Commonly used methods and relevance to the in vivo

force-velocity output

The majority of information on the force-velocity properties of shortening muscle have utilised either constant load (‘isotonic’) (e.g. Fenn and Marsh, 1935; Hill, 1938; Jewell and Wilkie, 1958) or constant velocity (‘isovelocity’) shortening contractions (e.g. Cecchi et al, 1978). Quick-release and afterloaded isotonic contractions have been shown to produce identical force-velocity results (Fenn and Marsh, 1935; Jewell and Wilkie, 1958). Isotonic contractions are performed with the muscle shortening against a constant external load (force). Muscle velocity takes a finite time after the imposition of a fixed force to adjust to a stable value. The force corresponding to the magnitude of the external load and the stable velocity of the preparation provide a single force- velocity observation. The process is repeated for a number of loads in order to obtain a force-velocity relationship. An isovelocity protocol has a muscle preparation shortening at a constant velocity while its force output is measured. Again one force-velocity observation per contraction is obtained. In both of these protocols shortening is allowed only over a limited range in order to

avoid the effects of sarcomere length change (e.g. Granzier et al, 1989) and shortening deactivation (e.g. Lânnergren, 1978) on the preparation’s output. Wilkie (1950) was one of the first researchers to investigate whether voluntary shortening contractions of human muscle also have a hyperbolic force-velocity relationship. In his experiments on the maximum voluntary flexion of the elbow, the set up was designed to provide a constant load to the movement (‘isotonic’). It was realised however that the load was not exactly constant due to the inertia of the apparatus and of the forearm. After corrections for the effects of inertia, the resulting force-velocity curves were typical of those obtained in vitro from isolated muscle preparations. The force-velocity output of human muscle has also been investigated during constant velocity movements. Cook and McDonagh (1996a) timed the release of the index finger into constant velocity abductions during the rising phase of maximal electrically-evoked tetanic contractions of the first dorsal interosseus (FDI) muscle-tendon complex, such that a constant-force phase was generated in each record. During these constant force phases the series elastic component was not expected to change length and the observed external velocity was therefore attributed to shortening of the FDI muscle. When the constant-force values of each volunteer were plotted against the corresponding constant- velocity values. Cook and McDonagh found the force-velocity relationship of the FDI muscle to be similar to the hyperbolic relationships observed in isolated muscle preparations. More recently (Ichinose et al, 2000) it was shown, using ultrasound imaging, during ‘isokinetic’ knee extension at two different angular velocities (30 and 150 ® sec'*) the muscle fascicle shortening velocity of the vastus lateralis changed throughout the movement. Moreover,

the change in the average muscle fascicle velocity in the vastus lateralis was not in the same proportion to the change in the angular velocity of knee extension. This was attributed mainly to the series elastic properties of the muscle-tendon complex. Thus, real ‘isotonicity’ and ‘isovelocity’ conditions at the level of the muscle fibres are very difficult to achieve under in vivo

conditions.

Even during isometric contractions in which there is no external movement (and hence the external velocity is zero), the muscle fascicles have been shown to shorten, stretching the muscle’s series elasticity (Ito et al, 1998). Macpherson (1953) using frog sartorius muscles determined their force- velocity (and their series elastic component load-elongation) properties by comparing in each muscle two isometric contractions, one with and another without added series compliance. The force-velocity relationship during such isometric contractions was also found to have a typical hyperbolic shape. The same method of comparing two isometric contractions with and without added compliance was also used by Wilkie (1950) to determine the series elastic properties of the elbow flexors in humans and use this compliance result to make corrections that improved the predictions of the velocity output during his ‘isotonic’ experiments. Cook and McDonagh (1996a) found the force- velocity relationships of the electrically-stimulated first dorsal interosseus muscle to be similar under isometric and ‘isokinetic’ conditions.

The force-velocity relationship during voluntary contractions performed in vivo has also been shown to depend on the level of muscle activation.

activation here referring to motor unit recruitment (Chow and Darling, 1999). A family of hyperbolic force-velocity curves was obtained for the wrist flexors of human volunteers, each different curve corresponding to a different level of ‘activation’ and reflecting the force-velocity properties of the currently recruited motor units. These results were not expected to have been affected, at least to any considerable extend, by the muscle group’s series elasticity as the wrist flexors have very stiff tendons and the force the muscles produced was too small to cause appreciable tendon elongation.

Force-velocity properties have also been investigated under different levels of activation (Cecchi et al, 1978; Curtin et al, 1998; De Haan, 1988, 1998) and more complex patterns of the preparation’s length change mimicking in vivo

movement (e.g. Askew and Marsh, 1997, 1998; Curtin et al, 1998). Although the findings of such studies are not dealt with in detail in this thesis, the general conclusion is that when certain factors are accounted for (e.g. activation levels, length change of the series elastic component etc) the force- velocity output of muscles can be fitted well, at least in most of its range, by a hyperbolic relationship.