| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1Institute for Fundamental and Clinical Human Movement Sciences, Vrije University, Amsterdam, The Netherlands; and 2Institute for Biophysical and Clinical Research into Human Movement, Manchester Metropolitan University, Cheshire, United Kingdom
Submitted 27 December 2004 ; accepted in final form 14 March 2005
 |
ABSTRACT |
|---|
 TOP ABSTRACT METHODSRESULTSDISCUSSIONREFERENCES |
|---|
O2) of the rectus femoris, vastus lateralis, and vastus medialis muscle was obtained with near-infrared spectroscopy. mO2 was measured during maximal isometric contractions and during contractions at 10, 30, and 50% of maximal torque capacity. During all contractions, blood flow to the muscle was occluded with a pressure cuff (450 mmHg). mO2 significantly (P < 0.05) increased with torque and at all torque levels, and for each of the three muscles mO2 was significantly lower at 30° compared with 60° and 90° and mO2 was similar (P > 0.05) at 60° and 90°. Across all torque levels, average (± SD) mO2 at the 30° angle for vastus medialis, rectus femoris, and vastus lateralis, respectively, was 70.0 ± 10.4, 72.2 ± 12.7, and 75.9 ± 8.0% of the average mO2 obtained for each torque at 60 and 90°. In conclusion, oxygen consumption of the knee extensors was significantly lower during isometric contractions at the 30° than at the 60° and 90° knee angle, which probably contributes to the previously reported longer duration of sustained isometric contractions at relatively short muscle lengths. energy utilization; submaximal isometric contractions; voluntary activation; fatigue
Recently, Place et al. (31) related the lower fatigability at short muscle lengths to a greater twitch potentiation at short compared with a longer length after a voluntary sustained contraction of the knee extensors at 20% maximal voluntary contraction (MVC). They suggested that the longer endurance time at the shorter muscle length could at least partly be explained by a slower development of central fatigue during the sustained contraction at short compared with longer knee extensor length. However, in a recent study (21) our laboratory consistently found longer endurance times in the knee extensors during isometric contractions at 50% MVC at short (30° knee angle) compared with long (90° knee angle) muscle lengths, but central fatigue was only marginal and similar among knee angles in that study. In the same study of Kooistra et al. (21), blood flow was totally abolished during the contractions by an external pressure cuff, and therefore it was refuted that length-dependent differences in internal muscle pressure contributed to differences in muscle perfusion and hence fatigability at different muscle lengths. By elimination of length-dependent differences in muscle activation and perfusion, it is hypothesized in the present study that the lower fatigability at short muscle lengths will be related to a reduced energetic cost of isometric force production at short compared with longer muscle lengths, as was suggested several decades ago by Fitch and McComas (13). The problem with this hypothesis seems that it apparently already has been falsified in studies on isolated skinned fibers (37) and mouse muscles (29). The lower rates of energy consumption found at short muscle lengths in other studies on isolated preparations may have been caused by submaximal activation of the preparation at short lengths (8, 18). Moreover, also in intact human tibialis anterior muscle with the use of 31P-MRS, similar rates of ATP usage have been reported at short and optimal muscle length for maximal force production (2, 34). Nevertheless, by elimination of the alternative explanations for the length-dependent fatigability (21), we decided to investigate the possibility of length-dependent energy consumption in vivo again, using the easily applicable noninvasive near-infrared spectroscopy (NIRS) technique.
With NIRS, changes in oxyhemoglobin and deoxyhemoglobin (and myoglobin) in superficial parts of the knee extensors muscles can be monitored (6, 25). When blood volume is kept constant, changes in the oxy and deoxy state of hemoglobin and myoglobin during steady torque production are a measure of oxygen consumption (40). Lower rates of oxygen consumption and therefore energy usage were expected during isometric contractions at a knee extensor muscle length shorter than the optimal length for maximal torque production.
|
METHODS |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
Torque measurements. Isometric knee extension torque of the right leg was measured by use of a custom-made dynamometer. Subjects sat in a backward-inclined (15°) chair, with a 100° hip angle. They were firmly secured with straps fastening hips and shoulders. The lower leg was tightly strapped to a strain-gauge transducer (KAP, E/200Hz, Bienfait, Haarlem, The Netherlands) placed 30 cm distally from the knee joint, which measured the force exerted at the shin. The real-time force applied to the force transducer was displayed online on a computer monitor and digitally stored (1 kHz). At each angle, the force signals were automatically corrected for gravity: the average force applied by the weight of the limb at the transducer during the first 50 ms after the start of a measurement, with the subject sitting relaxed in the dynamometer, was set at zero force by the computer program. Extension torque was calculated by multiplication of force with the lever arm.
