Neural fatigue
In addition to the metabolic causes of fatigue discussed above, there is also a neural
component of fatigue. When, a ‘diverting’ activity (such as light activity of another limb) is
performed in-between bouts of high intensity activity , force output is maintained at a higher
level than if the diverting activity is not performed (101,102). This has been attributed to
‘distracting’ the nervous system, allowing faster recovery.
During a weight training session, alternating exercises with different bodyparts (i.e. one set
for legs, one set for chest) allows for greater recovery because of this diverting activity (as well as
giving more rest time between sets). Physical therapists have long known that the contraction of
one muscle (for example the biceps) causes the antagonist muscle (in this case, the triceps) to
relax. So alternating sets for opposing muscle groups (i.e. one set for biceps, rest, one set for
triceps) may allow greater force production and decrease fatigue (103).
Other causes of fatigue
Finally, there are possible non-metabolic causes of fatigue. During muscular contraction,
signals are sent from the brain to the muscle, eventually reaching the muscle fibers through a
structure called the sarcoplasmic reticulum (SR). Normal SR function may be impaired during
exercise and this impairment may be related to a loss of intracellular potassium (71,104,105).
Muscle potassium levels are decreased on a ketogenic diet as a result of glycogen depletion (104)
further implicating glycogen depletion as one source of fatigue on a ketogenic diet.
Additionally a glycogen-SR complex has been proposed (106) such that depletion of
glycogen may physically impair conduction of signals to the muscle. Finally, the intracellular
dehydration seen with a ketogenic diet may also affect strength levels (52,104,107,108).
Summary
Regardless of the ultimate cause of fatigue during weight training, glycogen depletion has
the potential to decrease performance through one of several mechanisms. Until more research
is done, we can only speculate as to the exact cause of fatigue. For the purpose of the ketogenic
diet, the exact cause of fatigue is more an academic question than a practical one. It is a basic
physiological fact that Type II fibers require glycogen to function optimally. Therefore, a SKD
will eventually decrease performance as well as inhibit muscle growth. Individuals who wish to
weight train on a ketogenic diet will have to consume carbohydrates at some point.