It has been well documented that the steady-state isometric force generated by skeletal muscle is influenced by the contractile history of the muscle but there is very little agreement on the underlying mechanism. The most commonly held mechanism for the residual force enhancement following active muscle stretching involves sarcomere length re-distribution (and disruption). To gain insight into this history dependent behaviour of muscle contraction, specifically force-enhancement following active stretch, experiments were performed on rabbit psoas muscle myofibrils where all individual sarcomere lengths could be determined simultaneously with force during isometric and eccentric myofibrillar contractions. In single isometrically activated and then stretched myofibrils, force-enhancement following stretch occurred and while sarcomere length non-uniformities were detected, they exhibited stable behaviour and did not increase during stretch, therefore the development of sarcomere length non-uniformities were deemed to be an unlikely source of the enhanced force. Experiments were conducted on myofibrils reduced to a single sarcomere so as to preclude the possibility of non-uniform sarcomeres playing a role in the extra force following stretch. Enhanced force was observed in these single sarcomere preparations when compared to the isometric reference force uniquely demonstrating that force enhancement can occur in the absence of sarcomere overstretching. Finally, the role of titin in actively and passively lengthened myofibrils was investigated to determine whether the enhanced force following active stretch could be attributed to a molecular spring, as some have proposed. Single myofibrils were lengthened from the plateau region of the force-length relationship until mechanical failure (first observation of negative slope for the stress-time curve) was observed, with and without titin present, and it was determined that titin is crucial for force production in actively and passively lengthened myofibrils. While mechanical failure occurred at similar sarcomere length, failure force was four times higher in the actively compared to the passively stretched myofibrils. All myofibrils failed at sarcomere lengths beyond which actin-myosin based cross-bridge forces could be expected to contribute to the force at failure. These results suggest that the molecular spring titin plays a major role in sarcomere mechanics and that history dependent behaviour likely originates from a parallel elastic element and not from sarcomere length redistribution.