The axis of rotation of the knee was aligned with the axis of rotation of the dynamometer. Measurements were made at 30, 60, and 90° knee flexion angles (0° indicated straight leg). The knee angles were set manually by using a goniometer and using the greater trochanter of the femur, the lateral epicondyle, and the lateral malleolus as anatomical landmarks. The knee angles were set while the subjects exerted an isometric contraction at 
50% maximal torque at each angle. These knee angles were chosen to enable comparison with our laboratory's previous work (10, 21). The knee angle torque relation has its optimum around 60°, but in many studies for practical reasons the knee angle has been set at 90° (19, 24), which is 30° greater than 60°, and therefore also a 30° smaller knee angle was selected in the present study. Moreover, knee extension torque is similar at the 30° and 90° knee angle (21, present results).
Electrical stimulation. Constant-current electrical stimulation (200-µs pulses) was applied only during the first experimental day by use of a computer-controlled stimulator (model DS7H, Digitimer, Welwyn Garden City, UK) and a pair of self-adhesive surface electrodes (Schwa-medico, Leusden, The Netherlands). After shaving of the skin, the cathode (5 x 5 cm) was placed in the femoral triangle above the femoral nerve and the anode was placed transversely over the gluteal fold. At each knee angle, stimulation current was increased until force in response to a burst of three pulses applied at 300 Hz (triplet) leveled off. The latter always occurred between 300 and 500 mA, and it was assumed that at that point all of the knee extensor muscle fibers were activated.
Voluntary activation at each of the three knee angles was established as follows. Triplet stimulation on the relaxed knee extensor muscles was followed by triplet stimulation superimposed at the plateau of an MVC. This was performed three times with 3 min of rest. The force enhancement due to the superimposed triplet was expressed as a percentage of the force obtained when the triplet was applied on the resting muscle. This value was subsequently subtracted from 100%, resulting in a measure of voluntary activation. Voluntary activation = 1 – [triplet amplitude at MVC·(triplet amplitude at resting muscle)–1]·100% (9). Subsequently maximal torque capacity (MTC) was taken as the highest value of the three attempts calculated by the following formula: MTC = MVC·(voluntary activation)–1·100% (11). MTC is the torque the muscle will generate under conditions of real maximal activation.
Because the method of calculating MTC involves extrapolation to torque values at which superimposed stimulation on MVC does no longer lead to a torque increase, the higher the level of voluntary activation the more reliable the subsequent calculation of MTC will be. For the present study it was very important to have accurate values for MTC, because on days 2 and 3 oxygen consumption during constant torque production at fixed percentages of MTC was obtained. MTC rather than MVC values were taken as 100% because we did not want our results to be affected by possible knee angle-dependent differences in maximal voluntary activation. For example, if maximal voluntary activation would be lower during an MVC at the smaller knee angle compared with the other angles, as found previously (10, 22), MVC at 30° would be a greater underestimation of the true MTC of the muscle than MVC obtained at the other angles. This would subsequently lead to an underestimation of the measured oxygen consumption during submaximal isometric contractions at 30°, if the latter were to be obtained during torque production at a certain percentage of MVC rather than MTC.
NIRS. On the second and third visit, oxygen consumption of the vastus medialis (VM), rectus femoris (RF), and vastus lateralis (VL) during isometric contractions at different percentages of MTC was determined by use of a continuous-wave near-infrared spectrophotometer (Oxymon, Artinis Medical Systems, Arnhem, The Netherlands), which generated light at 780 and 850 nm (41). The three optode sets were each fixed in a mold with an interoptode distance of 45 mm. The molds were secured to the thigh with elastic Velcro straps such that the optodes did not move during contraction. The optodes were positioned on the center of the muscle bellies.
