Muscle health and performance in monozygotic twins with 30 years of discordant exercise habits.
Level 4 - case-series / case-control
Single case study of one monozygotic twin pair (Level 4).
PubMed 30006671 · doi:10.1007/s00421-018-3943-7
What was done
One pair of 52-year-old male monozygotic twins discordant for more than 30 years of exercise habits—one trained in chronic endurance exercise (TT) and one untrained with no consistent exercise (UT)—underwent physiological testing. The authors measured anthropometrics, blood profiles, cardiovascular and pulmonary function, skeletal muscle size, strength, power, fiber type composition (MHC isoforms), AMPK protein expression, and mRNA expression of PAX7, IGF1Ec, IGF1Ea, and FN14.
What was found
Compared to UT, TT demonstrated 8.6% lower body fat percentage, 12.4 ml/kg/min greater VO2 max, 55% more MHC I (slow-twitch) fibers, and fewer MHC IIa fibers. TT also had lower body mass, resting heart rate, blood pressure, cholesterol, triglycerides, and plasma glucose, as well as higher relative cycling power, anaerobic endurance, AMPK protein expression, and PAX7, IGF1Ec, IGF1Ea, and FN14 mRNA expression. However, TT showed lower muscle size, lower muscle strength, and poorer muscle quality than UT.
Why it matters
By isolating genetic background across three decades, this study provides an extreme human model demonstrating the substantial phenotypic plasticity of cardiorespiratory capacity, fiber type distribution, and metabolic markers in response to chronic endurance exercise.
Limits
The study is limited to a single twin pair (n = 2), precluding statistical generalization. Uncontrolled lifestyle differences across 30 years, including non-exercise physical activity, dietary intake, and environmental exposures, could not be experimentally standardized.
Cited by
- contradicts Monozygotic twin research shows that individuals who do not lift weights exhibit lower muscle quality and lose power and speed more frequently as they age.