When the whole leg got bigger from training, it was because the fast-twitch muscle fibers got larger—but only if the men did the upward lifting motion. If they did the slow lowering motion, the leg got bigger for other reasons, not because the fibers grew.
Scientific Claim
Whole muscle hypertrophy measured by MRI is associated with type II fiber growth only after concentric training, not eccentric training, suggesting that concentric exercise promotes muscle growth primarily through fiber enlargement, while eccentric training may involve other mechanisms like sarcomere addition.
Original Statement
“Significant correlations were observed in the Conc leg for both mean fiber (r = 0.45, P < 0.05) and type II fibers (r = 0.56, P < 0.01), while for the Ecc leg, no association was observed with mean fiber (r = −0.03, P = 0.90) or type II fibers (r = 0.04, P = 0.87).”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design supports claim
Appropriate Language Strength
association
Can only show association/correlation
Assessment Explanation
The claim correctly uses correlational language and accurately reflects the statistical relationships observed. No causal claims are made, and the design supports this type of inference.
Evidence from Studies
Supporting (1)
Influence of exercise contraction mode and protein supplementation on human skeletal muscle satellite cell content and muscle fiber growth.
This study found that lifting weights by pushing up (concentric) makes the big muscle fibers grow bigger, which is why your whole muscle gets larger. But lowering weights slowly (eccentric) doesn’t make those fibers grow bigger—even though your muscle still gets stronger, maybe by adding more tiny parts inside the fibers instead.