“Variability in Muscle Size and Strength Gain After Unilateral Resistance Training”, Monica J. Hubal, Heather Gordish-Dressman, Paul D. Thompson, Thomas B. Price, Eric P. Hoffman, Theodore J. Angelopoulos, Paul M. Gordon, Niall M. Moyna, Linda S. Pescatello, Paul S. Visich, Robert F. Zoeller, Richard L. Seip, Priscilla M. Clarkson2005-06 ()⁠:

Background: This study assessed variability in muscle size and strength changes in a large cohort of men and women after a unilateral resistance training program in the elbow flexors.

A secondary purpose was to assess sex differences in size and strength changes after training.

Method: 585 subjects (342 women, 243 men) were tested at one of 8 study centers. Isometric (MVC) and dynamic strength (one-repetition maximum (1RM)) of the elbow flexor muscles of each arm and magnetic resonance imaging (MRI) of the biceps brachii (to determine cross-sectional area (CSA)) were assessed before and after 12 week of progressive dynamic resistance training of the nondominant arm.

Results: Size changes ranged from −2 to +59% (−0.4 to +13.6 cm2), 1RM strength gains ranged from 0 to +250% (0 to +10.2 kg), and MVC changes ranged from −32 to +149% (−15.9 to +52.6 kg).

Coefficients of variation were 0.48 and 0.51 for changes in CSA (p = 0.44), 1.07 and 0.89 for changes in MVC (p < 0.01), and 0.55 and 0.59 for changes in CSA (p < 0.01) in men and women, respectively. Men experienced 2.5% greater gains for CSA (p < 0.01) compared with women. Despite greater absolute gains in men, relative increases in strength measures were greater in women versus men (p < 0.05). Conclusion:

Men and women exhibit wide ranges of response to resistance training, with some subjects showing little to no gain, and others showing profound changes, increasing size by over 10 cm2 and doubling their strength. Men had only a slight advantage in relative size gains compared with women, whereas women outpaced men considerably in relative gains in strength.

[Keywords: Hypertrophy, Gender Differences, Variation, 1RM, MRI]

Figure 1: Biceps cross-sectional area. Histogram of biceps cross-sectional area changes (relative to baseline) within each gender for the trained arm: Black bars denote responses of men while white bars denote responses of women.

…Furthermore, the number of subjects found to be outliers (±2 SD from the mean) were similar between genders. We found that 0.08% of both men (n = 2) and women (n = 3) were low responders, whereas 3% of men (n = 7) and 2% of women (n = 7) were high responders.

Figure 3: Isometric strength test. Histogram of isometric strength changes (relative to baseline) within each gender for the trained arm.: Black bars denote responses of men whereas white bars denote responses of women.

…No subject lost dynamic strength so that there were no low responders, whereas 3.4% of men (n = 8) and 2.6% of women (n = 9) were high responders.

…With regards to outliers, we found that 0.9% of men (n = 2) and 0.6% of women (n = 2) were low responders, whereas 3.6% of men (n = 8) and 3.8% of women (n = 12) were high responders.

Figure 3: Isometric strength test. Histogram of isometric strength changes (relative to baseline) within each gender for the trained arm: Black bars denote responses of men whereas white bars denote responses of women.

…Exercise training: Subjects underwent gradually progressive, supervised strength training of their nondominant arm in one of the 8 collaborating exercise sites…All training sessions were supervised and lasted ~45–60 min each.

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Variability in Muscle Size and Strength Gain After Unilateral Resistance Training