SHADWICK, R.E.; SYME, D.A.*: Mechanical Power Production by Red and White Swimming Muscles of Yellowfin Tuna (Thunnus albacares)
The mechanical performance of red and white muscle from yellowfin tuna was studied in vitro at 25oC using the work loop technique. Fibers from internal red muscle were taken from two axial locations, 0.5L and 0.65L (L=total body length), and subjected to sinusoidal strains at frequencies of 1-12Hz and activation patterns that mimicked those in vivo, as determined previously. White fibers from 0.45L and 0.65L were tested over frequencies of 3-15Hz. During steady swimming, where the muscle strain cycle (=tail beat) frequency is 2-4Hz, the activation phase and duration used by fish were nearly identical to those that elicited maximal net work and power from isolated red muscle. The greatest net work per cycle occurred at 1-2Hz, while peak power (=work X frequency) was obtained at 3-4Hz. Thus, during steady swimming the internal red muscle functions to maximize power production. At higher frequencies, the stimulation phase and duration that yielded maximal net work decreased, as expected, with power dropping to zero at about 9Hz. The twitch time (i.e from stimulus to peak force) of red fibers was about 100ms. Compared to skipjack tuna, yellowfin red muscle has slower contraction kinetics, so it requires longer and earlier activation to maximize power output. These results are in agreement with the differences in muscle activation timing previously reported for these two species in vivo. For yellowfin white fibers, tests with optimal activation produced the greatest work per cycle at 8Hz and peak power at 10Hz. Positive net work was still produced at frequencies up to14Hz, approximately the highest tail beat frequency recorded during burst swimming.
