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1Institute for Fundamental and Clinical Human Movement Sciences, and 3EMGO Institute and Department of Social Medicine, Vrije Universiteit, 1081 BT Amsterdam, The Netherlands; and 2Institute for Biophysical and Clinical Research into Human Movement, Manchester Metropolitan University, Alsager, Cheshire 5T7 2HL, United Kingdom
Submitted 11 November 2003 ; accepted in final form 7 April 2004
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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isometric; phosphocreatine-to-creatine ratio; shortening; single fibers
Studies using surface electromyography activity during maximal isometric and isokinetic exercise have shown that electromyography levels during maximal lengthening contractions are lower than during maximal shortening contractions, although higher force output is obtained (1, 20, 22, 35). With the use of superimposed electrical stimulation, it has been demonstrated that the torque of voluntary lengthening contractions can be increased (2, 12, 37). In addition, by means of the twitch interpolation technique, activation levels during maximal voluntary lengthening contractions have been shown to be lower than during isometric contractions (3). These results from studies using different methods support the notion of a reduced neural drive during lengthening contractions compared with isometric and shortening contractions. It has been suggested that the reduction in neural drive could be either due to a lower activation of all recruited fibers as a consequence of inhibition, or due to activation of selective fiber populations (and inhibition or derecruitment of other fiber populations) during lengthening contractions (14). This would lead to a reversal of the normal hierarchy of recruitment (17, 25, 26; but compare Refs. 4, 23, 31, 32).
In the present investigation, we have examined the level of activation during maximal voluntary lengthening, isometric, and shortening contractions and related this to evidence for activation of different fiber-type populations. For this purpose, the ratio of phosphocreatine to creatine (PCr/Cr) was measured in single characterized fiber fragments, which were isolated from needle biopsies obtained at rest and after 10 lengthening, isometric, and shortening contractions. In an earlier study (7), we have demonstrated that this ratio is a useful indicator of fiber activation after very brief exercise involving only a few contractions of 1-s duration. This technique has proven to be a valuable approach for assessing recruitment patterns during isometric contractions at different intensities (6). In the present study, this method enabled us to assess whether type II fibers had been selectively activated (associated with a derecruitment or inhibition of type I fibers), or whether all fibers were activated at a lower level during lengthening contractions.
Thus the first aim was to investigate the voluntary activation level during lengthening, isometric, and shortening contractions by using the superimposed stimulation technique. For this purpose, high-frequency triplets were applied to the femoral nerve during maximal voluntary knee extension contractions and on the relaxed muscle. The second aim was to investigate the activation of different fiber populations during these modes of contraction by using PCr/Cr in single fibers. We hypothesized that we would find lower activation levels of lengthening contractions compared with isometric and shortening contractions, as evidenced by superimposed stimulation, and that this would be associated with a lower decrease in PCr/Cr in all fiber types, indicating a maintenance of the normal hierarchy of fiber-type recruitment rather than any reversal of recruitment pattern.
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METHODS |
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Study Design
In experiment 1, the voluntary activation level was determined during lengthening, isometric, and shortening contractions by using superimposed electrical stimulation elicited on maximal efforts. Subjects came to the laboratory for two sessions. In the first session, the torque-knee angle relationship was determined for each subject. In addition, voluntary activation level during either lengthening, isometric, or shortening contractions was tested. In the second session, which was at least 2 days after the first, the voluntary activation level of the remaining two contraction modes was determined. The order in which the lengthening, isometric, and shortening protocols were carried out was randomized over the two sessions.
In experiment 2, five subjects came to the laboratory for a session in which needle biopsies were obtained after three separate series of 10 contractions performed under lengthening, isometric, and shortening conditions.
Torque Measurements
Isometric and isokinetic knee extension exercise was performed on a specially designed dynamometer, which allowed torque measurements at preset angular velocities. Subjects sat in an upright position with a hip angle of 
75° (0° = full extension), and the axis of rotation of the dynamometer was aligned with the lateral femoral condyle. Straps were applied to the pelvis and torso to stabilize the subject. For all subjects of experiment 1, the right leg was used for the exercise. A cuff was fastened around the lower leg, and this cuff was subsequently attached to the lever of the dynamometer. Torque of the contractions was analog-to-digital converted (1,000 Hz) and stored on disk for offline analysis. All recorded torque values were corrected for the effect of gravity. To prevent fatigue, a minimal rest of 2 min was allowed between all maximal efforts.
