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Influence of contractile tension development on dynamic strength measurements of the plantarflexors in man
J. 8romechanics Printed
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21. No. 2. pp. 89-96.
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1988 Pergamon
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INFLUENCE OF CONTRACTILE TENSION DEVELOPMENT ON DYNAMIC STRENGTH MEASUREMENTS OF THE PLANTARFLEXORS
IN MAN
DENIS GRAVEL, CAROL L. RICHARDS* and MICHEL FILION Faculty of Medicine, Lava1 University, Quebec, P. Que. GlK 7P4, Canada Abstract-The influence of the contractile tension rise time on isokinetic force-angle records has been inferred from static forc+timecurves but has not been experimentally determined. The purpose of this study is thus to describe the influence of the contractile rise time on the force-angle curves produced during maximal voluntary, acceleration controlled, isokinetic plantarflexions at 30-/s. Since we could not measure directly the period of force development unbiased by changes in muscle length during the movements, we devised an experimental strategy which allowed the computation of the dynamic force-time curve. Thus in five normal men, we first recorded force-angle curves produced during maximal voluntary plantarflexion movements preceded by maximal static pre-loading (D: - 10”Max) in order to eliminate the period of tension development from the force-angle record. Next, we recorded force-angle curves produced during maximal voluntary contractions initiated from two different starting angles without pre-loading (D: - lo” Min and D: 0”Min) to include the period of tension rise. The dynamic force-time curve was computed by correcting these force-angle curves (D: - lo” Min and D: 0”Min) for the hypothetical loss in force due to muscle shortening. We compared the relative (to remove the effects of force magnitude) computed dynamic force-time curves with relative static force-time curves measured at three different angles. We found the shape and several other parameters of all three static and both computed dynamic force-time curves to be similar (p > 0.05). The time to reach 90 ‘4 of maximum force in the dynamic tests was 365 and 439 ms (mean values), or 11” and 13” after initiation of the movement at 3O”/s, indicating the period of contractile tension rise. These results stress the importance of static pre-loading to improve the measurement of maximal strength capacity in the early part of an isokinetic contraction, especially when the excursion is limited and the speed is high. Use of pre-loading, however, may lead to underestimation of the maximal dynamic capacity in the last part of the isokinetic plantarflexion movements.
INTRODUCTION
When muscles are activated, by voluntary effort or by electrical stimulation, the contractile tension takes time to rise to maximum. The resulting force-time curve has been studied in both humans and animals in static conditions. The shape of the curve has been shown to depend on factors such as the total compliance of the series-elastic components (Wilkie, 1950), the type of muscle activation (Buller and Lewis, 1965; Budingen and Freund, 1976; Ranatunga, 1978; Miller et al., 1981) and the histochemical composition of the muscles (Buller and Lewis, 1965; Viitasalo et aI., 1981; Clarkson et al., 1981). The relative importance of each of these factors has not been completely elucidated. Nevertheless, since they are fundamental characteristics of a muscle or muscle group, they should not be modified by the fact that a contraction is made in static or in dynamic conditions. Therefore, the shape of static and dynamic force-time curves should be similar. In humans, during the performance of maximal voluntary contractions in static conditions, contractile tension requires about 0.5 s to reach 90 y0 of maximum (Royce, 1962; Clarke, 1968; Morris et al., 1983). In -
-
Receioed 5 May 1986; in revised form 13 January 1987. *Corresponde& address: Laboiatoire de Neurobiologie, H&pita1 de I’Enfant-J&us. 1401, 18e Rue Quibec, Que G 1J 124, Canada.
dynamic conditions, the hypothesis presented above predicts that it should require the same time to reach 90 7; of maximum, that is 15” of movement excursion at 3o”/s. However, the time course of the rise in contractile tension cannot be measured directly in dynamic conditions. In fact, dynamic force records show a rise of force at the beginning of movement, but this is rapidly followed by a decrease lasting until the end of movement. This decrease reflects mainly the mechanics of the joint and the length-tension relationship as the muscles are allowed to shorten to produce movement. The effects of the rise in contractile tension on the force record are therefore confounded with those of the length-tension relationship: the former factor tending to increase the force, the latter tending to decrease it. This problem is the subject of the present study. The isokinetic dynamometer used in most studies up to 1983 was the Cybex 11. The subject was asked to make and maintain a maximal voluntary effort for the duration of the movement. This was requested to obtain constant muscular activation, at levels comparable across individuals and testing conditions. This dynamometer imposed on the limb a load proportional to the force tending to increase the velocity and the magnitude of this force was therefore taken as a measure of contractile tension, provided adequate corrections were made for gravitational forces. The Cybex II, however, did not offer resistance and thus did 89
90
D. GRAVEL, C. L. RICHARDSand M. FILION
not measure force during the acceleration phase of movement. Moreover, when the selected speed was reached and the system suddenly braked the acceleration, it gave rise to an impact torque followed by a series of oscillations corrupting force records, markedly at high velocities (Winter et al., 1981; Sapega et al., 1983). This technical limitation incited investigators to avoid the first part of the torque-angle curve measured with the Cybex II dynamometer (Perrine and Edgerton, 1978; Knutsson and M&tensson, 1980; Richards, 1980, 1981). Although the development of contractile tension was implicit in these dynamic strength records, its influence on the torque-angle record was rarely discussed (Gransberg and Knutsson, 1983). In order to improve force records in the early part of movement, Gransberg and Knutsson (1983) added computer assistance to the speed control of a Cybex II dynamometer. The system offers two new features: control of acceleration and pre-loading. The acceleration control provides some resistance to movement during its acceleration phase. Thus, it prevents the impact torque and oscillations. However, the strength measurements taken during this period are not isokinetic. Pre-loading is the blockage of movement until a level of static force is reached. This results in the recording of higher force levels at the beginning of movement. These forces probably more correctly correspond to the maximal capacity of the muscle at the first angles of movement. Furthermore, the use of pre-loading tends to elude the problems posed by the development of contractile tension during dynamic contractions since it removes this rise of force from the dynamic conditions. In the present study, it will be assumed that a level of pre-loading equal to at least 90% of maximal static strength removes an equal percentage of the rise time of contractile tension from the immediately following dynamic conditions. The dynamic force records, obtained with such pre-loading, will therefore be taken as relatively free of the effects of the development of contractile tension. Such records will be used as reference to differentiate by computation, from other dynamic force records without pre-loading, the changes of force related to the development of contractile tension from those related to the effects of the length-tension relationship. These computed dynamic force-time curves will be statistically compared with force-time curves obtained in static conditions.
