Ok to clear up a few things...
Rest Between Sets
Muscles have one source of energy for contraction, that source is Adenosine Triphosphate (ATP). The body can produce ATP through three mechanisms, as shown below:
· By the phosphagen system - the chemical breakdown of phosphocreatine (PC) - an anaerobic mechanism
· By anaerobic glycolysis - an anaerobic mechanism
· By oxidative phosphorylation (Kreb's cycle and electron transport) - an aerobic mechanism
Certain fibers are optimized to use each method of energy production, as shown below:
Type I - Oxidative phosphorylation mechanism.
Type IIA - Both the phosphagen system and the anaerobic glycolysis mechanisms.
Type IIB - Phosphagen system.
All three of these take place primarily in the mitochondria. This is reason to stimulate Type I fibers and sarcoplasmic hypertrophy. Sarcoplasmic hypertrophy increases the volume of energy supplying tissue. More energy supplying tissue = more ATP = more energy for contraction! And we all know what that means - more muscle growth. This is an example of how sarcoplasmic hypertrophy indirectly contributes to muscular force production. After a heavy set of resistance exercise, your ATP will be momentarily depleted, in order to perform another set with maximal force you must allow enough time between sets for ATP to replenish. Here is a chart that shows the percentage of ATP replenishment after an maximal effort weight training set, assuming the muscle is completely at rest during the recovery period:
ATP replenishment times
Time since activity ended Percentage of ATP replenished
20 seconds 50.00%
40 seconds 75.00%
1 minute 87.50%
80 seconds 93.75%
100 seconds 96.88%
2 minutes 98.44%
140 seconds 99.22%
160 seconds 99.61%
3 minutes 99.81%
Again, only useful guidelines, not absolutes.
If a large amount of ATP production was required to perform an exercise (anaerobic glycolysis mechanism) then light activity would be of benefit because it would require the body to use some of the lactic acid to fuel the light activity which means less lactic acid in the muscle. This activity must be light enough not to involve any anaerobic mechanisms of energy production as this would only serve to further deplete ATP in the muscles prolonging the recovery period.
You must make sure that this activity would not be intense enough to use phosphagen or anaerobic glycolysis mechanisms for energy - this would deplete ATP further. Something such as a light walk between sets of squats would suffice.
Application
Based on this information, we can see that maximal ATP replenishment is essential if we are to maximally stimulate muscle hypertrophy. ATP replenishment times will vary slightly among individuals, but the 3-minute rule is a good guideline. If you would like to be more accurate, just experiment a bit to find out how long it takes your own ATP to replenish in various muscle groups. You know ATP has been replenished when you can complete another set with maximum weight (the weight you used in your previous set).