The short range stiffness of active mammalian muscle and its effect on mechanical properties

Abstract

SUMMARY1. The tension in tetanized cat soleus and lateral gastrocnemius muscles was measured during alternating lengthening and shortening movements. Sinusoidal movements were sometimes used; on other occasions the movement was at a constant velocity but with periodic reversal of direction.2. With constant velocity movements of small amplitude the tension rose steeply during lengthening and fell during shortening in a relatively simple way. With longer movements the tension at first changed steeply as it had done with the smaller movement, but later in the movement the resistance of the muscles decreased so that the tension change became more gradual. The muscles resisted a small movement or the first part of a larger movement with a 'short range stiffness' which did not persist as the movement continued.3. So long as the constant velocity movement was not too slow the short range stiffness was independent of velocity though it lasted for more of a fast movement than of a slow one.4. In small movements the muscle was never extended beyond its short range stiffness, and the over-all peak-to-peak tension change was therefore large compared with the amplitude of movement. When, with larger movements, the muscle was stretched into a range in which it became more compliant, the peak-to-peak force fluctuation did not increase by an equivalent amount, and over the whole course of the movement the force change per unit extension was smaller.5. When the movement was confined to a short range, little work was expended in driving the muscle through a cycle of movement; its properties were essentially elastic. With larger amplitudes the muscle met the movement with a frictional resistance, the tension during lengthening then being greater than during shortening. A considerable amount of work had then to be done on the muscle to maintain the movement.6. The short range stiffness was also apparent in the response to sinusoidal movements. 332 PETER M. H. RACK AND D. R. WESTBURY 7. The short range stiffness was attributed to elastic properties of crossbridges between thick and thin filaments in the myofibrils.8. The effect of the short range stiffness on the mechanical properties of the limb is discussed.

Active tension changes in frog skeletal muscle during and after mechanical extension

“…As has been reported by several authors (Fowler and Crowe, 1976;Rack and Westbury, 1974;Sugi, 1972), the form of the tension change during the ramp phase of the stretch depends upon the parameters of the ramp i.e. upon the magnitude and speed of stretch.…”

Systematic nonlinear relations between displacement amplitude and joint mechanics at the human wrist

“…These calculations indicated that the six displacement amplitudes corresponded to 0.2, 0.25, 0.3, 1, 2, and 3 percent of average muscle length. This indicates that the 0.4º, 0.5º, 0.6º RMS displacements were within the region of short range stiffness (< 1% of resting length (Rack and Westbury, 1974) for FCR, while the 2º, 4º and 6º RMS displacements were beyond that range.…”

“…The intrinsic muscle structures (without stretch reflex input) exhibit high initial 'short range stiffness' for increases in length up to approximately 1% of the resting muscle length. This is followed by a sudden yield, after which the muscle stiffness remains constant at a lower level (Rack and Westbury, 1974). Studies of intact joint mechanics (with stretch reflex input) have reported that stiffness decreases as displacement amplitude increases, both in humans (Kearney and Hunter, 1982;Milner and Cloutier, 1998;Sinkjaer et al, 1988) and decerebrate cats (Nichols, 1985).…”

Simulation studies of contracting skeletal muscles during mechanical stretch

“…The short-range stiffness has been demonstrated (Rack and Westbury, 1974) in tetanized mammalian muscle, and Fig. 4 shows a record from our series of experiments on frog sartorious.…”

“…The simulated tension changes are compared with the experimental results presented in the previous article (Van Atteveldt and Crowe, 1980) and results of experiments on skeletal muscle presented by other authors (Edman et al, 1976;Edman et al, 1978a, b;Rack and Westbury, 1974;Sugi, 1972). * Received for publicatiort 22 May 1979.…”