Strength measurements
Plantarflexion strength measurements were made with the subjects seated on a specially designed chair, the hip angle flexed 1lo”, and restraining straps placed across the trunk, hips and right thigh (Fig. 1). The knee was extended and the right foot attached to a metal soleplate as described by Nistor et al. (1982). This soleplate was connected to a computer controlled KinCorn dynamometer so that the ankle rotational axis was aligned to the dynamometer axis. Plantarflexion force and ankle angle (sampled at 100 Hz) were recorded as the subjects performed a series of maximal voluntary static and dynamic contractions under standardized test conditions. Each series consisted of three specific contractions interspersed by 30 s rest periods, while 2-3 min rest periods separated the different series of contractions. Static contractions
Maximal voluntary static contractions were performed at three different ankle positions: -10” of dorsiflexion (S:- lo’), 0” (S:o”) and lo” (S:+ 10”). Subjects were told to exert maximal force as fast as possible and to maintain this level for a duration of 4 s. Dynamic contractions
Maximal voluntary dynamic contractions were performed at a constant velocity of 3O”/s. Test 1: the first dynamic test (I): - 10”Max) consisted of isokinetic contractions from - 10” of dorsiflexion to + 30” of plantarflexion preceded by a maximal static contraction with a duration of about 2 s. The purpose of this static pre-loading was to allow time for the contractile tension to rise before movement was permitted, thus eliminating its influence on the early part of the dynamic force record. Test 2: the second dynamic test (D: - 10”Min) required a maximal contraction through the same range of motion as in test 1, but this time the dynamic effort was not preceded by a maximal static contraction. Test 3: the third dynamic test (D: O”Min) was similar to test 2 except that the starting angle was 0” instead of - 10”. Since tests 2 and 3 began from a TESTS
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SUBJECTS AND METHODS
Subjects
Five healthy men participated in this study. The mean age was 30.6 + 4.0 (S.D.) yr. All were university graduate students without dysfunction of the right lower extremity.
Fig. 1. Schematic representation of the position of the subject and range of ankle movement evaluated in the different test conditions.
91
Measurements of the plantarflexors in man minimal force level, the dynamic force record includes a period of tension development in contrast to test 1
where the period of tension rise occurs during the static pre-loading. In order to limit the effects of fatigue and learning on performance, the order of the static and dynamic tests, as well as the sequence of these tests were systematically alternated. Verbal commands were standardized and given by the same person. The forces produced during the voluntary contractions were corrected for passive forces due to the weight of the footplate and the joint structures. These passive forces were obtained by measuring the resistance of passive ankle movements at 3o”/s. Measurements were made throughout the range of the plantarflexion movement, and after a pause of 1 s, during the reverse movement. Mean values (three repetitions) were used to calculate the correction, angle for angle, throughout the movement. Since the Kin-Corn dynamometer provides acceleration control in the first part of the movement, forces are measured during the period of contractile tension rise (when pre-loading is not used). The acceleration control period is, however, fixed (cannot be programmed) and its duration is dependent on the selected speed. Two degrees of movement are required to reach the selected speed of 3O”/s. The Kin-Corn system also allows the setting of a minimal force level which must be reached for movement to occur. This threshold force prevents movements from starting inappropriately, for example, due to the weight of the limb segment. The threshold force for the dynamic contractions (D:- 10”Min. D: 0”Min) was set equal to the passive resistance measured at - 10” of dorsiflexion plus 20 N (4.6 Nm). This threshold force is active throughout the movement, however, and thus precludes use of this feature for static pre-loading at the high force levels used in D: - 10”Max test. To provide high static preloading in the D: - 10” Max test, it was necessary to instruct the subjects to begin maximal voluntary static contractions 2 s (determined from the time display) before the end of the programmed rest period.
tions at three different ankle angles (A-C). These angular positions were chosen to correspond to the starting positions of the dynamic contractions (tests l-3). Furthermore, by recording the force-time curve at different angular positions, we were able to evaluate the effect of ankle angle on the characteristics of the static force-time curve. The three force-time curves (Fig. ZA-C) have a characteristic sigmoid shape as the force rises and levels off to a plateau. The maximum force value reached in C, was significantly lower (p < 0.05) than in A and B. The maximum force values and values for other parameters of the force-time curves in Fig. 2 are given in Table 1. As can be seen in the table, the
Data analysis
Although the force and angle data were sampled every 10 ms by the Kin-Corn recording system, we chose to compare data obtained in the different static and dynamic tests at every 30 ms, or a force reading at approximately each degree of movement at 3O”/s. Several force and time parameters of the static and dynamic contractions were statistically compared by means of an analysis of variance test (ANOVA) for repeated measures followed by a post hoc Tukey test.
RESULTS 1. Static force-rime
curves
Figure 2 illustrates the mean force-time curves recorded during the maximal voluntary static contrac-
,,.
..
B
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8.36
8.45 TIME
0.68
0.75
8.98
(S)
Fig. 2. Force-time curves in absolute values obtained during the first second of a maximal static contraction at three different ankle positions (A-C). Values given as mean + 1 SD. (N = 5).
92
D. GRAVEL, C. L. RICHARDSand M. FILION Table 1. Parameters of the force-time curves obtained in the test conditions Time (ms) to reach maximum force Test s:-10 s: 0” s:+ 10” D: - 10”Min D: 0”Min
Maximum force (Nm)
MRFD
MRFD
(Nmis)
( %I9
197 + 29* 186+ 12 147_+13 -
750 + 184 767 + 92 600+ 109
381_+86 414 * 56 409*91 438 k 86 553 + 50
-
30%
60%
125+21 123+28 134&32 143 + 28 128 + 30
90%
236 + 39 233 + 38 241 f 47 235+30 211+18
100%
605 k 194 2340 + 473 996 f 736 3080 + 1010 711+184 2740+713 439+46 365&18 -
*Values given as mean f 1 standard deviation (N = 5).
maximum rate of force development (MRFD) is least @ < 0.05) with the ankle in the + 10” position, which is also the position of least maximum force in absolute values. There were no significant differences (p > 0.05) among the three angular positions for the relative MRFD, or the times to reach 30,60,90 “/, of maximum force. Of particular interest to the evaluation of the effects of contractile rise time on the measurement of dynamic strength during contractions lasting about 1.3 s, are the times (mean) to reach maximum force in the three static contractions: 2340 to 3080 ms. In order to remove the effects of the magnitude of the static force developed at the three angular positions on the shape of the force-time curves, the force values were converted to relative values (percent of maximum). As can be seen in Fig. 3, the relative force-time curves obtained during the maximal static contractions at the three different ankle angles are nearly identical.
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Fig. 3. Comparison of relative force-time curves calculated from the foreetime curves in absolute values presented in Fig. 2.
