R. Bakels et D. Kernell, MEASURES OF FASTNESS - FORCE PROFILES OF TWITCHES AND PARTLY FUSED CONTRACTIONS IN RAT MEDIAL GASTROCNEMIUS AND TIBIALIS ANTERIOR MUSCLE UNITS, Pflugers Archiv, 431(2), 1995, pp. 230-236
Recordings of isometric force were obtained for twitches and (sub)maxi
mal tetani of gastrocnemius medialis (MG) and tibialis anterior (TA) m
uscle units in female Wistar rats. We assessed the relationships betwe
en unit properties that have all been associated with ''speed''. (1) t
he relative degree of peak force attained during repetitive activation
at 40 Hz (P-40/P-max), (3) the relative degree of final twitch fusion
during the same test burst (Fus-end), and (3) various measures of the
time-course of single twitches, including twitch time-to-peak and a p
arameter referred to as ''initial fusion ratio'' (Fus-in; relative dec
line from peak force at 25 ms from twitch onset). The various measures
of twitch time-course were significantly correlated to each other wit
h correlation coefficients varying over a fairly wide range (0.35-0.64
for MG; 0.50-0.80 for TA). Twitch time-course was also significantly
correlated with Fus-end during the 40-Hz repetitive activation; the hi
ghest correlation coefficient (0.69 for MG, 0.80 for TA) was obtained
for Fus-in, which was also numerically similar to Fus-end. Thus, the d
egree of fusion indeed seemed to be largely dependent upon aspects of
twitch time-course. However, the relative degree of force mobilization
obtained in the same contractions elicited by stimulation at 40 Hz wa
s not consistently better correlated with Fus-end than with measures o
f single twitch time-course. Furthermore, in fast-twitch units having
the same twitch time-to-peak, the force mobilization elicited by stimu
lation at 40 Hz (P-40/P-max) was the same for MG and TA, while the deg
ree of fusion was significantly smaller for TA than for MG units. The
results demonstrate the complexity of the concept of isometric ''speed
'' and underline the need for using several speed indicators in parall
el in studies concerning the differentiation of muscle (unit) properti
es.