The effects of static magnetic field (SMF) exposure on physiological factors affecting muscle performance were investigated during leg extension exercise at 2 different contraction intensities. 14 subjects participated in two double blind trials separated by one week and allocated by systematic rotation (sham or active arrangement 0.1 Tesla peak SMF intensity, 30 min pre-exposure), half (n=6, male and n=1, female) completed 10 sets of 15 dynamic contractions at 50% maximum lifted weight (1RM) (L), and the remaining subjects (n=7, male) completed 21 sets of 4 contractions at 90% 1RM (H). Custom-designed devices (sham or active) were attached to the quadriceps of the subject. Biomechanical data were collected using in-line force transducer and electrogoniometry, and rectus femoris activation was assessed using surface electromyography (EMG). Data normalised against 1RM values were averaged across the repetitions (n=15, L; n=4, H) for each set (n=10, L; n=21, H) and shown as population means±standard error of the mean (SEM). Data were analysed by repeated measures ANOVA, and post hoc paired Student’s t tests corrected for multiple comparisons using Holm-Šidák step-down procedure . Root mean square (RMS) amplitude of muscle EMG activity increased over time during sham trials for both protocols (20±7, L; 1±4, H; %); SMF exposure modified this response (3±4, L, P<0.001; -6±5, H; %). The mean frequency of the EMG power spectrum decreased over time to a greater degree in sham (-7±1, L; -2±2, H; %) than active trials (-3±5, L, P<0.05; 0±2, H; %). Mean power output per contraction was higher throughout active compared to sham trials although this reached significance only during H trials (4±1, L, 16±1, P<0.05, H; %). Functional efficiency (power per unit RMS EMG amplitude) decreased to a greater degree during sham (-28±1, L; -11±9, H; %) than active (-6±9, L, P<0.01; 9±6, P<0.05, H; %) trials. The magnitude of the differences between sham and active trials increased over time, presumably as fatigue developed. The differences in the temporal pattern of fatigue development between low- and high intensity exercise protocols may account for the smaller SMF effect on muscle EMG parameters in H compared to L trials. Muscle performance was preserved at both contraction intensities, by exposure to SMF. This effect appears to be mediated by alterations in the profile of fatigue development, as evidenced by the reduced change over time in muscle EMG parameters, power output and functional efficiency. It is possible that the magnetic field interacts with ionic membrane processes and muscle contractility. All procedures accord with current local guidelines and the Declaration of Helsinki. This study was supported by The Wellcome Trust.