Tom McCullough

Many wonder why so many professional bodybuilders use partial rep training to grow.  These athletes feel they allow heavier weight to be used and the avoidance of full ROM seem to keep more tension on the muscle during the set. Science explains this type of training in studies that show partial ROM equal to full ROM except the total work done is slightly less. They also point out that partial ROM training will allow the lifter to add more weight on the bar thus increasing the intensity.


For many years it was thought that increases in muscle size were largely attributed to two factors: the mechanical load (tension on muscle) and the growth factor environment that the muscle experiences (external hormones such as testosterone and GH). So lifting heavy weight not only increases the amount of tension being placed on the muscle, it also increases natural anabolic hormones in our body such as testosterone and GH.

However, new research has come out that tested both animals and humans and it was discovered that it is very possible to induce muscular growth without these anabolic hormone concentrations being available.

Researchers at the Exercise Metabolism Group at McMaster University found that when human subjects were tested after weight training there was no change in GH, testosterone or IGF-1 concentrations in the 90-minute period following exercise. Furthermore there was no influence of training on the anabolic hormones measured. McMaster University researchers concluded, unilateral training induced local muscle hypertrophy only in the exercised limb, which occurred in the absence of testosterone, GH, or IGF-1 circulating levels. So apparently all of these by doing short rest periods combined with no more than 45 minute training do very little to help increase muscle size as we once thought.


Well let’s go back to our original two factors thought to increase muscle growth: mechanical load (tension on muscle) and the growth factor environment that the muscle experiences (external hormones such as testosterone and GH). Since the later were found to not be so important, let’s take a look at the mechanical load or the amount of tension we place on the muscle. Perhaps this is the key?

We do know that exercise does stimulate some GH during the first 30 minutes post-exercise. GH in turn causes the body to release IGF-1. However, Dr. Goldberg, of the University of London, who has spent much of his academic career studying how muscles grow, claims that after IGF-1 is produced in the body it splits in to a variant called MGF.

Dr. Goldberg noticed through his experiments on rats that even after he removed their pituitary so they could not produce GH , IGF-1 or testosterone, mechanical tension placed on their muscles still cause muscular growth. He also didn’t feed them and they still grew. What he did find in the rats exercised muscle was a huge increase in what he called MGF.

This study showed that mechanical overload increases muscle hypertrophy independent of testosterone, IGF-1, or GH. MGF seemed to be a very important substance because as the mechanical stress was increased so was the amount of MGF present in the exercised muscle. Not only was this true in rats but later on it was found to be the same in humans. So Dr. Goldberg concluded that mechanical load leading to an eccentric stretch, combined with muscle contraction, leads to a rapid increase in MGF.

Through further studies Dr. Goldberg determined increases in MGF in an exercised muscle starts the local repair of muscle damage by activating the muscle stem cells, as well as activating other anabolic processes. He further stated that maximal tension development through exercise leads to greater increases in muscle hypertrophy. In other words the more mechanical load you place on a muscle when exercising the more MGF which will be released. The more MGF released through heavy weight training the better recovery as well and a greater increase in muscle growth.


Past research has determined muscle hypertrophy to be greatest in the 8-12 repetition range; this is likely due to the combined effects of myofibrillar and sarcoplasmic hypertrophy being greatest at this rep range. This does not mean the athlete should train exclusively within this rep range. Low rep range training increases muscle fiber density and higher repetition training (up to 20) has also recently been shown to up-regulate the genes involved in hypertrophy.


Use partial ROM motion training because you are able to add more weight per rep and keep the tension on the muscle for the entire set. It is important that you use controlled negatives and very explosive positive movements when you lift.

Use longer rest periods. We know now that resting 2-5 minutes between sets enable us to maximize the amount of weight we lift. This means more mechanical load is used causing more damage to muscle tissue and greater release of MGF in the exercised muscle.

So don’t be afraid to use heavier weights and rest longer.


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Goldspink G: Gene expression in muscle in response to exercise. Journal Of Muscle Research And Cell Motility 2003, 24(2-3):121-126.

Goldberg AL, Etlinger JD, Goldspink DF, Jablecki C. Mechanism of work-induced hypertrophy of skeletal muscle. Med Sci Sports, 1975 Fall;7(3):185-98.

Kraemer A, Cafarelli et al.: ACSM Position Stand: Progression Models in Resistance Training for Healthy Adults. Medicine & Science in Sports & Exercise 2002, 34(2).

Wilborn CD, Taylor LW, Greenwood M, Kreider RB, Willoughby DS: Effects of different intensities of resistance exercise on regulators of myogenesis. Journal Of Strength And Conditioning Research / National Strength & Conditioning Association 2009, 23(8):2179-2187.

Wilkinson SB, Tarnopolsky MA, Grant EJ, Correia CE, Phillips SM. Hypertrophy with unilateral resistance exercise occurs without increases in endogenous anabolic hormone concentration. Eur J Appl Physiol, 2006.