Experiment 1
Purpose. The aim of the first experiment was to investigate voluntary activation levels of the quadriceps muscle during lengthening, isometric, and shortening contractions by using the superimposed stimulation technique.
Subjects. Ten healthy subjects, six men and four women, with a mean ± SD age of 28 ± 8 yr, height 179 ± 12 cm, and weight 74 ± 9 kg, participated in this experiment. All were regularly active with a mean of 6 training h/wk.
Electrical stimulation. To determine the voluntary activation level of the quadriceps muscle, a superimposed nerve stimulation technique was used. In the present study, a triplet was used instead of a twitch (e.g., Refs. 3, 5), as it has been shown that the variability in superimposed torque is reduced with increasing number of stimuli (33). By using a twitch, superimposed torques may sometimes seem absent, although this may be due to the insensitivity of the method because of the small, transient extra torque rather than complete activation (21). Consequently, this may overestimate the voluntary activation level.
Electrical stimulation was applied to the femoral nerve during maximal voluntary knee extension efforts (isometric and isokinetic) and when the muscle was relaxed. A constant-current stimulator (model DS7, Digitimer, Hertfordshire, UK) was used with self-adhesive surface electrodes (Schwa-Medico, Nieuw Leusden, the Netherlands) placed on the skin. The cathode (5 x 5 cm) was placed in the trigonum femorale to stimulate the nervus femoralis; the anode electrode (8 x 13 cm) was placed on the most prominent part of the m. vastus medialis. The exact location for the stimulating electrode on the nerve was determined by using a ball probe electrode. Pulses of 30–50 mA were applied, and a different electrode position was used for each twitch. The smallest electrode was located at the position of the probe, which gave the largest visible muscle contraction. To determine voluntary activation level during maximal attempts, three square-wave pulses (triplet) of 200 µs were delivered to the muscle at a frequency of 300 Hz, with the use of supramaximal current. Maximal current was determined, by using the triplet, by raising the current until isometric torque at optimum knee angle did not increase further. The current was then increased by a further 20 mA to ensure supramaximality. In the second session, the current was determined again. The mean current used was 162 ± 26 mA, and the applied triplet evoked a muscle contraction of 40% of maximal voluntary isometric torque. To increase the reproducibility of the electrode positioning, the positions of the electrodes were marked on the skin before they were removed in the first session.
Most of the subjects had experienced electrical stimulation before. In the subjects who had not, the intensity of stimulation was gradually increased to accustom them to the sensation.
Isometric exercise.
In the first session, the torque-knee angle relationship was determined. Subjects performed maximal isometric knee extension contractions at 90, 80, 70, 60, 50, 40, and 30° knee flexion angle (0° = full extension) in a randomized order. One maximal effort lasted 3–4 s. At each knee angle, two attempts were performed. When the torque of these two attempts differed by more than 10%, a third attempt was performed. The maximal torque reached in these attempts was considered to be the subject's maximal isometric torque for the specific knee angle. The knee angle at which maximal torque was measured (optimal knee angle) was used for all isometric testing protocols.
Voluntary activation level was determined for isometric contractions by using superimposed stimulation. During maximal isometric knee extension contractions, triplets were elicited 1.5–2 s after the start of the contraction (e.g., see Fig. 1B). Because subjects sometimes had difficulties in performing a maximal effort when a triplet was to be superimposed, several attempts had to be performed until the maximal voluntary torque (before the triplet) did not further increase (usually not more than 5 attempts). For the isometric contractions, the triplets evoked at resting muscle were applied at the same knee angle as the voluntary contractions.
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As the movement was started, the dynamometer accelerated at 1,000°/s2 to the preset angular velocity. Subjects performed two to four maximal lengthening or shortening attempts to get accustomed to the movement and to determine the knee angle at which maximal torque was reached. Next, superimposed stimulation was applied on maximal lengthening or shortening attempts (e.g., see Fig. 1, A and C). For each subject, superimposed triplets were triggered such that the effect of the stimulus occurred at optimal knee angle for lengthening and shortening contractions. For the triplets evoked in resting muscle, the leg was passively moved from 30 to 90° for the lengthening mode and from 90 to 30° for the shortening mode. Similar to the triplets superimposed on maximal efforts, the triplet in resting muscle was elicited a few degrees before optimum knee angle was reached. With the use of this protocol, the highest torque of the triplet was reached at optimum knee angle (optimum angles for the three contraction types were similar; see RESULTS).
