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To escape from predators, sh use rapid, carefully timed and directed movements which involve the recruitment of the entire
white muscle (for a review, see Domenici and Blake, 1997).
Although escape responses are continuous events, they have
often been classied into distinct kinematic stages (Weihs,
1973; Webb, 1978). Initially, the body of the sh is bent into
a C or S shape, rapidly rotating the head so as not to provide
a stationary target (Eaton et al. 1977). The C-bend is then
followed by one or more complete tailbeats of variable
amplitude and duration which usually involve an overall
change in direction (see Domenici and Blake, 1997). Length-
specic swimming velocity decreases with increasing body
length (Videler and Wardle, 1991; Gibson and Johnston,
1995), whereas tailbeat duration increases (Bainbridge, 1958;
Webb, 1978). The scaling of kinematic parameters has been
attributed to systematic variations in the mechanical properties
of muscle (Wardle, 1980). For example, James et al. (1998)
found that in short-horn sculpin (Myoxocephalus scorpius L.)
the activation and relaxation times of muscle increased and the
maximum unloaded shortening velocity (V 0 ) decreased with increasing body length. Johnston et al. (1995) studied muscle performance during predation fast-starts in the short-horn sculpin. Successive
outlines of the body were digitised and used together with a
knowledge of the geometric arrangement of muscle bres to
calculate in vivo muscle strain changes. In order to estimate in
vivo muscle power output, an adaptation of the work-loop
technique (Josephson, 1985; Altringham and Johnston, 1990)
was used to measure power output in vitro in bre bundles
subjected to the calculated strain waveforms and measured in
vivo muscle activation patterns. In addition, instantaneous
force was plotted against velocity to produce a power loop
(Stevens, 1993), allowing comparisons of muscle performance
under in vivo and steady-state (isovelocity) conditions. The
amount of active lengthening of the muscle prior to shortening
(pre-stretch) was found to increase down the length of the sh,
resulting in a progressive increase in the amount of force
enhancement. Pre-stretch caused the muscle properties to
deviate from those found under steady-state conditions such
that higher V/V max (shortening velocity expressed as a proportion of maximum shortening velocity) values were used
in vivo than would have been predicted from forcevelocity
curves. The aim of the present study was to investigate the scaling 913 The Journal of Experimental Biology 201, 913923 (1998)
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