When young men did leg extensions with a slow upward motion and drank a protein shake right after, their fast-twitch muscle fibers got noticeably bigger—but this didn’t happen if they did the slow downward motion or drank sugar water instead.
Scientific Claim
Whey protein supplementation combined with concentric resistance training is associated with increased type II muscle fiber cross-sectional area by approximately 25% after 12 weeks, an effect not observed with eccentric training or placebo, indicating a synergistic interaction between contraction mode and protein intake for fast-twitch fiber growth.
Original Statement
“Type II fiber CSA increased exclusively in the Whey-Conc group by 25 ± 7% (P < 0.01) and this was significantly greater than both the Whey-Ecc (P < 0.01) and Placebo-Conc (P < 0.05) groups.”
Evidence Quality Assessment
Claim Status
overstated
Study Design Support
Design supports claim
Appropriate Language Strength
association
Can only show association/correlation
Assessment Explanation
The abstract and conclusion use causal language ('accentuated when combining'), but without confirmed randomization, causation cannot be established. Only an association between the combination and hypertrophy is supported.
Evidence from Studies
Supporting (1)
Influence of exercise contraction mode and protein supplementation on human skeletal muscle satellite cell content and muscle fiber growth.
When people lifted weights using a pushing motion (concentric) and drank whey protein, their fast-twitch muscle fibers got significantly bigger — but this didn’t happen with pulling motions (eccentric) or with just sugar water.