As in the isometric contractions, subjects performed lengthening or shortening attempts until voluntary torque did not increase further.
Torque analysis.
Maximal voluntary extension torque was the highest measured voluntary torque for lengthening, isometric, and shortening contractions, respectively. For each subject, the optimum knee angle was determined, i.e., the knee angle at which the maximal torque was reached. For the isometric contractions, the torque at the remaining knee angles was expressed relative to this maximal torque. Torque of superimposed triplet (a) for the different contraction modes was determined by subtracting the maximal voluntary extension torque during or after the triplet (just before an effect of the triplet was seen) (b) from the maximal torque evoked by the triplet (c). The superimposed torque was expressed relative to the torque of the triplet evoked on the relaxed muscle in the different modes. Maximal torque generating capacity (MTGC) of the muscle was then calculated by using the following formula:
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It must be noted that we have not corrected for the effect of muscle slack on the torque of the triplet on the relaxed muscle in the shortening mode. Neglecting this effect of muscle slack might have overestimated the MTGC and, consequently, underestimated the voluntary activation level.
Several attempts were made at each contraction mode to perform a maximal effort. For statistical analysis, we have used the attempt in which the highest voluntary torque before the triplet was reached.
Experiment 2
Purpose. The aim of the second experiment was to assess the activation pattern of type I, IIa, and IIax fibers during only 10 lengthening, isometric, and shortening contractions by using the PCr/Cr of single characterized fiber fragments as a measure for fiber activation.
Subjects. Five healthy subjects, four men and one woman with a mean ± SD age of 30 ± 9 yr, height 181 ± 11 cm, and weight 76 ± 8 kg, participated in this experiment. Four of these subjects (three men and one woman) also participated in experiment 1. All were regularly active with a mean of 4 training h/wk.
Protocol. The subjects performed three separate series of 10 maximal contractions under lengthening, isometric, and shortening conditions. Each contraction was of 1-s duration followed by 1-s rest before the next contraction. Each protocol was performed with either the right leg or the left leg. The order in which the series were performed and the leg that was tested were randomized, except that, after lengthening exercise, no other protocol was performed with that leg. Rest between series was at least 10 min. To impose the 1-s on/1-s off rhythm, an auditory signal was given during the contraction phase. Immediately after relaxation from the last contraction, a needle biopsy was taken from the m. vastus lateralis. The isometric protocol was performed at optimum knee flexion angle. The preload, range of motion, and angular velocity for lengthening and shortening contractions were the same as used in experiment 1. To be able to relate the average torque attained during the contractions to the MTGC of the muscle, activation levels during the three contraction modes were determined for both legs. This was performed in two separate sessions (presessions), which were carried out before the session in which biopsies were obtained.
Muscle biopsy.
With the subject seated on the dynamometer and the knee in the optimum knee flexion angle, small incisions were made after local anesthesia (2% lidocaine) of the skin and fascia. Two incisions were made (one on each leg) at one-third of the distance between the lateral femoral epicondyle and trochanter major. Immediately after the last contraction of a series, a muscle sample was collected from the m. vastus lateralis of the exercised leg by using a Bergström type biopsy needle (diameter of 5 mm, Popper Biomedical Instruments, Schuco International London Limited, London, UK) with suction. From each subject, a resting sample was also obtained. As two biopsies were taken from one leg, the needle was directed either proximal or distal. Biopsies were frozen in liquid nitrogen within an average of 3.8 s (range 2.2–6.7 s) from end of exercise to freezing and they were freeze-dried overnight. The freeze-dried samples were stored desiccated in tubes of which the lid was sealed with laboratory film. Each tube was placed in another small jar with some silica gel, the lid was sealed with laboratory film, and the small jar was stored in liquid nitrogen vapor until it was analyzed. Individual fiber fragments of 2- to 3-mm length (80 from each sample) were dissected under conditions of controlled ambient temperature and relative humidity (20°C and less than 20% relative humidity). Each fiber fragment was divided into two parts: one for histochemistry and the remaining part for analysis of metabolites. The methods used for single-fiber typing and muscle metabolite analysis are extensively described elsewhere (18, 29). Three biopsies were excluded from the study, because no fibers could be dissected from the sample or because freezing time (time from end of exercise to freezing of the sample) was too long (>10 s).
Histochemistry.
