Failure of stimulated skeletal muscle mainly contributed by passive force: an in vivo rabbit model.

OBJECTIVE: To evaluate the role of active and passive muscle forces in the failure mechanism of stimulated muscle. DESIGN: An in vivo rabbit model. BACKGROUND: Eccentric contractions can result in a greater incidence of muscle injury. However, the relative role of the active and passive muscle force in the failure mechanism of the activated muscle is not well elucidated. METHODS: After anaesthesia, New Zealand white rabbits were fixed in a frame on a materials testing machine. The triceps surae muscle-tendon units were passively stretched to rupture with our without continuous nerve stimulation. The force and muscle length were simultaneously recorded. Active muscle force, passive muscle force, and ratio of the active to passive muscle were calculated and depicted against strain. RESULTS: The results showed that the mean maximal passive force of triceps surae muscle was 293.1 N at a strain of 38%. The mean peak active muscle force was 21.5 N at a strain of 21%. The ratio of active to passive muscle force reached its peak first, followed by the active muscle force, and then the passive muscle force. The ratio of active to passive muscle force at the peak total force was only 3.3%. CONCLUSIONS: The stimulated muscle can exert its maximal response at extreme physiological extension. Injury of the stimulated muscle is caused mainly by passive muscle force.

Influences of configuration changes of the patella on the knee extensor mechanism.

Biomechanical analysis of the configuration changes of the patella was studied. Normal patellar tracking in the sagittal plane was obtained by recording and digitizing knee extension of six healthy adults with fluoroscopy. Based on the digitized data, the changes of patellar thickness were simulated by translating its axis and its forward and backward tilting by rotation in the sagittal plane. We assumed that the extensor moment remained constant before and after the patellar configuration was changed. Therefore, using a balance beam, model equilibrium was reached, after each simulated change of patellar configuration, of the forces involved in the extensor mechanism: the quadriceps muscle force, tension of patella tendon and patellofemoral joint reaction force. The results revealed that when the patellar thickness decreased, quadriceps force increased but patellofemoral joint reaction force decreased and the reverse was seen when the patella thickened. Backward tilting of the patella decreased quadriceps force and patellofemoral joint reaction force and increased the patella tendon/quadriceps force ratio, which produced a better mechanical advantage of force transmission of patella. The reverse was seen in forward tilting. These results suggest that the patella could be tilted backward for the older patient undergoing total knee replacement. For younger patients with relatively strong quadriceps musculature, we suggest that the patellar thickness could be decreased to reduce the patellofemoral joint reaction force so as to reduce the chance of failure of the patella prosthesis.