NIRS is an optical, noninvasive method that can be used to determine the tissue oxygenation level. NIRS actually measures the change in optical density of the tissue, which, by using a modification of the Lambert-Beer law (23), can be transformed into the change in concentrations of oxyhemoglobin ([O2Hb]) and -myoglobin ([O2Mb]) and deoxyhemoglobin and -myoglobin ([HHb] and [HMb]). Because of overlap of the spectrum, it is not possible to distinguish between hemoglobin and myoglobin; the oxygenated and deoxygenated form of the both proteins will respectively be denoted by O2Hb and HHb in the present study. The rates of concentration changes of O2Hb and HHb, or the slopes (Fig. 1) of the [O2Hb]- and [HHb]-time curves, represent the oxygen consumption per unit of time in the muscle (mO2). If blood volume in the tissue under the optodes remains constant, total Hb (the sum of O2Hb and HHb) will stay constant and the changes in [O2Hb] and [HHb] will be mirror images (Fig. 1). Blood volume was kept constant during the present experiments by rapid (<3 s) inflation of a cuff (Hokanson SC 10D), which was placed around the most proximal part of the thigh, to a pressure of 450–500 mmHg a few seconds before each contraction. To ensure that the cuff did not unwrap, an extra strap was secured around the cuff before inflation. The high pressure initially was uncomfortable for the subjects, but they did get used to it quickly. This cuff pressure was selected to ascertain that there was arterial occlusion throughout all contractions. It is known that arterial pressure during isometric contractions increases with force level and contraction duration (26). Moreover, it has been suggested (33) that a cuff pressure of 270 mmHg may be insufficient for complete arterial occlusion of the knee extensors. Furthermore, during pilot experiments at cuff pressure of 300 mmHg, the authors themselves could often actually feel the pulsation of the artery under the cuff, indicating that even 300 mmHg was too low for complete occlusion. The high pressure applied does not affect maximal torque production and maximal muscle activation (unpublished observations), nor does it affect muscle activation or endurance time at contraction intensities that in themselves are forceful enough (>50% MTC) to fully occlude the blood flow to the muscle (21). The cuff was deflated 5–10 s after contraction (Fig. 1).
|
O2 at all contraction intensities (39). The time window during which the changes in [O2Hb] and [HHb] remained linear (r2 > 0.98) decreased with contraction intensity (it was 8 s at 50% MTC, Fig. 1). The changes in [O2Hb] and [HHb] given by the spectrophotometer are in micromolar, with a differential path length factor of 4.0 used to correct for scattering of photons in the tissue (12), and can be further converted to milliliters O2 per minute per 100 g tissue (39, 40). Such a conversion necessitates some additional assumptions, e.g., that the molar volume of gas is 22.4 liters assuming STPD conditions. However, because we were not interested in the absolute mO2 values but only in their relative changes with knee angle and the effective path length of the NIRS signal remains uncertain (25), we expressed mO2 relative to maximal deoxygenation (%/s). Data were collected with a sample frequency of 10 Hz.
The NIRS signals are sensitive to changes in tissue temperature (4), but because in the present study mO2 was compared at different knee angles, studied in random order and within the same subjects, it is unlikely that there would be a systematic knee angle-dependent influence of either skin or muscle temperature that could affect our measurements.
Corrections for skinfold thickness.
Quantitative measurements to determine the absolute value of oxygenation using NIRS are difficult because they are based on the incorrect assumption that tissues are homogenous. Overlying tissues such as the subcutaneous adipose layer greatly affect the measurement sensitivity of NIRS (28, 39). Therefore, it is not possible to compare the absolute value of change between subjects. For the present study this is not a serious problem because we only make comparisons within subjects. However, any comparison of mO2 among the three different muscles investigated in the present study will have to take into account possible differences among muscles in subcutaneous fat under the optodes. Therefore, skinfold thickness was measured with a Harpenden skinfold caliper (John Bull; British Indicators, West Sussex, UK). Skinfold was measured with subjects seated in the dynamometer with a 60° knee angle. The average of three measurements at the position of the optodes on each muscle was used.
