Knee angle-dependent oxygen consumption of human quadriceps muscles during maximal voluntary and electrically evoked contractions.

Fatigability and muscle oxygen consumption (mVO(2)) during sustained voluntary isometric knee extensions are less at extended (30 degrees knee angle; 0 degrees , full extension) versus flexed knee angles (90 degrees). This lower energy consumption may partially result from lower neural activation at extended knee angles. We hypothesized a smaller difference in mVO(2) between extended and flexed knee angles during electrical stimulation, which guaranteed maximal activation, than during maximal voluntary contractions (MVC). In eight healthy young males, MVC extension torque was obtained at 30 degrees, 60 degrees and 90 degrees knee angles. mVO(2) of the rectus femoris (RF), vastus lateralis (VL) and medialis muscle was measured using near-infrared spectroscopy during tetanic (10 s) and maximal voluntary (15 s) contractions (MVC(15)). For electrically induced contractions, steady state mVO(2) was reached at similar (P > 0.05) times after torque onset (4.6 +/- 0.7 s) at all knee angles. In contrast, during MVC(15) at 30 degrees mVO(2) was reached at 7.1 +/- 1.1 s, significantly later compared to 60 degrees and 90 degrees knee angles. The knee angle dependent differences in mVO(2) were not lower in electrically induced contractions (as hypothesised) but were similar as in voluntary contractions. Normalized mVO(2) at 30 degrees (percentage 90 degrees knee angle) was 79.0 +/- 9.4% (across muscles) for electrically induced and 79.5 +/- 7.6% (across muscles) for voluntary contractions (P < 0.05). We conclude that the slower onset of mVO(2) during voluntary effort at 30 degrees may have been due to a lower maximal activation. However, because steady state mVO(2) both during electrically induced and voluntary contractions was approximately 20% less at extended versus flexed knee angles, the causes for the lower mVO(2) must reside within the muscle itself.

Conventionally assessed voluntary activation does not represent relative voluntary torque production.

The ability to voluntarily activate a muscle is commonly assessed by some variant of the twitch interpolation technique (ITT), which assumes that the stimulated force increment decreases linearly as voluntary force increases. In the present study, subjects (n = 7) with exceptional ability for maximal voluntary activation (VA) of the knee extensors were used to study the relationship between superimposed and voluntary torque. This includes very high contraction intensities (90-100%VA), which are difficult to consistently obtain in regular healthy subjects (VA of approximately 90%). Subjects were tested at 30, 60, and 90 degrees knee angles on two experimental days. At each angle, isometric knee extensions were performed with supramaximal superimposed nerve stimulation (triplet: three pulses at 300 Hz). Surface EMG signals were obtained from rectus femoris, vastus lateralis, and medialis muscles. Maximal VA was similar and very high across knee angles: 97 +/- 2.3% (mean +/- SD). At high contraction intensities, the increase in voluntary torque was far greater than would be expected based on the decrement of superimposed torque. When voluntary torque increased from 79.6 +/- 6.1 to 100%MVC, superimposed torque decreased from 8.5 +/- 2.6 to 2.8 +/- 2.3% of resting triplet. Therefore, an increase in VA of 5.7% (from 91.5 +/- 2.6 to 97 +/- 2.3%) coincided with a much larger increase in voluntary torque (20.4 +/- 6.1%MVC) and EMG (33.9 +/- 6.6%max). Moreover, a conventionally assessed VA of 91.5 +/- 2.6% represented a voluntary torque of only 79.6 +/- 6.1%MVC. In conclusion, when maximal VA is calculated to be approximately 90% (as in regular healthy subjects), this probably represents a considerable overestimation of the subjects' ability to maximally drive their quadriceps muscles.

Knee extensor muscle oxygen consumption in relation to muscle activation.

Recently, fatigability and muscle oxygen consumption (mVO(2)) during sustained isometric contractions were found to be less at shorter (30 degrees knee angle; 0 degrees = full extension) compared to longer knee extensor muscle lengths (90 degrees ) and, at low torques, less in the rectus femoris (RF) muscle than in the vastus lateralis and medialis. In the present study we hypothesized that these findings could be accounted for by a knee angle- and a muscle-dependent activation respectively. On two experimental days rectified surface EMG (rsEMG) was obtained as a measure of muscle activation in nine healthy young males. In addition, on day 1 maximal torque capacity (MTC) was carefully determined using superimposed nerve stimulation on brief high intensity contractions (> 70%MVC) at 30, 60 and 90 degrees knee angles. On day 2, subjects performed longer lasting isometric contractions (10-70%MTC) while mVO(2) was measured using near-infrared spectroscopy (NIRS). At 30 degrees , maximal mVO(2) was reached significantly later (11.0 s +/- 6.5 s) and was 57.9 +/- 8.3% less (average +/- SD, across intensities and muscles) than mVO(2) at 60 and 90 degrees (p < 0.05). However, rsEMG was on average only 18.0 +/- 11.8% (p = 0.062) less at the start of the contraction at 30 degrees . At 10%MTC at all knee angles, maximal mVO(2) of the RF occurred significantly later (28.8 +/- 36.0 s) and showed a significantly smaller increase in rsEMG compared to both vasti. In conclusion, it is unlikely that the tendency for less intense muscle activation could fully account for the approximately 60% lower oxygen consumption at 30 degrees , but the later increase in RFmVO(2) seemed to be caused by a less strong activation of the RF.

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.

Initial phase of maximal voluntary and electrically stimulated knee extension torque development at different knee angles.

We investigated the capacity for torque development and muscle activation at the onset of fast voluntary isometric knee extensions at 30, 60, and 90 degrees knee angle. Experiments were performed in subjects (n = 7) who had high levels (>90%) of activation at the plateau of maximal voluntary contractions. During maximal electrical nerve stimulation (8 pulses at 300 Hz), the maximal rate of torque development (MRTD) and torque time integral over the first 40 ms (TTI40) changed in proportion with torque at the different knee angles (highest values at 60 degrees ). At each knee angle, voluntary MRTD and stimulated MRTD were similar (P < 0.05), but time to voluntary MRTD was significantly longer. Voluntary TTI40 was independent (P > 0.05) of knee angle and on average (all subjects and angles) only 40% of stimulated TTI40. However, among subjects, the averaged (across knee angles) values ranged from 10.3 +/- 3.1 to 83.3 +/- 3.2% and were positively related (r2 = 0.75, P < 0.05) to the knee-extensor surface EMG at the start of torque development. It was concluded that, although all subjects had high levels of voluntary activation at the plateau of maximal voluntary contraction, among subjects and independent of knee angle, the capacity for fast muscle activation varied substantially. Moreover, in all subjects, torque developed considerably faster during maximal electrical stimulation than during maximal voluntary effort. At different knee angles, stimulated MRTD and TTI40 changed in proportion with stimulated torque, but voluntary MRTD and TTI40 changed less than maximal voluntary torque.