Extent of muscle inhibition as a function of knee angle

The present study was aimed at assessing muscle inhibition (MI) of the quadriceps muscles of healthy subjects as a function of knee angle. The extent of muscle activation and of twitch potentiation following maximal contractions at different knee angles were investigated. Six males and four females (mean age 29.5 +/- 6.2 yr) performed three maximal isometric knee extensions on a KinCom dynamometer with the right and left legs at 15, 30, 45, 60 and 90 degrees from full extension. MI was assessed using the interpolated twitch technique. This technique requires surface stimulation of the femoral nerve during the maximal contractions. MI was estimated by the amount of extra torque evoked by the superimposed twitch. Electromyographical (EMG) activity of the vastus lateralis muscle was measured in order to estimate muscle activation. Twitch potentiation was assessed at 5, 20, 60 and 120 s after the maximal contractions by applying a single electrical twitch to the relaxed muscle. Results revealed a strong dependency of MI on the knee angle tested; MI increased with increasing muscle lengths and was almost three times higher at 60 degrees than at 15 degrees of full extension. Muscle activation measured as the root mean square (RMS) values of the vastus lateralis EMG was the same for four out of the five muscle lengths measured; at a knee angle of 90 degrees , the RMS values were significantly higher. Twitch potentiation was observed at all knee angles following maximal contractions. Potentiation was highest immediately after contraction (i.e. 5 s after), and the amount of potentiation did not depend on the knee angle. Muscle activation and twitch potentiation only accounted for a small percentage of the differences in MI as a function of knee angle. It is suggested that increased patellofemoral pressure and increased ligament strain, which are highest between 45 and 60 degrees of full extension, might be responsible for the high MI measured at these knee angles. It is further hypothesized that the differences in MI as a function of muscle length are also associated with the shortening of the contractile elements during contractions, and the concomitant loss in force potential; a phenomenon which is probably more pronounced at short compared to long muscle lengths.