I read this on the supertraining list a while back - maybe a reason why they have been so effective for me.
Isometric contractions and fatigue
“Experiments studying fatigue under isometric conditions suggest that
the loss of force during sustained MVC is not due to impairment of
processes proximal to the neuromuscular junction, but is most likely
due to failure in the contractile machinery. Subjects attempted to
hold a maximal contraction for at least one minute (Fig 2). As
expected, force declined at a constant rate. Periodically during
this maneuver supramaximal electrical shocks were delivered to the
peripheral nerve supplying the muscle. The force record showed that
the shocks did not produce additional force. This means that the
muscle was being driven to its maximal force producing capacity by
the nervous system. According to the results of this experiment and
several others like it, it appears that the nervous system is capable
of activating a muscle to produce its true maximum force and that
loss of force is likely due to failure somewhere in the contractile
apparatus.”