Serial sections (10 µm) of gelatin-embedded, single-fiber fragments were cut and characterized for acid myofibrillar ATPase (mATPase) stability at pH 4.6 and 4.4 (adapted from 8). An image analysis system (KS 300 Imaging System 3.0, Carl Zeiss Vision) was used to measure the optical density of histochemically treated fiber sections (29). Based on the optical density values from mATPase, preincubation pH 4.4, the fibers were classified into type I and II fibers. The mATPase, preincubation pH 4.6, was then used to divide the type II fibers into two categories, type IIa (0–25% IIX) and type IIax (25–100% IIx) (15).
Analysis of metabolites.
From the characterized muscle fiber fragments of each muscle biopsy, a maximum of 20 fibers from each fiber type were analyzed for PCr and Cr by using reverse-phase HPLC with ultraviolet photometric detection (18) after overnight extraction in 60% methanol (11). PCr/Cr was used as a measure for fiber activation (7).
The advantage of using PCr/Cr as an indication of fiber activation is that it allows fiber recruitment to be investigated after only a few (dynamic) contractions (6, 7). However, some methodological considerations regarding this method need to be mentioned. Both physiological variation between subjects as well as differences in freezing time, and thus resynthesis of PCr, will cause variation in PCr/Cr. Because the half-time of PCr resynthesis is 30 s (16, 24, 28), we have set a maximal time limit for muscle sampling (<10 s). If this time was exceeded, no sample was obtained, or the sample was not analyzed. Unfortunately, this has led to missing data for some conditions. Moreover, due to the rather time-consuming procedure of fiber characterization and metabolite analysis, a relatively small number of subjects was included in experiment 2 of this study. Unfortunately, data of three samples were lacking due to poor quality of the samples and/or due to long freezing time. It should be realized that this is a difficulty using the present method, and it could be suggested that this would limit firm conclusions from being made. Despite these difficulties, the method has proven to be useful to detect fiber activation after only approximately seven contractions (7).
Torque analysis. For each series of contractions, the peak torque of each individual contraction was determined. To determine the voluntary activation level during the series of contractions, the mean of the 10 contractions for each contraction mode was expressed relative to the MTGC estimated for that leg.
Statistical Analysis
All results are presented as means ± SD. The Shapiro-Wilk test was used to check for normality of the data of experiment 1. To test for significant differences, parametric one-way ANOVA for repeated measures was applied with contraction (lengthening, isometric, and shortening) as a within-subjects factor. Bonferroni post hoc comparisons were made between contraction modes where appropriate. t-Tests for paired samples were performed to compare maximal voluntary torque with triplet torque evoked at rest, both expressed relative to their respective isometric torque, for lengthening and shortening conditions. There was one subject who had difficulties reaching a high-maximal voluntary torque in the shortening contractions when a triplet was superimposed. Therefore, it was difficult to determine this subject's MTGC and voluntary activation level, and these data were not used for analysis.
The Kolomogorov-Smirnov test was used to check for normality of the metabolite data of experiment 2. As the data were not normally distributed, nonparametric Kruskal-Wallis test was applied followed by a Mann-Whitney U-test for post hoc comparisons. For this analysis, the PCr/Cr of the three fiber populations (type I, IIa, and IIax fibers) were grouped and tested for significant differences compared with rest and among the three contractions modes (lengthening, isometric, and shortening).
To investigate the activation of the individual fiber populations (type I, IIa, and IIax fibers) during lengthening, isometric, and shortening contractions, cumulative distributions of PCr/Cr of individual fiber fragments were calculated for each fiber type, by using intervals of 0.1. To determine significant differences between activation of the fiber populations, Kolmogorov-Smirnov two-sample tests were performed on the cumulative distributions. This test detects differences in both the location and the shape of the distributions (30).
The level of significance for all statistical analysis was set at P < 0.05.
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RESULTS |
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Optimum knee angle. In seven subjects, the maximal torque was reached at 60° knee flexion angle. The mean ± SD maximal relative torque was obtained at 60° knee flexion angle (97.8 ± 4.1%). However, there were no significant differences with 70° (97.5 ± 2.6%) and 80° (92.4 ± 4.6%) knee flexion angle. The mean ± SD optimum angles for lengthening, isometric, and shortening contractions were not significantly different (66 ± 4, 64 ± 7, and 63 ± 7° knee flexion angle, respectively).
Maximal voluntary torque. Mean voluntary torque during lengthening (270 ± 55 N·m) was significantly higher than mean torque during shortening contractions (199 ± 47 N·m, P < 0.05) but not significantly different from mean isometric torque (252 ± 47 N·m) (Fig. 2A). Mean isometric torque was significantly higher than mean torque during shortening (P < 0.05).