2. Dynamic force-time curves Figure 4 illustrates the mean force-time curve recorded during maximal voluntary isokinetic plantarflexion movements at 3o”/s for the D: - lo” Min test. This movement was initiated from a minimal force level at a starting angle of - 10” dorsiflexion. The force-time curve (Fig. 4) thus includes a period of contractile tension rise at the beginning of the movement. The peak force (145 Nm) is reached in 450 ms after which the force declines gradually as the plantarflexion movement is completed. In the D: 0” Min test, it took 480 ms to reach the peak force of I23 Nm. 3. Derivation of dynamic contractile tension development curve Since the force development in dynamic contractions occurs in conjunction with ankle motion, the effects of force development on the force-angle record is confounded by changes due to the length-tension
relationship. To circumvent this problem we corrected (Fig. 5) the force-angle curves recorded in the D: - 10”Min and D: 0” Min tests for the force loss (at each angle) expected to occur with muscle shortening. The correction factor was computed from the D: - 10” Max force-angle curve, where the highest force measured at the end of maximum static pre-loading was assumed to be the maximum which would be maintained if the
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1.75
1.90
Fig. 4. Dynamic force-time curve recorded during the first second of the D: - lo” Min test. Values given as mean a 1 SD. (N=5).
muscles did not shorten with the movement. The shaded area in Fig. 5B illustrates the correction factor or ‘force loss’ which represents the angle specific difference between the assumed maximum and the measured force-angle curve. These ‘force loss’ values were then added to the force-angle curves measured in the D: - 10” Min and D: 0” Min tests. Since the plantarflexion movements were at a constant angular velocity (isokinetic), the x-axis can he converted to time.
93
Measurements of the plantarflexors in man
FORCE-ANGLE
- 18 -5
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18 ANGLE
28
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Fig. 6. Comparison of relative computed dynamic curves for the D: - 10”Min and D: WMin tests. Values given as mean of five subjects.
FORCE-ANGLE
CURVE
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E
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et -1a -5
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Fig. 7. Comparison of dynamic force-angle curves recorded during the different tests. The D: - 10”Min and D:O”Min curves were recorded from minimal force levels (without preloading), while the D: - 10”Max curve was recorded following a maximal static contraction (pre-loading). Values given as mean of five subjects.
. 188.
---..-B
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CURVE D
8 45 e 68 B 7s TIME
0.98
-1B’MIN
1.85
1.28
-
1.35
(S)
Fig. 5. illustration of the method used to compute the dynamic force-time curve. The forceeangle curve recorded without pre-loading (D: - 1O’Min) is given in A. In B, the shaded area represents the assumed ‘loss’ of force, angle for angle, resulting from muscle shortening in the I): - 10”Max test (with pre-loading). This shaded area was added to the force-angle curve in A to obtain the force-time curve represented in C (see text for details). Consequently, the computed force-angle curve also represents the computed dynamic force-time curve (Fig. 5C). The two dynamic force-time curves corrected for ‘force loss’ due to muscle shortening (D: - lo” Min and D:0"Min), when expressed in relative values (Fig. 6), have a similar shape and the time to reach 90 y0 of the maximum force is 439 and 365 ms respectively (Table 1). In terms of angular displacement, these times represent 13” and 11” of movement. The maximal rate of relative force development (MRFD in
Table 1) and the force values between 0.21 and 0.57 s (Fig. 6) were higher for the D:O” Min test (p < 0.05). The times to reach the different percent of maximal force values (see Table 1), however, were not statistically different (p > 0.05). 4. Dynnmic force-angle cwces The effect of the interaction of the starting angle and the contractile rise time on the measurement of the dynamic force-angle curves was evaluated by comparing the force profiles recorded in the three dynamic tests (Fig. 7). The D: - lo” Mm and D:0"Min force curves given in relation to the ankle position illustrate the pattern of force increase previously described (Fig. SA). When the computer indicated the first degree of movement (after an 80 ms delay), the force values were not zero but 52 and 62 Nm respectively in the D: - 10”Min and D:o"Min tests. When maxima1 force values are approached, these force-angle curves intersect the D: - 10” Max curve and remain above the D: - 10” Max curve until the end of the movement.
94
D.
GRAVEL,
C. L. RICHARDS and M. FILION
Comparison of force-angle values among the curves indicated that the force values of the D: - lo” Min curve were significantly (p < 0.05) higher than those of the D - 10”Max curve from +7” to + 30”. For the D: 0”Min curve, the force values were generally higher (p < 0.05) after the 9” angle. 5. Comparison of the relative force-time curves
static
and ‘dynamic’
As it can be seen in Figs 3 and 6, the relative static and computed dynamic force-time curves have similar shapes. The time needed to reach different percentages of maximal force values are similar (p > 0.05). Of the parameters compared, only the relative rate of force development (x/s in Table 1) obtained in test D:O”Min was greater (p < 0.05) than that obtained in all other tests. DISCUSSION
In this study we computed the dynamic force-time curve for the ankle plantarflexor muscles during maximal voluntary acceleration controlled isokinetic movements. The force-time curve was computed from force-angle curves recorded during maximal dynamic efforts with and without static pre-loading. Comparison of the force-angle curves recorded with and without pre-loading demonstrated marked differences in the force record in the first half of the movement. When preceded by maximal static preloading which eliminated most of the period of contractile tension rise in the subsequent dynamic contraction, the force-angle curve (Fig. 7) declines gradually from a maximum value (at the end of the preloading phase) as the muscles shorten with the movement. In contrast, the force-angle curve recorded for contractions initiated from the same angular position but at a minimal force level, thus including the period of contractile tension rise (D: - lO”Min), increases from the minimum level to reach a peak in or about 12 degrees of movement (time to reach 90’:; of maximum force) to a peak value before declining as the movement is completed. In the latter contractions (without preloading), the rise in contractile tension occurs concomitantly with a loss in tension due to muscle shortening with the plantarflexion movement. Thus, the force-time curve defining the rise in contractile tension phase in dynamic contractions was computed by removing the confounding effects of changes in muscle length (Fig. 5). By recording dynamic force-angle curves from different starting positions (Fig. 7), we also demonstrated that the period of tension rise (force-time curve) was not angle specific. Force-angle curves recorded during plantarflexions beginning 10” later (D: O”Min) without pre-loading intersected the other two curves later in the movement (more plantarflexion), indicating the duration of the force development phase to be relatively constant and equal to about
