Muscle Failure - What causes it?

Scientists are still baffled by exactly what causes muscle failure during a hard, heavy set in the weight room. However, the researchers that provided us with previous, excellent muscle-building information on weight training’s dramatic effect on stimulating protein synthesis and amino acid transport rates, are now investigating mechanisms of muscular fatigue.

Using experienced, male weight trainers performing strict one arm preacher curls, MacDougall and colleagues examined muscle fatigue after 1 heavy set (80% of 1RM) and after three heavy sets. In brief, they found that upon failure to complete another rep, ATP stores were not diminished, but a 62% decline in phoshocreatine was witnessed. Muscle lactate levels did increase dramatically, altering muscle and blood pH (H+) levels. This interferes with the chemical binding of contractile protein and neural impulses.

Interestingly, muscle glycogen levels dropped 24% after only 3 sets of 10 or so reps. The authors concluded that fatigue is possibly caused by a combination of these three determinants and it is apparent glycogenolysis (breakdown and utilization of muscle glycogen) is the major energy delivery pathway for heavy resistance exercise and significant muscle glycogen depletion can occur in as little as three heavy sets!

Ref: Muscle substrate utilization and lactate production during weight lifting. Can. J. Appl. Physiol. 24(3): 209-215.

Probably the clearest explanation can be seen from work by Hogan and colleagues at the university of California-San Diego and Auburn University. Their research investigated lactic acid accumulation (that burning in mucles when you reach failure) and pH levels.

Now, pH levels are pretty involved chemistry, however all we need to be concerned with is the fact muscle and blood pH are strictly controlled within narrow ranges. Physiological (normal) pH is 7.4. When we lift heavy weights large amounts of glucose are utilized to generate ATP (the energy molecule) very quickly. To obtain this large amount of energy the glucose molecules are split in half to produce two molecules of lactic acid.

The research on working muscle showed that accumulation of lactic acid causes pH levels to drop. A lower pH means an increase in Hydrogen ions in the blood and/or muscle (that’s what the ‘H’ stands for). This is shown to interfere with the contractile force of muscle fibers, gradually preventing further contraction until movement ceases. This is why, try as hard as you might, things grind to a halt for your last rep. This is why endurance athletes try bi-carbonate loading. It tends to ‘buffer’ (reduce) the increase in hydrogen ions and keep blood pH levels within the narrow ranges thus delaying fatigue.