Maximal voluntary force of bilateral and unilateral leg extension

The aims were: (1) to investigate whether the 10-20% lower force during bilateral (BL) as compared to unilateral (UL) leg extension could be due to a general inability to activate fully a large number of muscles simultaneously, (2) to analyse the EMG signal of the quadriceps femoris during leg extensions, (3) to study the BL/UL force ratio in extension of the knee, and (4) to study the BL/UL leg extension force ratio in untrained and trained subjects. A 10% lower maximal voluntary isometric force was demonstrated during BL as compared to UL leg extension. This force discrepancy did not change when a total arm load of 250 N was applied simultaneously. Nor did the absolute force levels change, which indicates that the lower BL leg extension force is not due to a general mechanism of reduced activation with an increased number of muscles recruited in maximal voluntary contractions. Integrated EMG activity, mean power frequency and root mean square value of the EMG amplitude did not differ between UL and BL leg extensions. The knee extension force was slightly greater (4%) during BL than UL contractions. These findings are arguments against a reduced activation of the knee extensor muscles being the cause of the lower bilateral leg extension force. No differences in BL/UL force ratio were noted between groups of untrained and trained subjects despite the fact that several of the trained groups do different forms of BL leg extensions regularly. Thus, it does not appear that training readily affects the BL/UL leg extension force ratio.