Weight lifting recruits all three fiber types. Although Type I fibers have little anaerobic
potential for energy production, all fiber types will produce energy anaerobically (through the
degradation of ATP-CP and glycogen) during weight training. Thus, a carbohydrate-free diet
should negatively effect performance.
Fatigue during weight training can have one of three potential causes: metabolic (related to
depletion of fuel or accumulation of waste products), neural (due to impairment in nervous
system activation of the muscle), and non-metabolic (everything else).
Metabolic causes of fatigue
The potential metabolic causes of fatigue during weight training depend on the length of the
set being performed and the energy system which is being primarily used. During very short
weight training sets (less than 20 seconds), the metabolic cause of fatigue is most likely depletion
of creatine phosphate (CP). Although ATP never drops more than 20% below resting values even
at exhaustion (93), CP may be fully depleted after 20 seconds of maximal intensity exercise.
Although CP may be 96% resynthesized within 3 minutes (55), force production capacity
frequently takes longer to recover to normal, as much as 4-5 minutes (8). This suggests an
additional component of fatigue during this type of exercise (see neural fatigue below).
Lactic acid is not generated to any great degree during exercise of this short duration and is
unlikely to be a cause of fatigue. However, in general, lactic acid impairs muscle fiber
recruitment (8) which has implications for warm-ups and workout design. Raising lactic acid with
high repetition sets or high intensity aerobics will impair performance during short rep sets. To
the contrary, moving to your heaviest weights (and lowest rep sets) first and then performing
high reps sets afterwards will prevent high lactic acid levels from causing early fatigue. This is
further discussed in the section on proper warm-ups and weight training systems.
For longer weight training sets of 20-60 seconds, anaerobic glycolysis is activated relying
primarily on glycogen breakdown for ATP production. Intramuscular triglycerides may also play
a role in energy production, especially when short rest periods are used (91). However, fatigue
can occur even when glycogen stores are still fairly high. This indicates that, in general, there is
another cause of fatigue during weight training, most likely the buildup of metabolites that affect
force production (90).
During weight training of 20-60 seconds duration, the exact cause of fatigue is not known
(53). It is most likely related to buildup of lactic acid, H+ and other metabolites within the cell
that directly affect force production. However, individuals suffering from McArdle’s disease (a
total inability to use glycogen for fuel) show fatigue during high intensity exercise with no change
in lactic acid levels suggesting other causes of fatigue (94).
Increases in H+ lowers muscle and blood pH both of which inhibit force production in the
muscle (95,96). During exercise, lactate and H+ is removed from the muscle cell where it can be
buffered in the bloodstream by bicarbonate and myoglobin (97). Lowering blood pH decreases
performance (98), while increasing the buffering capacity of the blood by ingesting sodium
bicarbonate improves performance (95,99,100).