It may not be a Tina Turner song, but it's a question were going to look at in this article.
A lot of this discussion is really a deeper look at the energetic theory, specifically, how blood flow affects fatigue, and then how fatigue affects recruitment and contraction.
In several occlusion studes, where mechanical pressure was used to restrict blood flow, subjects were able to increase strength and muscle size with lighter weights as well as non-resistricted subjects, who used higher resistances. (1,2) One particular study (1) observed a 7.7% increase in the quadriceps CSA within two weeks using a mere 20% of 1RM! Fibril size, glycogen, GH and IGF1 are also significantly increased during training with occluded blood flow. (1,7)
How can such a light weight actually stimulate strength and size increases? Where is the coveted mechancial tension? Could it be that the load on the bar is not 'the stimulus', but, is more of a means to an end?
The reason such light resistances still have the ability to stimulate size and strength increases, is thought to be from the following reasons; The extremely high 'backing up' of fatigue products, in addition to a lack of oxygen, reduces the fiber's ability to generate force very rapidly.(3) This leads to full activation levels (full recruitment and high rate coding) leading to tetanic contractions. (2) Further, muscle cells will consume ATP at rates equal to higher resistances. The importance of this was spoke of in the energetics article. To borrow a section,
"Also, a measurement variable, termed TTI (Tension Time Integral), where the average true tension is computated, (4,5) is directly proportional to the energetic cost of a contraction. Even tension signaling factors, such as P70 (70-kDa ribosomal S6 kinase, an important marker for hypertrophy) (9,10) can be tracked by calculating the TTI of the contractions. One can then extrapolate, that ATP turnover (the energetic cost/time) is proportional to the stimulation induced by resistance based contractions. (5,6,7,8,21)"
Interestingly, the actual level of ATP can be lower in ischemic conditions (3), possibly leading to a significant repression of mTOR. However, the slower twich fibers, being more dependant on oxygen, may be affected to a greater extent. (4,9)
In normal anisometric contractions, blood flow can be limited or even occluded at higher contraction tensions, and/or frequencies, if the whole muscle tension is high enough. (5) Isometric contractions can have the most profound effect due to the absence of lower tension periods. Higher force and increased frequencies do stimulate a need for increased blood flow, and until muscle pressure limits this, it does indeed increase. (5,6) Both peak tension and work performed are indicators of this. (6)
Besides external pressure (as with a cuff), internal muscle pressure can, and does occlude blood flow. With standard anisometric contractions variations in tension throughout the range of motion cause these effects to be intermitant. However, with isometric contractions, tension is maintained and non-variable, allowing a longer time period of impeded blood flow. Studies have tested various muscles with isometric contractions to find a minimum tension level where blood flow is occluded. Here are is some of the data,
There are two main points we can learn and/or use from this information;