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Force enhancement following stretching of skeletal muscle : a new mechanism

W. Herzog^* and T. R. Leonard

Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada T2N 1N4

View larger version (7K): [in a new window]   Fig. 1. Schematic illustration of force enhancement following stretch according to the sarcomere-length non-uniformity theory. A muscle is stretched on the descending limb of the force—length relationship from an initial average sarcomere length (open circle) to a final average sarcomere length (filled square). During stretching of the muscle, it is assumed that some sarcomeres are stretched less than average (filled circle, left) while others are stretched more than average (filled circle, right). The sarcomeres that are stretched less than average are stronger than an average sarcomere would be, because of the slope of the force length relationship. The sarcomeres that are stretched more than average become weaker initially, but then are `caught' by the passive force of the muscle, and they elongate until a force equilibrium is established between the short and long sarcomeres. This force at equilibrium (dashed line) is greater than the expected force at the average sarcomere length, and therefore, this mechanism can potentially account for the observed force enhancement following muscle stretch.

View larger version (12K): [in a new window]   Fig. 2. Schematic representation of the stretching tests on the ascending limb of the force—length relationship of the cat soleus muscle. Muscles were stretched by 8 mm (approx. 20 % of optimal fibre length) at a speed of 4 mm s-1 (approx. 10 % of optimal fibre length s-1). For all tests, the muscle was activated for 8 s at 3T (3x the alpha-motoneuron threshold) starting at time 0 s. Steady-state total force enhancement following stretching was always determined 4.5 s after the end of the dynamic phase of contraction (i.e. at 7.5 s, see first arrowhead). Passive force enhancement following stretching was always determined 3 s after activation was stopped (i.e. at 11 s, see second arrowhead).

View larger version (13K): [in a new window]   Fig. 3. Schematic representation of the stretching tests on the descending limb of the force—length relationship of the cat soleus muscle. Muscles were stretched for 3, 6 and 9 mm (approx. 7, 14 and 21 % of optimal fibre length) at speeds of 3, 9 and 27 mm s-1 (approx. 7, 21 and 63 %, respectively, of optimal fibre length s-1). For all tests, the muscle was activated for 9 s at 3T (3x the alpha-motoneuron threshold) starting at time 0 s. Steady-state total force enhancement following stretching was always determined 4.5 s after the end of the dynamic phase of contraction (i.e. at 8.5 s, see first arrowhead). Passive force enhancement following stretching was always determined 3s after activation had been stopped (i.e. at 12s, see second arrowhead).

View larger version (19K): [in a new window]   Fig. 4. Example results of force enhancement following muscle stretching on the ascending limb of the force—length relationship for stimulation frequencies of 5 Hz (A) and 30 Hz (B). A summary of the corresponding mean results across all muscles is shown in Table 1.

View larger version (17K): [in a new window]   Fig. 5. Example force—time traces of active force (total — passive force) on the descending limb of the force—length relationship. The 9 mm stretch was associated with a steady-state active force following stretching (s) that exceeded the corresponding active isometric forces obtained at the initial (i) and final (f) muscle length.

View larger version (19K): [in a new window]   Fig. 6. Total isometric force (active + passive force) at the final length (f), and total isometric force following muscle stretching of 3 mm, 6 mm and 9 mm (3, 6 and 9, respectively) to the final length (f) for representative tests at a stretching speed of 3 mm s-1. Note that the steady-state force enhancement following stretching increases with increasing magnitudes of stretching on the descending limb of the force—length relationship of cat soleus muscle. The corresponding mean results of force enhancement across all muscles, speeds of stretching, and activation frequencies are summarized in Table 2.

View larger version (19K): [in a new window]   Fig. 7. Total isometric force (active + passive force) at the final length (f), passive force for a muscle stretched from 0 mm to 9 mm (p), and total isometric force following muscle stretching of 3 mm, 6 mm, and 9 mm (3, 6 and 9, respectively) to the final length for representative tests at a stretching speed of 3 mm s-1 (A), 9 mm s-1 (B), and 27 mm s-1 (C). Note the passive force enhancement following deactivation of the muscle for the actively stretched tests at all speeds, but not for the passively stretched muscle. These steadystate passive forces persisted for the duration of observation (5-10 s after deactivation) but were abolished following shortening of the muscle to -10 mm. Also note the increase in passive force enhancement with increasing magnitude of active muscle stretching, and the similarity in magnitude of total force enhancement (while the muscle was activated), and passive force enhancement for the 9 mm stretch magnitudes. A summary of the corresponding results across all muscles is given in Table 3.