11” movement or 365 ms to reach 90 “/:,of the maximum force level at 3O”/s. We compared the computed dynamic force-time curves to static force-time curves. When the force-time curves were represented in percent of maximum to remove the effect of absolute force amplitude, the computed dynamic and static force-time curves were similar in shape (Figs 3 and 6) and the various parameters (Table 1) describing these curves were, in general, not statistically different (p > 0.05). This statistical comparison thus supports our hypothesis that the dynamic and static force-time curves are similar. Such a finding was not unexpected since compliance of the series-elastic components (Wilkie, 1950), maximal voluntary activations (Budingen and Freund, 1976) and the histochemical composition of the muscles (Viitalaso et al., 1981; Clarkson et al., 1981) were most likely similar in both type of contractions. Statistical comparison of the dynamic force-time curves D: - 10”Min and D 0”Min did, however, reveal a greater rate of relative force development for D: O”Min, suggesting an angle specific difference. This finding contrasts with the lack of significant differences in the relative force-time parameters (Table 1 and Fig. 3) for the three angles evaluated in the static tests. These contradictory findings for static and dynamic contractions therefore suggest that factors other than angular position influence the relative rate of force development. For instance, complex interactions of contraction duration and initial muscle length (Bahler et al., 1968) might have promoted a greater relative rate of force development in the D:O”Min tests. In the static tests, the times to reach maximum force levels for the ankle plantarflexions are rather long in comparison to the values reported for other muscle groups (Royce, 1962; Stothart, 1973; Caldwell et al., 1974; Morris et al., 1983). Our mean values (230&3080 ms) are, however, similar to those reported by Kamen (1983) in the same muscle group. A possible explanation might be the compliance associated with the different parts of the measurement system. Lengthening of the fixation straps holding the foot to the soleplate is difficult to prevent under the high external moment of force (1855230 Nm) generated. Another factor which may have influenced the force-angle records is the difference between the ankle angle given by the apparatus and the actual position of the foot at high force levels. When the plantarflexion forces are high, the heel leaves the footplate so that it is no longer parallel to the soleplate. In fact, using photographic measurements, we found a difference of 5” between the recorded and actual ankle position at force levels equal to and greater than 115 Nm (n = 2). This implies a complex shifting of all the recorded force curves (once the force exceeds 115 Nm) about 5” toward more plantarflexion and may have influenced the determination of the force and time parameters. Calculations which include this apparent plantarflexion shift, however, indicate that the differences (not
Meastirements
of the plantarflexors
statistically significant) observed between the static and dynamic force-time parameters become even smaller. Compliance of the joints of the foot is a second factor. Transmission of force occurs across many joints and deformation of the longitudinal arch of the foot absorbs energy (Tardieu et al., 1981). Compliance of the internal series-elastic components of the muscular-tendinous complex, which varies in different muscles (Wells, 1965), is also important. The force rises more slowly with time when compliance is present in the system. Finally, one muscle of the plantarflexor group, the soleus, has a very high percentage of slow fibers (Johnson et al., 1973; Gollnick et al., 1974; Edgerton er al., 1975; Fugl-Meyer et al., 1975; Elder et al., 1982). Since the mechanical contribution of the soleus is estimated to be at least 40”/6 of the plantarflexor group (Murray et al., 1976; Alexander and Vernon, 1976; Ripperger et al., 1980), the long contraction time in this muscle should contribute to the long force rise time measured. As observed in the isokinetic force-angle curves recorded without pre-loading (Fig. 7), the initial period of force development results in a peak force value which is, in fact, an equilibrium point where the rate of force development is equal to the rate of force lost due to the change of angle position. Consequently, this ‘peak’ value is not a valid measurement of the maximum force capacity. The duration of the contractile rise period is related to the nature and speed of the effort deployed by the subject and underlines the importance of the instructions given prior to the effort. The force recorded during this period of contractile tension rise thus represents a maximal effort (when starting from a minimal force level) but does not reflect the maximal strength capacity of the ankle plantarflexors in the early part of the isokinetic movement. Strength capacity in the early part of the movement will be greater in dynamic contractions preceded by pre-loading. Moreover, considering that maximum static tension is attained 2-3 s after initiation of the effort, it is possible that the true maximum is not reached in a dynamic contraction with a duration of 1.3 s. The use of static pre-loading before the beginning of the isokinetic movement removes the effect of the period of contractile tension rise (force-time curve) and consequently improves the estimate of maximal dynamic capacity (Gransberg and Knutsson, 1983), in the early part of the movement. Use of pre-loading IS thus especially important when evaluating the force produced over a limited angular displacement at high movement velocities. Although static pre-loading may improve the measurement of maximal dynamic capacity in the early part of an isokinetic movement, our findings suggest that opposite effects may occur in the last part of the movement. Thus, in the dynamic contractions preceded by static pre-loading, the forces produced from 7 to 30” of plantarflexion were less than for the two dynamic tests performed without pre-
95
in man
loading. Clarke (1968) also found that the maximal static handgrip force produced from resting tension was significantly greater than that developed after static pre-loading. Since maximal pre-loading involves a maximal voluntary static contraction of about 2 s duration which is then followed by a dynamic contraction of an additional duration of about 1.3 s (at 3O”is). the total duration of the effort is about 3.3 s. It may be that the dropout of fast-twitch fatiguable motor units (Grimby and Hannerz, 1977), and fatigue-related factors (Caldwell, 1964; Bonde-Petersen et al., 1975) may contribute to a decreased force capacity in the last part of the movement. Acknowledgements-The authors would like to thank Mr Vincent Piette and Mr Stephen Desjardins for their collaboration. This work was supported by a grant from the Institut de recherche en sank et en skurith du travail du Quebec (IRSST).
REFERENCES
Alexander, R. McN. and Vernon, A. (1975) The dimensions of knee and ankle muscles and the forces they exert. J. hum. Mov. Stud. 1, 115-123. Bahler, A. S., Fales, J. T. and Zierler, K. L. (1968) The dynamic properties of mammalian skeletal muscle. J. gen. Physiol. 51, 369-384. Bonde-Petersen, F., Work, A. L. and Nielsen, E. (1975) Local muscle blood flow and sustained contractions of human arm and back muscles. Europ. J. appl. Physiol. 34, 43-50. Budingen, H. J. and Freund, H. J. (1976) The relationship between the rate of rise of isometric tension and motor unit recruitment in a human forearm muscle. Pflugers Arch. 362,6167. Buller, A. J. and Lewis, D. M. (1965) The rate of tension development in isometric tetanic contractions of mammalian fast and slow skeletal muscle. J. Physiol. 176, 337-354. Caldwell. L. S. (1964) Measurement of static muscle endurance. J. Eng. Psych. 3, 1622. Caldwell, L. S., Chaffin, D. B., Dukes-Dobos, F. N., Kroemer, K. H. E., Laubach. L. L., Snook, S. H. and Wasserman, D. E. (1974) A proposed standard procedure for static muscle strength testing. Amer. ind. Hyg. Ass. J. 35, 201-206. Clarke, D. H. (1968) Force-time curves of voluntary muscular contraction at varying tensions. Res. Quart. 39, 900-907. Clarkson, P. M., Kroll, W. and Melchionda, A. M. ( I98 1) Age, isometric strength, rate of tension development and fiber type composition. J. Geront. 36, 648-653. Edgerton, V. E., Smith, J. L. and Simpson, D. R. (1975) Muscle fibre type populations of human leg muscles. Hist. J. 7. 259-266. Elder, G. C. B., Bradbury, K. and Roberts, R. (1982) Variability of fiber type distributions withm human muscles. J. appl. Physiol. 53, 1473-1480. Fugl-Meyer, A. R., Sjiistrom, M. and Wahlby. L. (1979) Human plantar flexion strength and structure. Actu physiol. Scand. 107, 47-56. Gollnick, P. D., Sjodin. B., Karlsson, J., Jansson, E. and Saltin, B. (1974) Human soleus muscle: a comparison of fiber composition and enzyme activities with other leg muscles. Pjiugers Arch. 348, 247-255. Gransberg, L. and Knutsson, E. (1983) Determination of dynamic muscle strength in man with acceleration controlled isokinetic movements. Acta physiol. Stand. 119. 317-320. Johnson, M. A., Polgar, J., Weightman, D. and Appleton. D.
