Epstein 2014, The Sports Gene, ch6 "Superbaby, Bully Whippets, and the Trainability of Muscle", pg68:

[...description of Bouchard/HERITAGE Family Study...]

A series of studies in 2007 and 2008 at the University of Alabama-Birmingham's Core Muscle Research Laboratory and the Veterans Affairs Medical Center in Birmingham showed that individual differences in gene and satellite cell activity are critical to differentiating how people respond to weight training. Sixty-six people of varying ages were put on a four-month strength training plan-squats, leg press, and leg lifts-all matched for effort level as a percentage of the maximum they could lift. (A typical set was eleven reps at 75 percent of the maximum that could be lifted for a single rep.) At the end of the training, the subjects fell rather neatly into three groups: those whose thigh muscle fibers grew 50 percent in size; those whose fibers grew 25 percent; and those who had no increase in muscle size at all. A range from 0 percent to 50 percent improvement, despite identical training. Sound familiar? Just like the HERITAGE Family Study, differences in trainability were immense, only this was strength as opposed to endurance training. Seventeen weight lifters were "extreme responders," who added muscle furiously; thirty-two were moderate responders, who had decent gains; and seventeen were nonresponders, whose muscle fibers did not grow.* ...The Birmingham researchers took a HERITAGE-like approach in their search for genes that might predict the high satellite cell folk, or high responders, from the low responders to a program of strength training. Just as the HERITAGE and GEAR studies found for endurance, the extreme responders to strength training stood out by the expression levels of certain genes. Muscle biopsies were taken from all subjects before the training started, after the first session, and after the last session. Certain genes were turned up or down similarly in all of the subjects who lifted weights, but others were turned up only in the responders. One of the genes that displayed much more activity in the extreme responders when they trained was IGF-IEa, which is related to the gene that H. Lee Sweeney used to make his Schwarzenegger mice. The other standouts were the MGF and myogenin genes, both involved in muscle function and growth. The activity levels of the MGF and myogenin genes were turned up in the high responders by 126 percent and 65 percent, respectively; in the moderate responders by 73 percent and 41 percent; and not at all in the people who had no muscle growth.

• Bamman, Marcas M., et al. (2007). "Cluster Analysis Tests the Importance of Myogenic Gene Expression During Myofiber Hypertrophy in Humans". Journal of Applied Physiology, 102:2232-39.
• Petrella, John K., et al. (2008). "Potent Myofiber Hypertrophy During Resistance Training in Humans Is Associated with Satellite Cell-Mediated Myonuclear Addition: A Cluster Analysis". Journal of Applied Physiology, 104: 1736-42

Every similar strength-training study has reported a broad spectrum of responsiveness to iron pumping. In Miami's GEAR study, the strength gains of 442 subjects in leg press and chest press ranged from under 50 percent to over 200 percent. [108 GEAR study data was generously shared by members of the University of Miami research team.] A twelve-week study of 585 men and women, run by an international consortium of hospitals and universities, found that upper-arm strength gains ranged from zero to over 250 percent.

• Hubal, M. J., et al. (2005). "Variability in Muscle Size and Strength Gain After Unilateral Resistance Training". Medicine & Science in Sports & Exercise, 37(6):964-72