In principle, [O2Hb], [HHb], and total Hb concentration can be corrected for fat layer thickness (28). However, as will be shown in the present study, differences in subcutaneous fat can also be taken into account by expressing the slope of the linear part of the changes in [O2Hb] and [HHb] relative to the maximal possible changes in [O2Hb] and [HHb] (maximal deoxygenation). Maximal deoxygenation is defined as the absolute difference in [O2Hb] (and [HHb]) just after contraction onset (which often leads to a small changes in blood volume under the optodes, probably because of a redistribution of blood) and at the end of a contraction, when virtually all O2Hb will be converted into HHb (Fig. 1). To make this correction for subcutaneous fat possible, all isometric contractions in the present study had to be maintained until the [O2Hb]- and [HHb]-time curves had leveled off (which usually was near the point of torque failure). From the HHb and O2Hb time signals obtained during each contraction, oxygen consumption was respectively calculated by expressing the slopes of regression lines r1 and r2 (Fig. 1) relative to the maximal change in [HHb] and [O2Hb], respectively. mO2 was subsequently taken as the average of the absolute values of these two relative slopes (which were not statistically different in any circumstances). The values for maximal deoxygenation that are presented are the mean values of maximal change in [HHb] and [O2Hb] (which also were not statistically different in any of the muscles at any of the knee angles).
Protocol. As indicated in the above, during the first visit subjects' MVC and MTC were determined at the 30, 60, and 90° knee angle. Moreover, some familiarization trials with the NIRS measurements were made for the subjects to practice stable constant torque production with visual feedback and to familiarize them with the pressure cuff during the contractions. Before the optodes were placed, the skin was shaved and cleaned with ethanol. In addition, skinfold thickness was obtained. The positions of the optodes were marked to guarantee that optode position were similar during the second and third visit.
During the second and third visit two 5-s MVCs (2-min rest) were made to warm up. Subsequently, sustained isometric contractions with pressure cuff were made at the 30, 60, and 90° knee angle and at either 10 and 100% MTC (on 1 day) or at 30 and 50% MTC at the other day. The days were in random order, the knee angles and torque levels were randomly chosen among subjects, but the same order was maintained within each subject on both days. The third visit ended with an extra measurement of mO2 at rest (0% MTC) at the 60° knee angle. In addition, one extra measurement was made at 70% MTC at the 30° knee angle. This measurement was made at the same day as the measurements performed at 30 and 50% MTC. There was at least 10 min rest in between measurements. This setup was chosen to minimize fatigue and to attain similar degrees of subjective fatigue on both experimental days; the measurements at 10 and 100% MTC were, respectively, rated to be the easiest and most fatiguing task during pilot experiments.
The reason for including an extra measurement at 70% MTC at the 30° knee angle was that from recent results we knew that time to torque failure at 30°–70% MTC would be similar to time to torque failure for a sustained contraction at 90°–50% MTC (21). Thus in the present study, we expected mO2 at 90°–50% MTC to be higher than mO2 at 30°–50% MTC but similar to mO2 obtained at 70% MTC at the 30° angle (assuming that time to torque failure was related to the metabolic cost of the contraction).
Torque levels were maintained until the [O2Hb] and [HHb] time curves leveled off; measurement duration ranged from 20 s at 100% MTC to 8 min at 0% MTC (rest). Leveling off was established by eye during the measurements, but post hoc analysis showed that the mean change in [O2Hb] and [HHb] at the plateau was 0.12 ± 0.31%/s (mean ± SD), which was not significantly different from zero.
Statistics.