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(1973) Data on the distribution of fibre types in thirty-six human muscles. An autopsy study. J. neural. Sci. 18, 111-129. Kamen, G. (1983) The acquisition of maximal isometric plantar flexor strength: a force-time curve analysis. J. mot. Behao. 15, 6373. Knutsson, E. and Mgrtensson, A. (1980) Dynamic motor capacity in spastic paresis and its relation to prime mover dysfunction, spastic reflexes and antagonist coactivation. Stand. J. Rehab. Med. 12,93-106. Miller, R. G., Mirka, A. and Maxfield, M. (1981) Rate of tension development in isometric contractions of a human hand muscle. Exp. Neurol. 73, 267-285. Morris, A. F., Clarke, D. H. and Dainis, A. (1983) Time to maximal voluntary contraction (MVC) for five different muscle groups in college adults. Res. Quart. 54, 163168. Murray, M. P., Guten, G. N., Baldwin, J. M. and Gardner, G. M. (1976) A comparison of plantar flexion torque with and without the triceps surae. Acta orthop. Stand. 47,122-124. Nistor, L., Markhede, G. and Grimby, G. (1982) A technique for measurements of plantar flexion torque with the Cybex II dynamometer. &and. J. Rehab. Med. 14, 163166. Perrine, J. J. and Edgerton, V. R. (1978) Muscle force-velocity relationships under isokinetic loading. Med. Sci. Sports 10, 159-166. Ranatunga, K. W. (1978) Characteristics of tension recruitment and mechanical activation in mammalian skeletal muscle. Exp. Neurol. 61, 175-184. Richards, C. L. (1980) Dynamic muscle function in human normal, pathological and prosthetic knee joints. Ph.D. Thesis, McGill University, Montreal.
Richards, C. L. (1981) Dynamic strength characteristics during isokinetic knee movements in healthy women. Physiother. Can. 33, 141-150. Ripperger, R. R., Chao, E. Y. and Stauffer, R. N. (1980) A quantitative analysis of leg musculature. Trans. 26th Annual Meeting of the Orthopaedic Research Society, p. 52. Royce, J. (1962) Force-time characteristics of the exertion and release of hand grip strength under normal and fatigued conditions. Res. Quart. 33, 444-450. Sapega, A. A., Nicholas, J. A., Sokolow, D. and Saranti, A. (1982) The nature of torque ‘overshoot’ in Cybex isokinnetic dynamometry. MedI Sci. Sports 14, 36&375. Stothart. J. P. (1973) Relationshius between selected biomechanical paramkters of stabc and dynamic muscle performance. Medicine and Sport: Biomechanics, Vol. III (Edited by Cerquiglini, S., Venerando, A. and Wartenweiler, J.) pp. 210-217. Karger, Basel. Tardieu, C., Colbeau-Justin, P., Bret, M. D., Lespargot, A. and Tardieu, G. (1981) Effects on torque angle curve of differences in the recorded tibia-calcaneal angle and the true anatomical angle. Eur. J. appl. Physiol. 46, 41-46. Viitasalo, J. T., Hakkinen, K. and Komi, P. V. (1981) Isometric and dynamic force production and muscle fibre composition in man. J. hum. Mov. Studies 7, 199-209. Wells, J. B. (1965) Comparison of mechanical properties between slow and fast mammalian muscles. J. Physiol. 178, 252-269.
Wilkie, D. R. (1950) The relation between force and velocity in human muscle. J. Physiol. 110, 249-280. Winter, D.A., Wells, R. P. and Orr, G. W. (1981) Errors in the use of isokinetic dynamometers. Eur. J. appl. Physiol. 46, 397408.
in Great
Vol
21. No. 2. pp. 89-96.
OOZI-9290/88
1988. q
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1988 Pergamon
$3.00 + Journals
00 Ltd.
INFLUENCE OF CONTRACTILE TENSION DEVELOPMENT ON DYNAMIC STRENGTH MEASUREMENTS OF THE PLANTARFLEXORS
IN MAN
DENIS GRAVEL, CAROL L. RICHARDS* and MICHEL FILION Faculty of Medicine, Lava1 University, Quebec, P. Que. GlK 7P4, Canada Abstract-The influence of the contractile tension rise time on isokinetic force-angle records has been inferred from static forc+timecurves but has not been experimentally determined. The purpose of this study is thus to describe the influence of the contractile rise time on the force-angle curves produced during maximal voluntary, acceleration controlled, isokinetic plantarflexions at 30-/s. Since we could not measure directly the period of force development unbiased by changes in muscle length during the movements, we devised an experimental strategy which allowed the computation of the dynamic force-time curve. Thus in five normal men, we first recorded force-angle curves produced during maximal voluntary plantarflexion movements preceded by maximal static pre-loading (D: - 10”Max) in order to eliminate the period of tension development from the force-angle record. Next, we recorded force-angle curves produced during maximal voluntary contractions initiated from two different starting angles without pre-loading (D: - lo” Min and D: 0”Min) to include the period of tension rise. The dynamic force-time curve was computed by correcting these force-angle curves (D: - lo” Min and D: 0”Min) for the hypothetical loss in force due to muscle shortening. We compared the relative (to remove the effects of force magnitude) computed dynamic force-time curves with relative static force-time curves measured at three different angles. We found the shape and several other parameters of all three static and both computed dynamic force-time curves to be similar (p > 0.05). The time to reach 90 ‘4 of maximum force in the dynamic tests was 365 and 439 ms (mean values), or 11” and 13” after initiation of the movement at 3O”/s, indicating the period of contractile tension rise. These results stress the importance of static pre-loading to improve the measurement of maximal strength capacity in the early part of an isokinetic contraction, especially when the excursion is limited and the speed is high. Use of pre-loading, however, may lead to underestimation of the maximal dynamic capacity in the last part of the isokinetic plantarflexion movements.
