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 90 degrees knee angles (full extension = 0 degree). 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 (30 degrees : 229.6 +/- 39.3 N.m vs. 90 degrees: 215.7 +/- 13.2 N.m). Endurance times, however, were significantly longer (P < 0.05) at 30 vs. 90 degrees (87.8 +/- 18.7 vs. 54.9 +/- 12.1 s, respectively) despite the CAR not differing between angles at torque failure (30 degrees: 0.95 +/- 0.05 vs. 90 degrees: 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 degrees : 56.5 +/- 12.5% vs. 90 degrees : 58.3 +/- 15.2%), whereas rectus femoris EMG activity was lower at 30 degrees (44.3 +/- 12.4%) vs. 90 degrees (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.