Appropriate measurement of maximal voluntary contraction force of a single limb muscle or of a muscle group is important in clinical and research situations. To measure muscle force, one segment of an isometric measuring brace is fixed to a support and force is applied to the other. The output of this arrangement is affected by additional contributions such as pushing or pulling with the whole body. This paper quantifies the differences between measurements of torque produced by the biceps brachii with the brace fixed versus those produced when the elbow joint was isolated by suspending the brace from cables. No statistically significant differences were found between MVC values observed with the two methods within subjects. However, a statistically significant difference in EMG fatigue indices was observed and attributed to a different sharing of force production among different muscles in the two conditions. We conclude that different brace arrangements may lead to the same maximal force but to different rates of myoelectric manifestations of muscle fatigue since the effort may be shared differently among the muscles of synergic or stabilizing groups.