INTRODUCTION
When muscles are activated, by voluntary effort or by electrical stimulation, the contractile tension takes time to rise to maximum. The resulting force-time curve has been studied in both humans and animals in static conditions. The shape of the curve has been shown to depend on factors such as the total compliance of the series-elastic components (Wilkie, 1950), the type of muscle activation (Buller and Lewis, 1965; Budingen and Freund, 1976; Ranatunga, 1978; Miller et al., 1981) and the histochemical composition of the muscles (Buller and Lewis, 1965; Viitasalo et aI., 1981; Clarkson et al., 1981). The relative importance of each of these factors has not been completely elucidated. Nevertheless, since they are fundamental characteristics of a muscle or muscle group, they should not be modified by the fact that a contraction is made in static or in dynamic conditions. Therefore, the shape of static and dynamic force-time curves should be similar. In humans, during the performance of maximal voluntary contractions in static conditions, contractile tension requires about 0.5 s to reach 90 y0 of maximum (Royce, 1962; Clarke, 1968; Morris et al., 1983). In -
-
Receioed 5 May 1986; in revised form 13 January 1987. *Corresponde& address: Laboiatoire de Neurobiologie, H&pita1 de I’Enfant-J&us. 1401, 18e Rue Quibec, Que G 1J 124, Canada.
dynamic conditions, the hypothesis presented above predicts that it should require the same time to reach 90 7; of maximum, that is 15” of movement excursion at 3o”/s. However, the time course of the rise in contractile tension cannot be measured directly in dynamic conditions. In fact, dynamic force records show a rise of force at the beginning of movement, but this is rapidly followed by a decrease lasting until the end of movement. This decrease reflects mainly the mechanics of the joint and the length-tension relationship as the muscles are allowed to shorten to produce movement. The effects of the rise in contractile tension on the force record are therefore confounded with those of the length-tension relationship: the former factor tending to increase the force, the latter tending to decrease it. This problem is the subject of the present study. The isokinetic dynamometer used in most studies up to 1983 was the Cybex 11. The subject was asked to make and maintain a maximal voluntary effort for the duration of the movement. This was requested to obtain constant muscular activation, at levels comparable across individuals and testing conditions. This dynamometer imposed on the limb a load proportional to the force tending to increase the velocity and the magnitude of this force was therefore taken as a measure of contractile tension, provided adequate corrections were made for gravitational forces. The Cybex II, however, did not offer resistance and thus did 89
90
D. GRAVEL, C. L. RICHARDSand M. FILION
not measure force during the acceleration phase of movement. Moreover, when the selected speed was reached and the system suddenly braked the acceleration, it gave rise to an impact torque followed by a series of oscillations corrupting force records, markedly at high velocities (Winter et al., 1981; Sapega et al., 1983). This technical limitation incited investigators to avoid the first part of the torque-angle curve measured with the Cybex II dynamometer (Perrine and Edgerton, 1978; Knutsson and M&tensson, 1980; Richards, 1980, 1981). Although the development of contractile tension was implicit in these dynamic strength records, its influence on the torque-angle record was rarely discussed (Gransberg and Knutsson, 1983). In order to improve force records in the early part of movement, Gransberg and Knutsson (1983) added computer assistance to the speed control of a Cybex II dynamometer. The system offers two new features: control of acceleration and pre-loading. The acceleration control provides some resistance to movement during its acceleration phase. Thus, it prevents the impact torque and oscillations. However, the strength measurements taken during this period are not isokinetic. Pre-loading is the blockage of movement until a level of static force is reached. This results in the recording of higher force levels at the beginning of movement. These forces probably more correctly correspond to the maximal capacity of the muscle at the first angles of movement. Furthermore, the use of pre-loading tends to elude the problems posed by the development of contractile tension during dynamic contractions since it removes this rise of force from the dynamic conditions. In the present study, it will be assumed that a level of pre-loading equal to at least 90% of maximal static strength removes an equal percentage of the rise time of contractile tension from the immediately following dynamic conditions. The dynamic force records, obtained with such pre-loading, will therefore be taken as relatively free of the effects of the development of contractile tension. Such records will be used as reference to differentiate by computation, from other dynamic force records without pre-loading, the changes of force related to the development of contractile tension from those related to the effects of the length-tension relationship. These computed dynamic force-time curves will be statistically compared with force-time curves obtained in static conditions.
Strength measurements
Plantarflexion strength measurements were made with the subjects seated on a specially designed chair, the hip angle flexed 1lo”, and restraining straps placed across the trunk, hips and right thigh (Fig. 1). The knee was extended and the right foot attached to a metal soleplate as described by Nistor et al. (1982). This soleplate was connected to a computer controlled KinCorn dynamometer so that the ankle rotational axis was aligned to the dynamometer axis. Plantarflexion force and ankle angle (sampled at 100 Hz) were recorded as the subjects performed a series of maximal voluntary static and dynamic contractions under standardized test conditions. Each series consisted of three specific contractions interspersed by 30 s rest periods, while 2-3 min rest periods separated the different series of contractions. Static contractions
Maximal voluntary static contractions were performed at three different ankle positions: -10” of dorsiflexion (S:- lo’), 0” (S:o”) and lo” (S:+ 10”). Subjects were told to exert maximal force as fast as possible and to maintain this level for a duration of 4 s. Dynamic contractions
Maximal voluntary dynamic contractions were performed at a constant velocity of 3O”/s. Test 1: the first dynamic test (I): - 10”Max) consisted of isokinetic contractions from - 10” of dorsiflexion to + 30” of plantarflexion preceded by a maximal static contraction with a duration of about 2 s. The purpose of this static pre-loading was to allow time for the contractile tension to rise before movement was permitted, thus eliminating its influence on the early part of the dynamic force record. Test 2: the second dynamic test (D: - 10”Min) required a maximal contraction through the same range of motion as in test 1, but this time the dynamic effort was not preceded by a maximal static contraction. Test 3: the third dynamic test (D: O”Min) was similar to test 2 except that the starting angle was 0” instead of - 10”. Since tests 2 and 3 began from a TESTS
PmmJE38 ‘6
STClTIC S;-18’
DYtwlIC
38
‘4
Da-WMAX O:-1B’MIN 0: 8’NIN
SUBJECTS AND METHODS
Subjects
Five healthy men participated in this study. The mean age was 30.6 + 4.0 (S.D.) yr. All were university graduate students without dysfunction of the right lower extremity.
