Muscle health and performance in monozygotic twins with 30 years of discordant exercise habits.

INTRODUCTION: Physical health and function depend upon both genetic inheritance and environmental factors (e.g., exercise training).

PURPOSE: To enhance the understanding of heritability/adaptability, we explored the skeletal muscle health and physiological performance of monozygotic (MZ) twins with > 30 years of chronic endurance training vs. no specific/consistent exercise.

METHODS: One pair of male MZ twins (age = 52 years; Trained Twin, TT; Untrained Twin, UT) underwent analyses of: (1) anthropometric characteristics and blood profiles, (2) markers of cardiovascular and pulmonary health, and (3) skeletal muscle size, strength, and power and molecular markers of muscle health.

RESULTS: This case study represents the most comprehensive physiological comparison of MZ twins with this length and magnitude of differing exercise history. TT exhibited: (1) lower body mass, body fat%, resting heart rate, blood pressure, cholesterol, triglycerides, and plasma glucose, (2) greater relative cycling power, anaerobic endurance, and aerobic capacity (VO2 max), but lower muscle size/strength and poorer muscle quality, (3) more MHC I (slow-twitch) and fewer MHC IIa (fast-twitch) fibers, (4) greater AMPK protein expression, and (5) greater PAX7, IGF1Ec, IGF1Ea, and FN14 mRNA expression than UT.

CONCLUSIONS: Several measured differences are the largest reported between MZ twins (TT expressed 55% more MHC I fibers, 12.4 ml/kg/min greater VO2 max, and 8.6% lower body fat% vs. UT). These data collectively (a) support utilizing chronic endurance training to improve body composition and cardiovascular health and (b) suggest the cardiovascular and skeletal muscle systems exhibit greater plasticity than previously thought, further highlighting the importance of studying MZ twins with large (long-term) differences in exposomes.