The contribution of contractile pre-activation to loss of function after a single lengthening contraction

Abstract

Purpose: Some muscle injuries are the result of a single lengthening contraction. Our goal was to evaluate the contributions of angular velocity, arc of motion, and timing of contractile activation relative to the onset of joint motion in an animal model of muscle injury using a single lengthening contraction.

Methods: The intact tibialis anterior (TA) muscle of rats was activated while lengthened, preceded by a maximal isometric contraction of 0, 25, 50, 100, or 200 ms. The lengthening contraction was performed at two different angular velocities (300 or 900 degrees/s) and through two different arcs of motion (90 degrees or 45 degrees).

Results: Muscle contractile function, as measured by maximal isometric tetanic tension, was significantly decreased only when the TA was activated at least 50 ms prior to the motion, regardless of angular velocity or arc of motion.

Conclusion: The data indicated that the duration of an isometric contraction prior to a single lengthening contraction determined the extent of muscle injury irrespective of two different angular velocities.

Fig. 1

Muscle Injury Model: the tibia of the hindlimb was stabilized and the foot secured to a plate driven by a stepper motor. The peroneal nerve was used to stimulate the TA anterior (a dorsiflexor) at a selected time while the plate forced the foot into plantar flexion at a selected velocity.

Fig. 2

Real time torque measurements during the lengthening contraction. The torque shown is the average at each data point (1 ms intervals) from five different animals. The muscle injury was created with an isometric contraction of 200 ms prior to a lengthening contraction at 900°/s over a 90° arc. An initial rise in total torque is followed by a plateau phase only to rise sharply near the end of the motion. Passive torque was obtained by moving the ankle through the 90° arc without stimulation. The maximal active torque was obtained by subtracting the passive torque from the total torque at each data collection point (1 ms). After a sharp rise in active torque (contributed nearly exclusively by the TA), a yield point is observed (open arrow) and a smaller second peak could represent the contribution of the EDL (closed arrow) during the lengthening contraction of the dorsiflexors.

Fig. 3

Maximal tetanic isometric tension after a single lengthening contraction with different pre-activation times. A significant loss of force occurred only when the TA was activated 50 ms or longer prior to (and throughout) the lengthening contraction (mean ±SD, n = 5 per group, *p<0.05).