Written by Michael J. Rudolph, Ph.D.
16 July 2019

19kickstart

Kickstart Muscle Gains

Rep Out for Strength & Size

 

 

Effective strength training typically involves lifting heavy weights at approximately 80 to 100 percent of your one-repetition maximum (1RM) within the two- to five-repetition range. This level of intensity is ideal for strength gains because if preferentially activates fast-twitch muscle fibers. Fast-twitch fibers do exactly what the name implies, contract faster, giving them the ability to generate much more force, or strength, than the other major type of muscle fiber, slow-twitch.

In addition to fast-twitch fibers generating more strength, the recruitment of fast-twitch fibers stimulates anabolic-signaling proteins, such as mTOR, more potently— increasing muscle protein synthesis to a greater degree, ultimately resulting in a greater capacity for muscle growth.1 Altogether, this makes the recruitment of fast-twitch fibers an essential requirement to maximize muscle hypertrophy and strength in response to weight training.

On the other hand, slow-twitch muscle fibers generate considerably lower amounts of force and don’t grow anywhere near as well as fast-twitch muscle fibers do, making them a suboptimal target for weight-training protocols striving to increase muscle size and strength.2 Consequently, attempts have been made within the scientific community to identify novel training techniques that more potently activate fast-twitch muscle fibers, promoting more robust gains in muscle growth and strength.

 

Kaatsu Training Triggers Strength While Using Light Weights

While exercise intensity, i.e., heavy weight, strongly stimulates muscle strength by activating the more powerful fast-twitch muscle fibers, there is good evidence indicating that other types of stimuli, such as metabolic stress, can also trigger extensive strength gains— in large part by recruiting fast-twitch muscle fibers. This effect has clearly been shown in earlier studies investigating the Kaatsu training method. This mode of training requires lifting lighter loads of roughly 30 percent of your 1RM to muscle failure while restricting blood flow to the exercised muscle groups.

The restricted blood flow during Kaatsu training causes the muscle cell to produce energy without oxygen, or anaerobically. This preferentially activates fast-twitch fibers over slow-twitch fibers, because fast-twitch muscle fibers prefer anaerobic respiration over aerobic respiration.3,4In addition, the greater level of anaerobic respiration increases metabolic stress by producing more lactic acid, which rapidly decreases the muscle cell’s ability to contract. As a result, additional fast-twitch muscle cells are activated in order to maintain muscular contraction, effectively increasing fast-twitch fiber activity. In fact, several studies have demonstrated that Kaatsu training increases muscle activation to levels found when performing high-intensity training.5,6,7Furthermore, this form of training has also been shown to produce considerable gains in size and strength8,9, despite using such a low level of intensity.

So taking everything into consideration, it seems that the enhanced recruitment of fast-twitch fibers caused by the added metabolic stress induced during Kaatsu training is the primary reason why this type of exercise triggers a very potent hypertrophic response.

 

Pre-exhaust Weaker Muscle Fibers by Repping-out First

Since different training methods, such as Kaatsu training, specifically increase fast-twitch muscle fiber recruitment by a different mechanism that doesn’t involve the use of heavy weights and, more importantly, results in considerable gains in size and strength, a group of scientists at North University in Brazil investigated another training approach to see if it too could increase fast-twitch muscle fiber recruitment.10

This approach used by the Brazilian scientists boosts fast-twitch fiber activity in a somewhat counterintuitive way, by exclusively pre-exhausting slow-twitch muscle fibers before performing sets with heavier weight. It accomplishes this with an initial set performed to exhaustion with very light weight, which preponderantly activates, and therefore exhausts, only slow-twitch muscle fibers. The pre-exhaustion of the slow-twitch fibers is key because normally, slow- and fast-twitch muscle fibers are both activated during the early stages of most lifts when the intensity level is relatively low. This, of course, effectively lowers the recruitment of fast-twitch fibers, because the slow-twitch muscle cells are contributing to some degree to the overall force production of the muscle, meaning less fast-twitch activity is necessary. Therefore, the ability of this approach to pre-exhaust slow-twitch fibers, thus mitigating their contribution to contraction, should plausibly increase fast-twitch fiber activation. This effect will be even more evident if all subsequent sets performed after pre-exhausting the slow-twitch fibers are done with heavier weights which, as previously mentioned, predominantly recruits fast-twitch muscle.