The results are presented as mean values ± SD. Tests for significance (P < 0.05) were done using a three-factor (muscle, knee angle, torque) repeated-measures ANOVA. With respect to maximal deoxygenation and mO2, in the first instance we did an "overall" ANOVA for repeated measures that included the data obtained at all torque levels, for all three muscles and at all three knee angles. Only if we found significant effects in this overall analysis did we separately perform three subsequent ANOVAs for repeated measures (either for the data obtained at each of the knee angles or for each of the muscles) to test for differences among muscles at each knee angle or for differences among the angles for each muscle. On significance of these latter tests, these were followed by Bonferroni post hoc tests. Pearson's correlation coefficient was calculated to establish significance of correlation.
|
RESULTS |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
|
O2 was determined, was significantly lower than 100% MTC. The respective values at 30, 60, and 90° knee angles were 84.5 ± 9.0, 75.9 ± 7.8, and 73.0 ± 11.8% (P > 0.05 across angles).
Skinfold thickness (mm) was significantly higher for RF (11.4 ± 3.2 mm) than for VM (9.7 ± 2.7 mm) and VL (9.5 ± 3.7 mm). As expected, there were significant negative linear relations between skinfold thickness and maximal deoxygenation (average values across all measurements) for VM (r2 = 0.71), RF (r2 = 0.71), and VL (r2 = 0.66). The relations were similar for the three muscles, and hence a single regression line was calculated (r2 = 0.69, P < 0.05) using all (3 muscles x 10 subjects) data points (Fig. 2). The relation in Fig. 2 clearly shows that the measured maximal deoxygenation (on average 51.0 µM) when using NIRS is always an underestimation of the maximal deoxygenation that could have been obtained in theory when there would be no skin and fat between the optodes and the muscle tissue. This also illustrates that differences in subcutaneous fat over different muscles and/or among subjects can be taken into account by expressing the mO2 relative to the maximal deoxygenation reached in the same contraction, as was done in the present study.
|
|
O2 significantly increased with torque in all three muscles at each knee angle (Fig. 4). However, and this is the main finding of the present study, in each muscle mO2 was significantly lower at 30° (Fig. 4, 
) compared with the other angles. In all three muscles and at all torque levels, mO2 was similar (P > 0.05) between 60 and the 90° knee angle (Fig. 4). The knee angle effect on mO2 was quite substantial; mO2 at 30° for VM, RF, and VL, respectively, was 70.0 ± 10.4, 72.2 ± 12.7, and 75.9 ± 8.0% of the average (60° and 90°) mO2 obtained at 10, 30, and 50% MTC and during MVC.
|
O2 at 30°–70% MTC was significantly higher than mO2 at 30°–50% MTC in all three muscles. In addition, and in accordance with our prediction, mO2 at 30°–70% MTC was similar (P > 0.05) compared with mO2 at 50% MTC at the 60 and 90° angle. This suggest that the last three conditions (30°, 70% MTC; 60°, 50% MTC; and 90°, 50% MTC) were metabolically equally demanding.
We will now focus on potential differences among the muscles. In the overall analysis, there were significant main effects of the factors torque level, muscle, and their interaction. Subsequent analysis of the data at each of the knee angles separately revealed that at 10 and 30% MTC, mO2 was or tended to be lower in RF muscle compared with both vasti. At 30° (Fig. 5, top), there was no significant difference among the muscles but there was a tendency for mO2 of the RF to be lower compared with mO2 of both vasti at the lower torque levels (muscle x torque interaction effect, P = 0.08). At 60° (Fig. 5, middle) and 90° (Fig. 5, bottom), there were significant differences among the muscles and the interaction of torque and muscle was also significant, meaning that mO2 of the rectus femoris muscle was significantly lower than that of both vasti during constant torque deliverance at 10% (and 30%) MTC. During maximal voluntary effort, any significant differences or tendencies for differences that were found among the muscles at 10 and 30% MTC had completely disappeared (Fig. 5).
|
|
DISCUSSION |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
O2 during constant isometric knee extensions was lower at the 30° knee angle compared with sustained torque production at greater knee flexion angles. This finding is in accordance with our hypothesis and may explain why time to torque failure in previous experiments using sustained isometric contractions was found to be longer at smaller knee angles (16, 21, 26, 31).
