SKELETAL MUSCLE FATIGABILITY AND MYOSIN HEAVY CHAIN FIBER TYPE IN RESISTANCE TRAINED MEN

Forty years ago, Thorstensson and Karlsson in 1976 described the link between muscle fatigability and fiber type, finding that more fast-twitch fibers were associated with a quicker onset of quadriceps fatigue. This provided the foundation for the Classic Thorstensson Test of fatigability and subsequent noninvasive fiber type prediction equation. This equation was developed with data from recreationally active (REC) men but has been implemented in participants with heterogeneous physical activity/exercise backgrounds. The accuracy of this approach in resistance trained (RET) men has not been established. Moreover, muscle fiber typing techniques have evolved considerably since this seminal work. Therefore, we reexamined this relationship using RET men and a more sensitive fiber typing method (single fiber myosin heavy chain [MHC] isoform classification). Fifteen RET men (age = 24.8 +/- 1.3 years) performed maximal knee extensions (via isokinetic dynamometry) to determine peak torque (PT) and quadriceps fatigue percentage (FP) after 30 and 50 repetitions. Vastus lateralis (VL) single fiber MHC type was determined and fibers were grouped as % Fast (expressing MHC IIa, IIa/IIX, or IIx; no MHC I containing fibers). Resistance trained men exhibited 46% greater PT (RET = 207 +/- 28 N.m vs. REC = 130 +/- 8 N.m) and 28% more % Fast (RET = 61 +/- 4% vs. REC = 44 +/- 4%) than REC men. Additionally, RET men had a relatively homogeneous FP (64 +/- 1%) ranging from 53 to 72%. No relationship was found between FP and MHC fiber type (R-2 = 0.01, p > 0.05). The Classic Thorstensson Test may not accurately estimate VL fiber type composition in RET men, highlighting the (a) unique phenotypical/functional adaptations induced by chronic RET and (b) the need for more sensitive cellular/molecular analyses in RET muscle.