Identify a true statement about endurance training-induced changes in myosin isoforms.

• A. It Results In A Slow-To-Fast Shift In Myosin Isoforms
• B. It Results In A Fast-To-Slow Shift In Myosin Isoforms
• C. No Change In Isoforms
• D. It Increases Cardiac Myosin Concentration

Answer: (B)

Endurance training promotes a shift toward slower, more oxidative myosin heavy chain isoforms in skeletal muscle. With regular aerobic work, the muscle reduces expression of the fast, glycolytic isoforms and increases expression of the slower isoforms, moving from the fast end of the spectrum toward Type I (and more IIa) fibers. This change aligns contractile properties with sustained activity: slower cross-bridge cycling and lower ATPase activity support longer, fatigue-resistant contractions, while oxidative adaptations like higher mitochondrial density and capillarization accompany this shift. So the true statement is that endurance training induces a fast-to-slow shift in myosin isoforms. The idea of a slow-to-fast shift would not support endurance performance; a lack of isoform change contradicts well-documented adaptations; and a simple increase in cardiac myosin concentration misstates the primary skeletal muscle isoform changes seen with endurance training.