In the present study, the mO2, measured under conditions of complete arterial occlusion, was taken as a measure of energy consumption. This was considered to be a reasonable assumption because the flux through the electron transport system of oxidative phosphorylation is known to be regulated by some index of the energetic state, such as the ratio of free ATP over free ADP concentration (5). Therefore, the lower mO2 during isometric contractions at 30° strongly suggests that energy consumption and the consequent metabolic changes (decrease in pH and increase of inorganic phosphate) were lower at 30° than at 60 and 90°. Indeed, good correspondence between measurements with NIRS and 31P-MRS has been reported (15, 27, 35).
It is unlikely that the relatively low mO2 during submaximal contractions at 30° was due to an underestimation of maximal torque at 30° because torque levels were set at a percentage of MTC, thus including a (small) correction of the knee angle-dependent difference in maximal voluntary activation (Table 1). Moreover, even during maximal effort, mO2 was lower at 30° even though average torque during sustained MVC tended to be higher at 30° (85% MTC) compared with the other angles (75% MTC, Fig. 4). Furthermore, to obtain a value for mO2 during isometric contraction at 30° that was similar to the mO2 obtained during a sustained contraction at 50% MTC at 60° and 90°, torque at 30° had to be increased from 50 to 70% MTC (Fig. 4). Clearly this increase is far too great to be accounted for by any error in the calculation of MTC and the subsequent setting of the relative torque level. Finally, as found by others (15, 32, 39), mO2 increased with torque level in each of the muscles, which strongly suggests that mO2 is a good indicator of the number of force-generating cross bridges. Therefore, we think it is valid to conclude that energy consumption during constant torque production was lower at 30° than at the other angles.
With NIRS, the near-infrared light reaches a depth below the skin of about half the optode distance (7), which would be 22.5 mm in the present study. With an average skinfold of 10 mm, this means that on average only the superficial 17.5 mm [= 22.5 – (10/2)] of muscle tissue was sampled from. Consequently, possible regional differences between superficial and deeper muscle parts cannot be taken into account by using NIRS. However, we do not think that this will influence our main conclusion that mO2 was knee angle dependent, because measurements were taken from the same (superficial) muscle parts at all knee angles.
In the present study, we normalized mO2 (the slopes of the linear regression lines of the [O2Hb] and [HHb] time curves) to the maximal possible changes in [O2Hb] and [HHb] (maximal deoxygenation). This was done to correct for local differences in subcutaneous fat under the optodes. The highly significant negative linear relation between skinfold thickness and maximal deoxygenation (Fig. 2) illustrates that the measured mO2 critically depends on the kind of tissue below the optodes (28, 39). In the present study, we carefully fixed the optodes on the skin, but muscles probably slide below the skin when exerted torque changes and certainly when knee angle is changed. However, there were neither main nor interaction effects of torque and knee angle on maximal deoxygenation (Fig. 3), indicating that the same, or at least qualitatively the same kind of, tissue was sampled from at all knee angles and torque levels. Note that, because maximal deoxygenation was independent of knee angle (and torque), the normalization of mO2 to maximal deoxygenation did not affect our main conclusion.
