Muscle fiber composition and respiratory capacity in triathletes

In an effort to describe the skeletal muscle characteristics of trained triathletes, biopsies were obtained from the gastrocnemius, vastus lateralis, and posterior deltoid muscles of 11 triathletes, and 4 normally active controls. Each specimen was analyzed for muscle fiber composition, respiratory capacity (QO₂), and citrate synthase activity. The mean (±SE) percentage of type I fibers for the triathletes was 59 (4.0), 63 (3.3), and 60 (2.8) in the gastrocnemius, vastus lateralis, and deltoid, respectively (P > 0.05). The mean (±SE) QO₂ values in the gastrocnemius(4.4 ± 0.3 ml O₂ · min^-1 · g^-1) and the vastus lateralis (4.1 ± 0.2) were not significantly different while the QO₂ values of the deltoid (3.6 ± 0.2) were significantly lower than the gastrocnemius (P < 0.05). The mean citrate synthase activity of the deltoid (27.7 ± 1.7 μmol · min^-1 · g^-1) was significantly lower than both the vastus lateralis (36.0 ± 3.2) and the gastrocnemius (45.8 ± 2.1) (P < 0.05). There was a high correlation between the percentage of type I fibers and the citrate synthase activity within the vastus lateralis (r = .760) and deltoid (r = .610) (P < 0.05) but not the gastrocnemius (r = .200). No significant relationships were observed between skeletal muscle characteristics and V̇O₂ max nor between skeletal muscle characteristics and performance. The results of this study demonstrate that: (1) these triathletes have a high percentage of type I fibers in all three muscle groups; (2) skeletal muscle characteristics were not highly related to laboratory or competitive performance; (3) despite a similar fiber composition, the deltoid appears to have a lower oxidative potential than the leg muscles, possibly due to the lesser use of the deltoid in normal daily activity.