65
explain the increase in Rt 1/2 seen in ischemic fatigue
(see Figure 13). Since Rt 1/2 was not prolonged following
a period of fatiguing contractions with an intact blood
supply, it would appear that ATP was available within the
muscle.
The evidence presented above suggests that the fatigue
observed in these experiments did not occur as a result
of reduced ATP levels. Other evidence supports this
suggestion. Although PC levels were low during the
contraction period, (see Table III), resynthesis had
occurred before significant recovery of AT. Rapid
resynthesis of PC during recovery from a period of
contractions was also observed by Piiper and Spiller (51).
There seems to be no direct relationship between PC levels
and AT. This is contrary to a report by Spande and
Schottelius (56), but supports the observations of Fitts
and Holloszy (29). It should also be pointed out that
glycogen was still available after 30 minutes of stimulation
at 10/sec (14) and lactate production had declined (or
even reversed lactate uptake) (60). It would seem that
energy was available, but demand for energy was reduced.
In support of this conclusion, PvC>2 was not below minimum
critical PO2 (59) at any time that PvC>2 was measured.
The muscle is capable of developing more force than
that seen in a twitch. Twin impulse stimulation causes
developed tension to be double that seen with single