12
believe that the muscle has a reduced amount of Ca2+
released. In the latter, it is assumed that relaxation
occurs simultaneously with Ca2 + reuptake (4). Under these
circumstances, a reduction in contraction time would
result from a reduction in duration of activation (more
f
rapid reaccumulation or shorter duration of release). A
reduction in peak rate of force development without a
concommitant reduction in contraction time indicates
reduced activation, and this is interpreted as a reduced
amount of Ca2+ released. Brust has made observations
similar to these (reduced rate of force development in
fatigue with no change in contraction time) on mouse
soleus muscles in vitro, (10) and concluded that fatigue
was due to reduced Ca2+ release.
Similar observations would be expected if there was
an increase in the Ca2+ concentration at which binding to
troponin and subsequent contractile activity occurs. It
has been observed by Fuchs et al. (32) that the affinity
of troponin for Ca2+ can be altered by pH. This
possibility must be considered when dealing with
inferences from measurements of contraction time and
rate of force development. Fitts and Holloszy (29) have
presented data indicating that reduced pH may be
associated with fatigue. They support the theory that
reduced activation (and reduced rate of force development)
is due to a reduced affinity of troponin for Ca2+.