Antagonist muscle coactivation during isokinetic knee extension

The aim of the present study was to quantify the amount of antagonist coact ivation and the resultant moment of force generated by the hamstring muscle s during maximal quadriceps contraction in slow isokinetic knee extension, The net joint moment at the knee joint and electromyographic (EMG) signals of the vastus medialis, vastus lateralis, rectus femoris muscles (quadricep s) and the biceps femoris caput longum and semitendinosus muscles (hamstrin g) were obtained in 16 male subjects during maximal isokinetic knee joint e xtension (KinCom, ROM 90-10 degrees, 30 degrees.s(-1)). Two types of extens ion were performed: [1] maximal concentric quadriceps contractions and [2] maximal eccentric hamstring contractions. Hamstring antagonist EMG in [I] w ere converted into antagonist moment based on the EMG-moment relationships determined in [2] and vice versa. Since antagonist muscle coactivation was present in both [1] and [2] a set of related equations was constructed to y ield the moment/EMG relationships for the hamstring and quadriceps muscles, respectively, The equations were solved separately for every 0.05 degrees knee joint angle in the 90-10 degrees range of excursion (0 degrees = fall extension) ensuring that the specificity of muscle length and internal musc le lever arms were incorporated into the moment/EMG IG relationships establ ished, Substantial hamstring coactivation was observed during quadriceps ag onist contraction. This resulted in a constant level of antagonist hamstrin g moment of about 30 Nm throughout the range of motion. In the range of 30- 10 degrees from full knee extension this antagonist hamstring moment corres ponded to 30-75% of the measured knee extensor moment. The level of antagon ist coactivation was 3-fold higher for the lateral (Bfcl) compared to media l (ST) hamstring muscles. The amount of EMG crosstalk between agonist-antag onist muscle pairs was negligible (R-XY(2) < 0.02-0.06). The present data s how that substantial antagonist coactivation of the hamstring muscles may b e present during dow isokinetic knee extension. In consequence substantial antagonist flexor moments are generated. The antagonist hamstring moments p otentially counteract the anterior tibial shear and excessive internal tibi al rotation induced by the contractile forces of the quadriceps near full k nee extension. In doing so the hamstring coactivation is suggested to assis t the mechanical and neurosensory functions of the anterior cruciate Ligame nt (ACL).