Fig. 1. Schematic representation of the position of the subject and range of ankle movement evaluated in the different test conditions.
91
Measurements of the plantarflexors in man minimal force level, the dynamic force record includes a period of tension development in contrast to test 1
where the period of tension rise occurs during the static pre-loading. In order to limit the effects of fatigue and learning on performance, the order of the static and dynamic tests, as well as the sequence of these tests were systematically alternated. Verbal commands were standardized and given by the same person. The forces produced during the voluntary contractions were corrected for passive forces due to the weight of the footplate and the joint structures. These passive forces were obtained by measuring the resistance of passive ankle movements at 3o”/s. Measurements were made throughout the range of the plantarflexion movement, and after a pause of 1 s, during the reverse movement. Mean values (three repetitions) were used to calculate the correction, angle for angle, throughout the movement. Since the Kin-Corn dynamometer provides acceleration control in the first part of the movement, forces are measured during the period of contractile tension rise (when pre-loading is not used). The acceleration control period is, however, fixed (cannot be programmed) and its duration is dependent on the selected speed. Two degrees of movement are required to reach the selected speed of 3O”/s. The Kin-Corn system also allows the setting of a minimal force level which must be reached for movement to occur. This threshold force prevents movements from starting inappropriately, for example, due to the weight of the limb segment. The threshold force for the dynamic contractions (D:- 10”Min. D: 0”Min) was set equal to the passive resistance measured at - 10” of dorsiflexion plus 20 N (4.6 Nm). This threshold force is active throughout the movement, however, and thus precludes use of this feature for static pre-loading at the high force levels used in D: - 10”Max test. To provide high static preloading in the D: - 10” Max test, it was necessary to instruct the subjects to begin maximal voluntary static contractions 2 s (determined from the time display) before the end of the programmed rest period.
tions at three different ankle angles (A-C). These angular positions were chosen to correspond to the starting positions of the dynamic contractions (tests l-3). Furthermore, by recording the force-time curve at different angular positions, we were able to evaluate the effect of ankle angle on the characteristics of the static force-time curve. The three force-time curves (Fig. ZA-C) have a characteristic sigmoid shape as the force rises and levels off to a plateau. The maximum force value reached in C, was significantly lower (p < 0.05) than in A and B. The maximum force values and values for other parameters of the force-time curves in Fig. 2 are given in Table 1. As can be seen in the table, the
Data analysis
Although the force and angle data were sampled every 10 ms by the Kin-Corn recording system, we chose to compare data obtained in the different static and dynamic tests at every 30 ms, or a force reading at approximately each degree of movement at 3O”/s. Several force and time parameters of the static and dynamic contractions were statistically compared by means of an analysis of variance test (ANOVA) for repeated measures followed by a post hoc Tukey test.
RESULTS 1. Static force-rime
curves
Figure 2 illustrates the mean force-time curves recorded during the maximal voluntary static contrac-
,,.
..
B
e.15
..I
8.36
8.45 TIME
0.68
0.75
8.98
(S)
Fig. 2. Force-time curves in absolute values obtained during the first second of a maximal static contraction at three different ankle positions (A-C). Values given as mean + 1 SD. (N = 5).
92
D. GRAVEL, C. L. RICHARDSand M. FILION Table 1. Parameters of the force-time curves obtained in the test conditions Time (ms) to reach maximum force Test s:-10 s: 0” s:+ 10” D: - 10”Min D: 0”Min
Maximum force (Nm)
MRFD
MRFD
(Nmis)
( %I9
197 + 29* 186+ 12 147_+13 -
750 + 184 767 + 92 600+ 109
381_+86 414 * 56 409*91 438 k 86 553 + 50
-
30%
60%
125+21 123+28 134&32 143 + 28 128 + 30
90%
236 + 39 233 + 38 241 f 47 235+30 211+18
100%
605 k 194 2340 + 473 996 f 736 3080 + 1010 711+184 2740+713 439+46 365&18 -
*Values given as mean f 1 standard deviation (N = 5).
maximum rate of force development (MRFD) is least @ < 0.05) with the ankle in the + 10” position, which is also the position of least maximum force in absolute values. There were no significant differences (p > 0.05) among the three angular positions for the relative MRFD, or the times to reach 30,60,90 “/, of maximum force. Of particular interest to the evaluation of the effects of contractile rise time on the measurement of dynamic strength during contractions lasting about 1.3 s, are the times (mean) to reach maximum force in the three static contractions: 2340 to 3080 ms. In order to remove the effects of the magnitude of the static force developed at the three angular positions on the shape of the force-time curves, the force values were converted to relative values (percent of maximum). As can be seen in Fig. 3, the relative force-time curves obtained during the maximal static contractions at the three different ankle angles are nearly identical.
I,.:..,_ .__, ___, ,__ *
:
2e
8
.
s:-,p s: 8’
.
S:tll
.
(
D n’
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0.15
0
@.45 1.61 TIME
0.75
(
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Fig. 3. Comparison of relative force-time curves calculated from the foreetime curves in absolute values presented in Fig. 2.
2. Dynamic force-time curves Figure 4 illustrates the mean force-time curve recorded during maximal voluntary isokinetic plantarflexion movements at 3o”/s for the D: - lo” Min test. This movement was initiated from a minimal force level at a starting angle of - 10” dorsiflexion. The force-time curve (Fig. 4) thus includes a period of contractile tension rise at the beginning of the movement. The peak force (145 Nm) is reached in 450 ms after which the force declines gradually as the plantarflexion movement is completed. In the D: 0” Min test, it took 480 ms to reach the peak force of I23 Nm. 3. Derivation of dynamic contractile tension development curve Since the force development in dynamic contractions occurs in conjunction with ankle motion, the effects of force development on the force-angle record is confounded by changes due to the length-tension
relationship. To circumvent this problem we corrected (Fig. 5) the force-angle curves recorded in the D: - 10”Min and D: 0” Min tests for the force loss (at each angle) expected to occur with muscle shortening. The correction factor was computed from the D: - 10” Max force-angle curve, where the highest force measured at the end of maximum static pre-loading was assumed to be the maximum which would be maintained if the
tfli iii
4t
,L.+’ I
a.15
L3a
8.45 8.6s TIRE (S)
1.75
1.90
Fig. 4. Dynamic force-time curve recorded during the first second of the D: - lo” Min test. Values given as mean a 1 SD. (N=5).
muscles did not shorten with the movement. The shaded area in Fig. 5B illustrates the correction factor or ‘force loss’ which represents the angle specific difference between the assumed maximum and the measured force-angle curve. These ‘force loss’ values were then added to the force-angle curves measured in the D: - 10” Min and D: 0” Min tests. Since the plantarflexion movements were at a constant angular velocity (isokinetic), the x-axis can he converted to time.
