The effect of knee angle on maximal isometric torque rise was investigated with voluntary and electrically activated contractions of the knee extensors.

Seven healthy male subjects (19-40 years) with different training backgrounds, signed informed consent and the local ethics committee approved the study. Following a practice session, subjects were tested at 30 deg and 60 deg, and at 90 deg and 60 deg (random order) knee angles (full extension 0 deg) on separate days. Subjects were firmly secured in a chair with straps on hips and shoulders, and the lower leg was tightly strapped to a force transducer. Before each muscle contraction the upper leg was firmly strapped to the seat, this strap was released between contractions (3 min rest in between). Stimulation current was increased until force measured at the shin in response to burst femoral nerve stimulation (eight 100 Ás pulses applied at 300 Hz) levelled off. Thereafter, maximal voluntary extension and flexion torque was obtained. This was followed by three to six voluntary attempts to increase knee extension torque from a fully relaxed state, without any pre-flexion, as fast (and hard) as possible. Finally, two maximal extensions with superimposed burst stimulation were performed to calculate the maximal torque generating capacity (MTGC) of the knee extensors. Surface EMG electrode pairs were placed over the vastus lateralis, rectus and biceps femoris muscles. Angle effects (means±S.D.) were tested for significance (P < 0.05) with repeated measures ANOVA followed by Bonferroni post-hoc tests.

MTGC at 30 deg, 60 deg and 90 deg knee angle was 188±45 Nm, 315±64 Nm and 245±35 Nm (P < 0.05) respectively. Torque time integral over the first 40 ms of the voluntary fast contractions (TTI40) was similar (P = 0.62) across knee angles. When expressed as a percentage of TTI40 obtained with burst stimulation (an indication of the muscles maximum potential), the fastest and slowest subjects used 83.3±3.2 % (mean across angles) and 10.5±3.1 % of their muscles’ maximal potential, respectively. Furthermore, a positive linear relation was found (r = 0.87) between average (across extensor muscles and knee angles) rectified surface EMG (% MVC) of the knee extensors during the 40 ms before torque rise and fast voluntary TTI40.It was concluded that in contrast to maximal torque, maximal initial rate of voluntary isometric torque rise (absolute values) was independent of knee angle. In addition, there were substantial and consistent differences among subjects with respect to their ability for maximal voluntary activation of the knee extensors at the start of a fast isometric contraction.