in muscle injury. These unique features are not explained adequately by current (cross-bridge) theories of muscle contraction. Using specific myosin inhibitors and different temperatures the author has examined the molecular mechanisms of stretch-induced force enhancement. The results, which suggest that lengthening force arises from the strain of both cross-bridge and non-cross-bridge components of the sarcomere, help to refine understanding of the molecular mechanisms of muscle contraction.