How can the lower energy consumption during a sustained contraction at 30° compared with 60° and 90° knee angle be accounted for? It has been proposed that at short muscle lengths energy consumption would be lower because of fewer cross-bridge interactions compared with lengths that are closer to optimal overlap of the actin and myosin filaments (13). However, two studies that explicitly investigated this hypothesis using 31P-MRS failed to show length-dependent energy consumption in human muscle (2, 34). The results of Sacco et al. (34) are somewhat difficult to interpret because they electrically activated the tibialis anterior muscle, which led to length-dependent high-frequency fatigue in their study. On the other hand, during a 2-min MVC of the same muscle, Baker et al. (2) also found similar energy utilization at short compared with longer muscles lengths. Unfortunately, muscle fatigue during MVC of the short muscle could not be assessed in that study, and consequently it is unclear whether there was length-dependent muscle fatigue. In addition, it is possible that length-dependent differences in muscle activation affected the results of Baker et al. In the present setup, length-dependent differences in muscle activation (and blood flow) do (can) not occur (21). However, also in studies on isolated skinned fibers (37) and mouse muscle (29), energy consumption was similar at short and optimal sarcomere length for force production, at least when activation was maximal (29). Consequently it is doubtful whether the lower energy consumption at the 30° knee angle in the present study can be the result of fewer cycling cross bridges at that angle. It is, however, difficult to compare the present in vivo observations with results obtained from isolated muscle (fibers). For instance, because of a lack of consensus in the literature on the knee angle-dependent internal moment arm of the extensors (Refs. 14, 36 vs. Refs. 3, 20), we do not know where exactly on the knee extensor length tension relation the contractions at the different knee angles took place. At 30° knee angle most knee extensor muscle fibers will probably operate on the ascending limb of their length-tension relation, whereas at 60 and 90° the knee extensor muscles may operate close to their optimum for maximal isometric force production (17, 38). The latter may account for the similar mO2 of the knee extensors at the 60 and 90° knee angle, but it does not readily explain the longer time to torque failure during an isometric contraction at 60 compared with 90° (unpublished observations and Ref. 26). It is, however, of interest that endurance time of isometric contractions at 50% MTC seems inversely related to the energetic cost of the contraction relative to the torque level. If we set the values of torque and mO2 at the 60° knee angle at 100%, torque at both 30 and 90° is 66%, but mO2 at 30° is only 63%, whereas mO2 at 90° is 100%. This gives the following ratios of torque over mO2 at 30°, 60°, and 90°: 1.05, 1.00, and 0.66, respectively, illustrating that torque production at the 90° knee angle is less economical. Having noted this, it is also clear that the underlying cause(s) for the present finding remains to be elucidated because it seems questionable that it is a lower number of cycling cross bridges at short muscle lengths. Moreover, at present it cannot be excluded that joint angle-dependent energy consumption is different for different muscle groups.
At each of the knee angles and at torques lower than and/or equal to 30% MTC, mO2 tended to be lower (30°) or was significantly lower (60 and 90°) in RF than in VL and VM. Skinfold was significantly greater above RF than above the other muscles, and the sensitivity of the NIRS signal decreases with increasing skinfold (Fig. 2). Therefore, it is important to note that we have corrected our mO2 values for skinfold thickness; otherwise the calculated mO2 would be underestimated in RF (27, 28, 39). The finding that during MVC mO2 of RF was no longer lower than mO2 of the vasti suggests that oxidative capacity of the three muscles was similar but that the biarticular rectus was recruited to a lesser extent at low torque levels than the monoarticular vasti in the present setup. This is, however, speculative because we did not obtain EMG of the different muscles. However, task-dependent, differential activation among the knee extensors has been reported before (1, 30, 42).
In conclusion, the present study clearly shows that oxygen consumption of the knee extensors is lower during isometric contractions at the 30° knee angle than at the 60° and 90° knee angle. The lower oxygen utilization at the 30° angle probably contributes to the longer duration of sustained isometric contractions at shorter knee extensor muscle length that has been reported in the past. To establish whether the presented effect is a general phenomenon during in vivo activation or specific for the knee extensors, it should be investigated whether energy consumption and fatigability are also lower at short muscle lengths in other muscles. In addition, it would be interesting to relate the differences in mO2 found among the knee extensors in the present study to differences in activation strategy among these synergists.
|
FOOTNOTES |
|---|
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
|
REFERENCES |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  T. M. Altenburg, A. de Haan, P. W. L. Verdijk, W. van Mechelen, and C. J. de Ruiter Vastus lateralis single motor unit EMG at the same absolute torque production at different knee angles J Appl Physiol, July 1, 2009; 107(1): 80 - 89. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
), and rectus femoris muscle (
). For reasons of clarity, SDs have been omitted. There was a significant (r2 = 0.69, P < 0.05) negative linear relationship (y = –2.61x + 77.6) between skinfold thickness and the measured maximal deoxygenation.
, 90° 
) for vastus medialis (vm; top), rectus femoris (rf; middle), and vastus lateralis (vl; bottom) muscle. Note that the measurement in resting muscle (0% MTC) was only made at the 60° knee angle and that maximal deoxygenation at 70% MTC was only determined at the 30° knee angle.
