HAMSTRINGS AND GASTROCNEMIUS COCONTRACTION PROTECTS THE ANTERIOR CRUCIATE LIGAMENT AGAINST FAILURE - AN IN-VIVO STUDY IN THE RAT

An anesthetized rst model was used to study the effects of muscle cont raction on the ultimate tensile load and the energy absorption at fail ure of the anterior cruciate ligament. In both knees, the joint capsul e and ligaments, except for the anterior cruciate ligament, were divid ed, and the menisci were removed with the aid of a stereomicroscope. T he cruciate ligament of the right knee was tested in tension until fai lure by femorotibial distraction during contraction of the hamstrings and calf muscles induced by electrical stimulation of the ischiatic ne rve. The cruciate ligament of the left knee, which was loaded to failu re with nonstimulated (relaxed) muscles, served as the control. The me an ultimate tensile load during muscle contraction was 86 N compared w ith 53 N when tested with relaxed muscles (p < 0.001). The energy abso rption at failure was 0.41 and 0.19 J during contraction and relaxatio n, respectively (p < 0.05). This study suggests that previous investig ations evaluating the force and energy necessary to rupture the anteri or cruciate ligament (with use of a femur-anterior cruciate ligament-t ibia complex stripped of all soft tissues and without gastrocnemius-ha mstring muscle contractions) are incomplete and probably not represent ative of the in vivo situation.