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Triplet torque. Mean torque of the triplet evoked in resting muscle was significantly different between all modes of contraction (respectively, 136 ± 36, 105 ± 22, and 82 ± 18 N·m for lengthening, isometric, and shortening modes; P < 0.05) (Fig. 2A). When triplet torque and maximal voluntary torque for lengthening and shortening contractions were expressed relative to their respective isometric torque, it appeared that mean relative triplet torque (128 ± 11%) was significantly higher than mean relative maximal voluntary torque (106 ± 11%, P < 0.05) for lengthening contractions but not for shortening contractions (79 ± 5 and 78 ± 7%) (Fig. 2B). This implies that voluntary activation was not complete during lengthening contractions.
Voluntary activation level. Mean voluntary activation level during lengthening contractions (79 ± 8%) was significantly lower than during isometric (93 ± 5%) and shortening contractions (92 ± 3%) (P < 0.05). There was no significant difference in voluntary activation level of isometric and shortening contractions. There were no differences in voluntary activation level between men and women; both groups had a mean of 79% with a SD of 10 and 6%, respectively.
Experiment 2
Maximal voluntary torque. Mean maximal voluntary torque data measured in the presessions were not different from the data measured in experiment 1: i.e., 260 ± 58, 235 ± 34, and 208 ± 39 N·m, respectively, during lengthening, isometric, and shortening contractions (one-way ANOVA). Mean maximal torque during the series of 10 lengthening, isometric, and shortening contractions was, respectively, 252 ± 61, 240 ± 20, and 213 ± 36 N·m.
Voluntary activation level. The mean activation levels during the series of lengthening (76 ± 12%), isometric (90 ± 9%), and shortening (91 ± 10%) contractions were not different from the voluntary activation level of the three modes of exercise obtained in experiment 1 (one-way ANOVA).
PCr/Cr. A total of 720 single-fiber fragments (293 type I, 289 type IIa, and 138 type IIax) was characterized and analyzed for PCr and Cr content. Figure 3A shows an example of the PCr/Cr of the different fiber fragments of one subject at rest and after exercise.
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Cumulative distributions of the PCr/Cr of the individual fiber populations are shown in Fig. 4. Kolmogorov-Smirnov statistics showed that, at rest, the curves for type I, IIa, and IIax fibers were significantly different (P < 0.05). For each fiber type, the distribution after either contraction mode was significantly different from rest (P < 0.05). In addition, curves for lengthening, isometric, and shortening contractions were significantly different for each fiber type (P < 0.05). For all fiber types, there was an almost parallel shift in the curves for lengthening and isometric contractions, with a preservation of the shape of the curves at rest. However, the curves of type I and IIa fibers after shortening contractions show only a moderate rate of increase above the PCr/Cr of 1.0. This is caused by the results of two subjects (e.g., see Fig. 3B), who show a relatively large variation in the PCr/Cr after shortening exercise compared with the other three subjects (e.g., see Fig. 3A).
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DISCUSSION |
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76% MTGC), there was a less marked change in the PCr/Cr in the higher hierarchy type IIax fibers, as might be normally expected from the size principle (see Fig. 4). Neural Inhibition
In the present study, voluntary torque during maximal lengthening contractions was not higher than isometric torque. This is in contrast to what could be expected from force-velocity relationships of isolated muscle fibers (13) and whole muscle preparations (19) in which torque during lengthening has been shown to be higher than during isometric contractions. On the other hand, our results are comparable with studies on humans that also did not find an increased torque during maximal lengthening contractions (3, 12, 36, 37). A reduced neural drive during lengthening contractions compared with isometric and shortening contractions is suggested to limit maximal voluntary torques (34, 37). Several results from the present study confirm the notion of a limited neural activation during lengthening contractions. First, direct evidence comes from the voluntary activation level calculated from superimposed stimulation. Our data show that the voluntary activation level for lengthening contractions was lower than for isometric and shortening contractions. Second, whereas voluntary torque during lengthening was not different from isometric torque, triplet torque during lengthening was 30% higher than the isometric triplet torque (Fig. 2A). Third, the triplet torque during lengthening contractions was 20% higher compared with maximal voluntary torque when they were expressed relative to isometric torque (Fig. 2B), whereas there was no difference for shortening contractions. Fourth, estimated MTGC was 27 and 62% higher in lengthening contractions compared with isometric and shortening contractions, respectively (Fig. 2A, P < 0.05).
