The role of two-joint muscles in complex movements is still under discussion. Therefore, the aim of this study was to determine activation patterns of such muscles with respect to localisation of force transmission as well as force level. For this in 24 healthy subjects biceps brachii and rectus femoris muscles were investigated by multichannel monopolar surface EMG (SEMG). Electrodes were arranged in two rows over the muscles, each consisting of 8 and 14 electrodes, respectively. For every muscle proximal and distal force transmissions were applied with loads of 1, 2, 4, 8, 12, and 16 kg for 10 s of static contraction. Highest amplitude levels were always localized near the force transmission site, more pronounced for rectus femoris muscle. Increasing load except for distal force transmission of rectus femoris did not systematically change relative SEMG amplitude differences between distal and proximal muscle regions. For rectus femoris together with increasing force also relative differences between both muscle regions increased, but this effect might be caused by neighbouring muscles, i.e. crosstalk.

Therefore, depending on force initiation two-joint muscles use variable activation strategies which are independent from load level. This qualifies two-joint muscles more towards a tuning function instead of isolated force production.