In male athletes from different sports, isokinetic knee extensor, and in orienteers also plantar flexor peak torque (PT), contractional work (CW) and integrated surface electromyograms (iEMG) were analysed. Single contraction PT, CW and iEMG in sprinters and marathon runners were signi­ficantly correlated to the cross-sectional area (CSA) of m. quadriceps, and to the Type II fibre area of m. vastus lateralis. When correcting PT, CW and iEMG for CSA of m. quadriceps, such correlations were found only for Type IIA fibre area at 180° s~1. Elec- tromyographically, m. vastus lateralis (biopsied muscle) was representative for m. quadriceps. Calculated optimal mean power (CW s~1) and electrical efficacy (CW/iEMG) approximated for sprinters 450° s-1 and for marathon runners 270° s~1, i.e. velocities at or above the upper limit of the dynamometers. In orienteers, plantar flexor PT increased during winter training, but decreased during competitive season. Knee extensor PT increased over the whole year. At 30 and 60° s~1 only knee extensor PT was negatively associated with the running velocity at onset of blood lactate accu­mulation (VOBLA)- Changes in VOBLA during winter period were negatively associated with changes in knee extensor PT at 180° s~1. During competitive season, changes in Vobla were negatively associated with the ratio quality : quantity running. In ice hockey players PT varied non-systematically with training and games. The biopsy specimens of marathon runners showed irregular fibre shapes, an in­creased amount of connective tissue and central fibre nuclei, indicating an early strain disease or functional adaptation to extreme demands. During repetitive contractions in sprinters and marathon runners, fatigue, i.e. slope of decline in CW, was significantly associated with the Type II fibre area of m. vastus lateralis. For knee extensors of sprinters, ice hockey players and orienteers, a steep de­crease in CW/iEMG was observed. In contrast, knee extensors of marathon runners and plantar flexors of orienteers showed an almost unaltered CW/iEMG throughout the test. The knee extensor endurance level (CW/iEMG) was significantly correlated to the maximal oxygen uptake. In orienteers, an increase in endurance level of both tested muscle groups during winter training parallelled an increase in VOBLA and V02obla- In hockey players, fatigue and endurance pattern (CW and CW/iEMG) changed non-systematically with training and games. In conclusion, isokinetic measurements and iEMG reflect the structural properties of the knee extensor muscles in sprinters and marathon runners. The demonstrated characteristics and changes in leg muscle function in different groups of athletes apparently reflect varying demands from different sports activities. / <p>S. 1-31: sammanfattning, s. 33-84: 6 uppsatser</p> / digitalisering@umu