It’s “Heavy Day,” and you’ve just finished your first set of bench presses with a solid six reps. Excited about the prospect of increasing the weight on your next chest workout, you wait a full two minutes before beginning your next set expecting another solid six reps. Unfortunately, something happens. You only get four reps, and the weight feels like you just increased it by 50 pounds!
You sit up breathing heavily perplexed by your situation and blame your performance on having a “bad day.” Sound familiar?
So what happened?
Well, as most questions such as this, there is more than one cause. For the purposes of this article, discussion will be directed to several of the more prominent causes of muscular fatigue and inhibitors of performance.
The Neuromuscular Junction
The neuromuscular junction is the point at which the motor nerve connects to the muscle fibers it innervates. There is a gap between the muscle fiber and the nerve called a synapse across which the nervous impulse must travel. It does this through the excretion of a neurotransmitter, acetylcholine, which binds to receptors on the muscle side of the synapse causing the muscle cell membrane to depolarize and initiate contraction.
While you rest between sets, an enzyme called acetylcholinesterase on the muscle side of the synapse hydrolyzes (breaks down) the acetylcholine into acetate and choline. About 50% of this choline is taken back up into the nerve to make more acetylcholine to be used during your next set. Choline must also be taken up into the nerve cell from the surrounding extracellular fluid. Why is this important?
If you initiate another set before acetylcholinesterase has a chance to remove the acetylcholine from the muscle membrane or before the nerve has time to synthesize sufficient acetylcholine for the next contraction, the resulting depolarization on the muscle side of the synapse will be inefficient due to fewer receptors being available on the muscle membrane or insufficient acetylcholine excretion. In other words, a weaker contraction of the muscle results, and you get fewer reps than on the first set.
Lactic Acid Accumulation
During muscular contractions of at least 60-70% of maximum strength, blood flow to the muscle is shut off. This results in the utilization of anaerobic metabolism to create the ATP required for muscular contraction. During anaerobic glycolysis without the presence of oxygen a reserve energy substrate called lactic acid accumulates faster than it an be used and its buildup is directly related to the intensity and duration of the muscular contraction.
The accumulation of lactic acid in its “trapped” environment during contraction, lowers the pH of the muscle making it more acidic. This acidity, in turn, inhibits calcium binding which is necessary for muscular contraction and for several key enzymes used in anaerobic glycolysis to produce ATP.
If you initiate your next set before sufficient lactic acid is cleared by the circulation, the results are obvious. The acidity in the muscle prevents you from producing sufficient ATP for contraction, and even if you had enough ATP, contraction would still be reduced by the inability of the calcium to bind and initiate contraction. The end result is a set cut several reps short due to further accumulation of lactic acid.
Even in a perfect world, between set recovery takes much longer than you might think. During higher intensity exercise, creatine phosphate is utilized in large quantities to replenish depleting ATP stores. In a work-rest interval pattern typically associated with resistance exercise, creatine phosphate can approach a state of complete depletion. It may then take up to seven minutes for creatine phosphate stores to refill to over 95% of previous capacity.
How Long is Long Enough?
Regardless of your chosen repetition range, you can easily determine what your appropriate rest period should be over the next several workouts. You’ll need a training log and a stopwatch.
After your first true working set for each exercise, start the stopwatch. Give yourself a little extra rest time than you would typically take before you perform your next set. Document in your rest period in your training log, let’s say five minutes. Perform your next set and log the number of reps you completed. If you didn’t get the same number of reps on the second set, you didn’t rest long enough (assuming adequate nutrition). If you got the same number of repetitions as the first set, then you did rest long enough, but you may not have to rest the full five minutes. Make an adjustment of 30-60 seconds depending on your results and repeat your rest/set process. Based on your results, make any further adjustments in rest periods. If you must perform more than your typical number of sets per body part to really fine tune your rest periods, continue your “experiment” over several workouts for that same body part to avoid glycogen depletion and overtraining. Don’t forget to repeat this process for each body part and rep range as muscle size and fiber type will affect recovery capacity.
If you find that the rest periods you require are rather lengthy, arrange your workouts such that you train antagonistic body parts on the same day. By utilizing NFPT Antagonistic Multi-set Training, you can easily get sufficient rest between sets and shorten your total workout time. For further information about Antagonistic Multi-set Training consult your NFPT Study & Reference Manual.
Berne, R. Levy, M., Ed. Physiology. The C. V. Mosby Company. St. Louis. Farber, S. Neurorehabilitation: A Multisensory Approach. W. B. Saunders Company. St. Louis. Lamb, D. Physiology of Exercise: Responses and Adaptations. Macmillan Publishing Company. New York.