Département d'Education Physique et de Réadaptation, Faculté de Médecine, Université Catholique de Louvain, Louvain-la-Neuve, Belgium Submitted 23 August 2005 ; accepted in final form 24 November 2005 Functional performance of lower limb muscles and contractile properties of chemically skinned single muscle fibers were evaluated before and after 8 wk of maximal effort stretch-shortening cycle (SSC) exercise training. Muscle biopsies were obtained from the vastus lateralis of eight men before and after the training period. Fibers were evaluated regarding their mechanical properties and subsequently classified according to their myosin heavy chain content (SDS-PAGE). After training, maximal leg extensor muscle force and vertical jump performance were improved 12% ( P < 0.01) and 13% ( P < 0.001), respectively. Single-fiber cross-sectional area increased 23% in type I ( P < 0.01), 22% in type IIa ( P < 0.001), and 30% in type IIa/IIx fibers ( P < 0.001). Peak force increased 19% in type I ( P < 0.01), 15% in type IIa ( P < 0.001), and 16% in type IIa/IIx fibers ( P < 0.001). When peak force was normalized with cross-sectional area, no changes were found for any fiber type. Maximal shortening velocity was increased 18, 29, and 22% in type I, IIa, and hybrid IIa/IIx fibers, respectively ( P < 0.001). Peak power was enhanced in all fiber types, and normalized peak power improved 9% in type IIa fibers ( P < 0.05). Fiber tension on passive stretch increased in IIa/IIx fibers only ( P < 0.05). In conclusion, short-term SSC exercise training enhanced single-fiber contraction performance via force and contraction velocity in type I, IIa, and IIa/IIx fibers. These results suggest that SSC exercises are an effective training approach to improve fiber force, contraction velocity, and therefore power. skinned fibers; fiber hypertrophy; plyometry; passive stretch Address for reprint requests and other correspondence: D. Theisen, 1 Place P. de Coubertin, B-1348 Louvain-La-Neuve, Belgium (e-mail: daniel.theisen{at}edph.ucl.ac.be )