The contractile properties of the tibial anterior (TA) of Wistar rats were measured by means of a multipurpose testing machine. The muscle was isolated from the connective tissues, preserving the proximal insertion. The distal tendon was transected and fixed to the machine actuator. The leg was inmobilised using a pin drilled through the femoral condyle. In this way the force response was studied in vivo at different constant lengths for some voltages and frequencies. Mathematical functions are proposed for adjusting the force-length, force-frequency and force-time relations. The model includes a novel formulation for the depression response during muscle tetanisation.
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