Calcium sensitivity of human single muscle fibers following plyometric training

Purpose: To study the effect of plyometric training on Ca²⁺ sensitivity and the influence of troponin T (TnT) isoforms on Ca ²⁺-activation properties in skinned human muscle fibers. Methods: Biopsies were obtained from the vastus lateralis of eight men before and after the training period. Chemically skinned fibers were evaluated regarding their Ca²⁺-activation properties and were classified according to their myosin heavy chain (MHC) contents and analyzed regarding their slow and fast TnT isoforms. Results: After training, significant improvements (P < 0.05) were found for static jump, countermovement jump, 6 × 5-m shuttle-run test, and leg-press performances. An 8% increase in the proportion of type IIa fibers (P < 0.05) was observed. Single-fiber diameters increased by 11% in type I (P < 0.01), 10% in type IIa (P < 0.001), and 15% in type IIa/IIx fibers (P < 0.001). Peak fiber force increased by 35% in type I (P < 0.001), 25% in type IIa (P < 0.001), and 57% in type IIa/IIx fibers (P < 0.01). The Ca²⁺-activation threshold was not altered by training, but the Ca²⁺ concentration required to elicit half-maximal activation showed a decreasing trend, with significant changes in type I fibers (P < 0.001). Cooperativity at low Ca²⁺ concentrations was increased in type I and type IIa/IIx fibers (P < 0.05). Type I fibers exclusively expressed slow TnT isoforms, and type II fibers were always associated with fast TnT isoforms, independent of training status. Therefore, changes in Ca²⁺ sensitivity after training could not be explained by differential fast or slow TnT isoform expression. Conclusion: Plyometric training increased single-fiber Ca²⁺ sensitivity, especially in type I fibers. These changes could not be explained by a modified TnT isoform expression pattern.