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First published online September 9, 2003

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Characterization of the passive component of force enhancement following active stretching of skeletal muscle

W. Herzog^*, R. Schachar and T. R. Leonard

University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada

View larger version (13K): [in a new window]   Fig. 1. Representative force-time histories of two isometric and one experimental stretch contraction held for approximately 30 s beyond deactivation. The isometric contractions were performed at lengths of +9 and +11 mm (9, 11, respectively). The stretch test (s) was performed from 0 to +9 mm at a constant speed of 3 mm s-1. Note that the passive force following active stretch is greater than the corresponding passive force following isometric contraction (9), and decays at a greater rate than those of the two isometric contractions (9, 11).

View larger version (13K): [in a new window]   Fig. 2. Representative force-time histories of a passive stretch test (p), the corresponding active stretch test (a, 0 to +9 mm at 3 mm s-1), and the corresponding isometric contraction (i) at a length of +9 mm. Stiffness of the deactivated muscle was determined by a 1 mm stretch at 50 mm s-1 at about 5 s following cessation of muscle stimulation. Passive stiffness was significantly greater following the active stretch tests compared to the passive stretch tests and the isometric reference contractions.

View larger version (12K): [in a new window]   Fig. 3. Representative force-time histories of two identical stretch tests (0 to +9 mm at 3 mm s-1) and the corresponding isometric reference contraction (i) at +9 mm. Approximately 5 s following deactivation, the actively stretched muscles were released and immediately stretched again by 4.5 or 9 mm (4.5 and 9, respectively). When shortened-stretched by 4.5 mm (50% of the active stretch), passive force enhancement was almost completely maintained. When shortened-stretched by 9 mm, passive force enhancement was abolished `instantaneously'.

View larger version (13K): [in a new window]   Fig. 4. Representative force-time histories of an isometric reference contraction (i) and an experimental stretch contraction (s, 0 to +9 mm at 3 mm s-1). Following the first activation, the muscle was left deactivated for 5 s before it was activated again at the final length (+9 mm for the isometric reference contraction and the experimental stretch test). Note the passive force enhancement following the first (2) and second (4) deactivation, and the decreased `passive' force enhancement during the second activation period (3) compared to the passive force enhancement prior to and following the second activation.

View larger version (15K): [in a new window]   Fig. 5. Representative force-time histories of three experimental stretch contractions (0 to +9 mm at 3 mm s-1) that were preceded by active shortening of 0, 6 and 9 mm (0, 6 and 9, respectively). Also shown is an isometric reference contraction (i, +9 mm). Note how increasing the amount of shortening decreases the total and the passive force enhancement to a similar degree.

View larger version (12K): [in a new window]   Fig. 6. Representative force-time histories of an experimental stretch test (s, 0 to +9 mm at 3 mm s-1) and the corresponding isometric reference contraction (i) at the final stretch length (+9 mm). At 4.8 s following the end of the active stretch, muscle stiffness was determined by a quick stretch (1 mm at 50 mm s-1). The average stiffness for the experimental stretch contractions was 11.5% greater than the stiffness for the isometric reference contractions.

View larger version (11K): [in a new window]   Fig. 7. Representative force-time histories of the active force of an experimental stretch test (s, -6 to +3 mm at 3 mm s-1), and an isometric contraction at the optimum length of the muscle (o). Note that the steady-state isometric force following the active stretch contraction is greater than the purely isometric force at muscle optimum length.