Mechanical efficiency of fast- and slow-twitch muscle fibers in man during cycling.

The influence of different percentages of slow-twitch (ST) and fast-twitch (FT) fibers in vastus lateralis on delta efficiency expressed by delta work (x)/delta energy liberation (y) in y = a + bx was studied in six subjects during cycling on an ergometer at 60 or 100 rpm at work loads below 80% of VO2max. Three subjects had an average of 78% ST fibers (ST group) and the other subjects had an average of 76% FT fibers (FT group). There was no difference between the two groups in delta efficiency at 60 rpm, but at 100 rpm the efficiency of the ST group was significantly lower than that of the FT group (19.6 vs. 28.8%, P less than 0.01). In the ST group respiratory exchange ratio (R) was higher at 100 rpm than at 60 rpm, but the FT group had similar R values at both pedal revolution rates. The most important finding was the reduced efficiency when pedaling frequency was increased from 60 to 100 rpm in the ST group (23.3 to 19.6%). Predominant use of ST fibers at rapid pedal rates may require a substantial increase in energy expenditure.