Muscle activation and blood flow do not explain the muscle length-dependent variation in quadriceps isometric endurance

We investigated the role of central activation in muscle length-dependent endurance. Central activation ratio (CAR) and rectified surface electromyogram (EMG) were studied during fatigue of isometric contractions of the knee extensors at 30 and 90degrees knee angles ( full extension = 0degrees). Subjects ( n = 8) were tested on a custom-built ergometer. Maximal voluntary isometric knee extension with supramaximal superimposed burst stimulation ( three 100-mus pulses; 300 Hz) was performed to assess CAR and maximal torque capacity (MTC). Surface EMG signals were obtained from vastus lateralis and rectus femoris muscles. At each angle, intermittent ( 15 s on 6 s off) isometric exercise at 50% MTC with superimposed stimulation was performed to exhaustion. During the fatigue task, a sphygmomanometer cuff around the upper thigh ensured full occlusion ( 400 mmHg) of the blood supply to the knee extensors. At least 2 days separated fatigue tests. MTC was not different between knee angles (30degrees: 229.6 +/- 39.3 N . m vs. 90degrees: 215.7 +/- 13.2 N . m). Endurance times, however, were significantly longer ( P < 0.05) at 30 vs. 90° (87.8 +/- 18.7 vs. 54.9 +/- 12.1 s, respectively) despite the CAR not differing between angles at torque failure (30°: 0.95 +/- 0.05 vs. 90°: 0.96 +/- 0.03) and full occlusion of blood supply to the knee extensors. Furthermore, rectified surface EMG values of the vastus lateralis ( normalized to prefatigue maximum) were also similar at torque failure (30°: 56.5 +/- 12.5% vs. 90°: 58.3 +/- 15.2%), whereas rectus femoris EMG activity was lower at 30° (44.3 +/- 12.4%) vs. 90° (69.5 +/- 25.3%). We conclude that differences in endurance at different knee angles do not find their origin in differences in central activation and blood flow but may be a consequence of muscle length-related differences in metabolic cost.