Fig. 2.
Sample concentric (A) and eccentric (B) contractions. (A) Isotonic concentric contraction at 30% of maximum isometric force (Po). Phase 1 represents a period of force development and phase 2 represents the constant force (isotonic) period of the contraction. The ratio of MTU velocity (gray) and fascicle velocity (red) measured during the isotonic period is used to quantify AGR. (B) Isotonic eccentric contraction at 170% of Po. Phase 1 represents a period of force development. During phase 2 the muscle is stretched by an isovelocity ramp until the muscle force reaches a designated force level. Phase 3 represents an isotonic period where the muscle is being stretched while the motor maintains a constant force level. Gear ratio is quantified during phase 3, where force is constant. By convention, shortening is defined as positive velocity and lengthening is defined as negative velocity.