To see if they could induce this effect, Aguiar et al.10 had test subjects complete an initial set of knee extensions to complete exhaustion, using only 20 percent of their 1RM to specifically pre-exhaust slow-twitch muscle fiber fatigue. The test subjects then did several sets of knee extensions at high intensity. After eight weeks of training with this approach, each subject had their strength tested with a 1RM lift and their muscles probed with an MRI machine to visualize any newly formed muscle mass.

The results of the study clearly showed that the group performing the pre-exhaustion set experienced a much greater increase in muscle growth, along with a superior increase in their 1RM, relative to the control group that did no pre-exhaustion work. Altogether, this study demonstrated that an initial set to exhaustion with very light weight conceivably exhausted a significant percentage of slow-twitch fibers within the target muscle groups, which likely resulted in the additional recruitment of fast-twitch muscle fibers, ultimately promoting superior increases in muscle hypertrophy and strength.  

In conclusion, the use of unconventional training approaches that more productively trigger fast-twitch muscle fiber activity should more potently promote hypertrophy and strength increases, even without the use of heavy weights in some instances. Furthermore, the use of these unorthodox training methods calls for the acceptance of a few nontraditional concepts on the subject of weight training, such as the notion that the use of heavy weights is absolutely required for strength gains.

Now, don’t get me wrong— I’m not saying that heavy weight training is unimportant. In fact, it is definitely the best method for strength development. However, no serious lifter hits heavy weights all year long. So, during your next deloading phase or just to change things up a bit, you might want to rep out with light weight before hitting heavier weight— in order to more forcefully trigger fast-twitch muscle fibers and boost gains in muscle size and strength.

For most of Michael Rudolph’s career he has been engrossed in the exercise world as either an athlete (he played college football at Hofstra University), personal trainer or as a research scientist (he earned a B.Sc. in Exercise Science at Hofstra University and a Ph.D. in Biochemistry and Molecular Biology from Stony Brook University). After earning his Ph.D., Michael investigated the molecular biology of exercise as a fellow at Harvard Medical School and Columbia University for over eight years. That research contributed seminally to understanding the function of the incredibly important cellular energy sensor AMPK— leading to numerous publications in peer-reviewed journals including the journal Nature. Michael is currently a scientist working at the New York Structural Biology Center doing contract work for the Department of Defense on a project involving national security.

 

References:

1. Koopman R, Zorenc AH, et al. Increase in S6K1 phosphorylation in human skeletal muscle following resistance exercise occurs mainly in type II muscle fibers. Am J Physiol Endocrinol Metab 2006;290, E1245-1252.

2. Malisoux L, Francaux M, et al. (2006). Stretch-shortening cycle exercises: an effective training paradigm to enhance power output of human single muscle fibers. J Appl Physiol 1985;100, 771-779.

3. Idstrom JP, Subramanian VH, et al. Energy metabolism in relation to oxygen supply in contracting rat skeletal muscle. Fed Proc 1986;45, 2937-2941.

4. Katz A and Sahlin K. Effect of decreased oxygen availability on NADH and lactate contents in human skeletal muscle during exercise. Acta Physiol Scand 1987;131, 119-127.

5. Takarada Y, Nakamura Y, et al. (2000). Rapid increase in plasma growth hormone after low-intensity resistance exercise with vascular occlusion. J Appl Physiol 1985;88, 61-65.

6. Takarada Y, Sato Y and Ishii N. Effects of resistance exercise combined with vascular occlusion on muscle function in athletes. Eur J Appl Physiol 2002;86, 308-314.

7. Takarada Y, Takazawa H, et al. (2000). Effects of resistance exercise combined with moderate vascular occlusion on muscular function in humans. J Appl Physiol 1985;88, 2097-2106.

8. Abe T, Kearns CF and Sato Y. (2006). Muscle size and strength are increased following walk training with restricted venous blood flow from the leg muscle, Kaatsu-walk training. J Appl Physiol 1985;100, 1460-1466.

9. Loenneke JP, Wilson JM, et al. Low intensity blood flow restriction training: a meta-analysis. Eur J Appl Physiol 2012;112, 1849-1859.

10. Aguiar AF, Buzzachera CF, et al. A single set of exhaustive exercise before resistance training improves muscular performance in young men. Eur J Appl Physiol 2015;115, 1589-1599.

 

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