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First published online September 5, 2008
Journal of Experimental Biology 211, 3001-3008 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.021204

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Is a parallel elastic element responsible for the enhancement of steady-state muscle force following active stretch?

S. R. Bullimore^*, B. R. MacIntosh and W. Herzog

Human Performance Lab, Faculty of Kinesiology, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4

View larger version (3K): [in this window] [in a new window]   Fig. 1. Schematic illustrating how residual force enhancement (rFE) could be generated by a parallel elastic component (PEC) that increases in stiffness when the muscle is activated. The solid line is the force–length relationship of the PEC in a relaxed muscle. The broken line is the force-length relationship of the PEC when the muscle is activated with the PEC at the length LA. The stiffness of the PEC, but not the force, increases upon activation. If an isometric contraction is performed when the PEC is at LB, it exerts a force F1. However, if the muscle is activated when the PEC is at LA and is then stretched until the PEC reaches LB, the PEC force will be F2. The rFE is F2–F1.

View larger version (10K): [in this window] [in a new window]   Fig. 2. Example of raw data for stress against time (A) and length against time (B) for one set of three contractions with a shortening distance equal to 100% of stretch distance. Lopt=optimal length. Isometric-shortening contraction–thick, solid, light-grey line; purely isometric contraction–thick, broken, dark-grey line; stretch-shortening contraction–thin, solid, black line. Force is transiently negative after shortening because of damped oscillations, which may have been caused by vibrations in the wire hook attached to the force transducer. Vertical broken lines indicate the period over which mean force was calculated before the stiffness test. Inset shows expansion of force records between broken lines, with double-headed arrows indicating the force enhancement calculated by two different methods (rFE1,rFE2) and the force depression (FD).

View larger version (7K): [in this window] [in a new window]   Fig. 3. Force depression (% isometric force) induced by rapid shortening through various distances (shortening distance expressed as a percentage of stretch distance in the corresponding stretch-shortening contraction for consistency with the other figures). Top axis gives corresponding final fibre length relative to optimal length (Lopt). Error bars indicate means ± s.e.m. Labels give number of fibres (N) when this was less than six. *Different from zero, P<0.05.

View larger version (8K): [in this window] [in a new window]   Fig. 4. Residual force enhancement remaining after rapid shortening by various distances (shortening distance expressed as a percentage of stretch distance). Force enhancement is calculated in two different ways: (A) relative to a contraction without stretch but with shortening (solid line); and (B) relative to a purely isometric contraction (broken line). These two methods represent maximum and minimum bounds, respectively, for the true residual force enhancement. In both cases, force enhancement was significantly greater than zero when shortening distance was 100% of stretch distance. The first data point shows the residual force enhancement without shortening. Top axis gives corresponding final fibre length relative to optimal length (Lopt). Error bars indicate means ± s.e.m. Labels give number of fibres (N) when this was less than six. *Different from zero, P<0.05.

View larger version (5K): [in this window] [in a new window]   Fig. 5. Effect of altering stimulation rate and number of periods of force redevelopment on the force depression in two muscle fibres. `Control', same conditions as in the rest of the study with shortening 1500 ms into activation so that there were two periods of force development; `+10Hz', same as `Control' except for a 10 Hz increase in stimulation frequency; `ES' (early-shortening), shortening performed 20 ms into activation so that there was only one period of force development; `DS' (double-shortening), shortening broken into two equal steps with a 1000 ms gap so that there were three periods of force development; `TS' (triple-shortening), shortening broken into three equal steps with 500 ms gaps so that there were four periods of force development (only performed on fibre 2). For all trials performed on each fibre, total shortening distance and final length were the same. However, shortening distance and final length were different in the two fibres (see Materials and methods), which may explain the different magnitudes of force depression observed. Force depression increased when stimulation frequency was increased, in contrast to what would be expected for the `movement effect' (Edman, 1975; Edman, 1980). Force depression increased as the number of periods of force development was increased but the relationship was not linear.

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