A comparison of the data from experiment 1 can be made with a study from Babault et al. (3). They also found lower activation levels during lengthening (88.3%) compared with isometric contractions (95.2%). Although in contrast to the present findings and previous studies (2, 30), they also found a significant difference in activation levels during isometric compared with shortening contractions (89.7%).
Activation Pattern
A reduced neural drive during maximal attempted lengthening contractions, as demonstrated by the present study, has been attributed to either preferential recruitment of type II motor units (that is, with a concomitant inhibition of type I fiber recruitment) or lower activation levels of all activated fibers (14, 34). In the present study, PCr/Cr values of single characterized fiber fragments were used to investigate the activation pattern of different fiber populations during maximal lengthening, isometric, and shortening contractions. We have calculated cumulative distributions, and a significant shift in the curve of a fiber type after exercise was interpreted as an activation of that fiber type (Fig. 4). The data show that all fiber types were active during attempted maximal lengthening, isometric, and shortening contractions because, in each fiber type, there was a significant shift in the distribution after each contraction mode. In addition, for all fiber types, there were significant differences between curves of the three modes of contractions. This was also seen in the mean decline of the PCr/Cr of grouped fiber populations, which was significantly smaller for lengthening (37%, P < 0.05) than for isometric (66%, P < 0.05) and concentric contractions (67%, P < 0.05). For lengthening contractions, it might have been expected that the cumulative distribution would show a smaller shift in the curve, because, in an earlier study on maximally stimulated rat muscle, we have shown that the PCr/Cr after 10 maximally stimulated lengthening contractions was smaller compared with shortening contractions (unpublished observation). Moreover, the mechanical data of the present study already showed that subjects had lower voluntary activation levels during lengthening contractions.
The fact that the distributions of the PCr/Cr after isometric and shortening contractions were also significantly different was less expected because voluntary activation levels were not different. Furthermore, in a study on rat muscle, we showed that there was no difference in the PCr/Cr after isometric and shortening contractions at similar relative contraction velocities of approximately one-third of optimum velocity (7a). It should, however, be noted that the cumulative distribution curves for shortening contractions are different in shape compared with lengthening and isometric contractions. This is attributed to the data of two subjects, who showed an unusually large variation in the PCr/Cr after shortening exercise (for example, see Fig. 3B). As the Kolmogorov-Smirnov two-sample test assesses the difference in both position and shape, it is most likely that the significant difference between isometric and shortening curves is caused by the change in shape. The difference in shape is mostly attributable to a lower number of fibers with PCr/Cr values around 1.0. Therefore, no strong conclusions can be made on the recruitment of fiber types during shortening contractions.
It has been suggested that, during submaximal lengthening exercise, there would be a selective activation of type II fibers and deactivation of type I fibers (17, 25, 26). However, the present study could not find evidence for this concept in attempted maximal exercise. In contrast, as can be seen in Fig. 4, the gray areas for the three fiber populations decreases from type I to IIa to IIax fibers. Thus, if there were a difference in activation between fiber types, our data seem to suggest that, in maximal attempted lengthening contractions, there is less activation of type IIax compared with type I fibers. This observation is entirely consistent with a hierarchy of fiber-type recruitment from type I to IIa to IIax.
Conclusion
The present study investigated the voluntary activation level during maximal attempted lengthening, isometric, and shortening contractions. In addition, the activation pattern of muscle fiber types was studied by using the PCr/Cr of single characterized fiber fragments as a measure of fiber activation. It was demonstrated that the voluntary activation level during maximal attempted lengthening contractions was significantly lower than during isometric and shortening contractions, whereas the degree of voluntary activation was not different between isometric and shortening contractions. The PCr/Cr of single fibers showed that there was no evidence for selective recruitment of type II fibers with concomitant derecruitment of type I fibers. In contrast, the data seem to suggest that, during maximal attempted lengthening contractions, all fiber populations were recruited, albeit at a lower rate. Furthermore, the PCr/Cr depletion pattern indicated a hierarchical pattern of motor unit recruitment.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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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.
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), triplet torque (
), and estimated maximal torque-generating capacity (
) during lengthening (–60°/s), isometric (0°/s), and shortening (60°/s) contractions. B: maximal voluntary torque and triplet torque expressed relative to their respective isometric torque. Values are means ± SD. Significant difference compared with *isometric or 
concentric contractions, P < 0.05. 
Significant difference from relative maximal voluntary torque, P < 0.05.