93
Measurements of the plantarflexors in man
FORCE-ANGLE
- 18 -5
e
5
CURUE
D--1E’MIN
I5
18 ANGLE
28
25
(‘)
38 (PF)
e
8.15
e.3e
0.4s TIME
8.66 (S)
a.75
9.99
Fig. 6. Comparison of relative computed dynamic curves for the D: - 10”Min and D: WMin tests. Values given as mean of five subjects.
FORCE-ANGLE
CURVE
D--1B’MAX
8 -10 -5
E
28
5
25
, 38 (PF)
et -1a -5
,’ .
59 .a FORCE-TIME
8.15
21
5 At&
P?
. 25
-*.I
38
Fig. 7. Comparison of dynamic force-angle curves recorded during the different tests. The D: - 10”Min and D:O”Min curves were recorded from minimal force levels (without preloading), while the D: - 10”Max curve was recorded following a maximal static contraction (pre-loading). Values given as mean of five subjects.
. 188.
---..-B
8.38
CURVE D
8 45 e 68 B 7s TIME
0.98
-1B’MIN
1.85
1.28
-
1.35
(S)
Fig. 5. illustration of the method used to compute the dynamic force-time curve. The forceeangle curve recorded without pre-loading (D: - 1O’Min) is given in A. In B, the shaded area represents the assumed ‘loss’ of force, angle for angle, resulting from muscle shortening in the I): - 10”Max test (with pre-loading). This shaded area was added to the force-angle curve in A to obtain the force-time curve represented in C (see text for details). Consequently, the computed force-angle curve also represents the computed dynamic force-time curve (Fig. 5C). The two dynamic force-time curves corrected for ‘force loss’ due to muscle shortening (D: - lo” Min and D:0"Min), when expressed in relative values (Fig. 6), have a similar shape and the time to reach 90 y0 of the maximum force is 439 and 365 ms respectively (Table 1). In terms of angular displacement, these times represent 13” and 11” of movement. The maximal rate of relative force development (MRFD in
Table 1) and the force values between 0.21 and 0.57 s (Fig. 6) were higher for the D:O” Min test (p < 0.05). The times to reach the different percent of maximal force values (see Table 1), however, were not statistically different (p > 0.05). 4. Dynnmic force-angle cwces The effect of the interaction of the starting angle and the contractile rise time on the measurement of the dynamic force-angle curves was evaluated by comparing the force profiles recorded in the three dynamic tests (Fig. 7). The D: - lo” Mm and D:0"Min force curves given in relation to the ankle position illustrate the pattern of force increase previously described (Fig. SA). When the computer indicated the first degree of movement (after an 80 ms delay), the force values were not zero but 52 and 62 Nm respectively in the D: - 10”Min and D:o"Min tests. When maxima1 force values are approached, these force-angle curves intersect the D: - 10” Max curve and remain above the D: - 10” Max curve until the end of the movement.
94
D.
GRAVEL,
C. L. RICHARDS and M. FILION
Comparison of force-angle values among the curves indicated that the force values of the D: - lo” Min curve were significantly (p < 0.05) higher than those of the D - 10”Max curve from +7” to + 30”. For the D: 0”Min curve, the force values were generally higher (p < 0.05) after the 9” angle. 5. Comparison of the relative force-time curves
static
and ‘dynamic’
As it can be seen in Figs 3 and 6, the relative static and computed dynamic force-time curves have similar shapes. The time needed to reach different percentages of maximal force values are similar (p > 0.05). Of the parameters compared, only the relative rate of force development (x/s in Table 1) obtained in test D:O”Min was greater (p < 0.05) than that obtained in all other tests. DISCUSSION
In this study we computed the dynamic force-time curve for the ankle plantarflexor muscles during maximal voluntary acceleration controlled isokinetic movements. The force-time curve was computed from force-angle curves recorded during maximal dynamic efforts with and without static pre-loading. Comparison of the force-angle curves recorded with and without pre-loading demonstrated marked differences in the force record in the first half of the movement. When preceded by maximal static preloading which eliminated most of the period of contractile tension rise in the subsequent dynamic contraction, the force-angle curve (Fig. 7) declines gradually from a maximum value (at the end of the preloading phase) as the muscles shorten with the movement. In contrast, the force-angle curve recorded for contractions initiated from the same angular position but at a minimal force level, thus including the period of contractile tension rise (D: - lO”Min), increases from the minimum level to reach a peak in or about 12 degrees of movement (time to reach 90’:; of maximum force) to a peak value before declining as the movement is completed. In the latter contractions (without preloading), the rise in contractile tension occurs concomitantly with a loss in tension due to muscle shortening with the plantarflexion movement. Thus, the force-time curve defining the rise in contractile tension phase in dynamic contractions was computed by removing the confounding effects of changes in muscle length (Fig. 5). By recording dynamic force-angle curves from different starting positions (Fig. 7), we also demonstrated that the period of tension rise (force-time curve) was not angle specific. Force-angle curves recorded during plantarflexions beginning 10” later (D: O”Min) without pre-loading intersected the other two curves later in the movement (more plantarflexion), indicating the duration of the force development phase to be relatively constant and equal to about
11” movement or 365 ms to reach 90 “/:,of the maximum force level at 3O”/s. We compared the computed dynamic force-time curves to static force-time curves. When the force-time curves were represented in percent of maximum to remove the effect of absolute force amplitude, the computed dynamic and static force-time curves were similar in shape (Figs 3 and 6) and the various parameters (Table 1) describing these curves were, in general, not statistically different (p > 0.05). This statistical comparison thus supports our hypothesis that the dynamic and static force-time curves are similar. Such a finding was not unexpected since compliance of the series-elastic components (Wilkie, 1950), maximal voluntary activations (Budingen and Freund, 1976) and the histochemical composition of the muscles (Viitalaso et al., 1981; Clarkson et al., 1981) were most likely similar in both type of contractions. Statistical comparison of the dynamic force-time curves D: - 10”Min and D 0”Min did, however, reveal a greater rate of relative force development for D: O”Min, suggesting an angle specific difference. This finding contrasts with the lack of significant differences in the relative force-time parameters (Table 1 and Fig. 3) for the three angles evaluated in the static tests. These contradictory findings for static and dynamic contractions therefore suggest that factors other than angular position influence the relative rate of force development. For instance, complex interactions of contraction duration and initial muscle length (Bahler et al., 1968) might have promoted a greater relative rate of force development in the D:O”Min tests. In the static tests, the times to reach maximum force levels for the ankle plantarflexions are rather long in comparison to the values reported for other muscle groups (Royce, 1962; Stothart, 1973; Caldwell et al., 1974; Morris et al., 1983). Our mean values (230&3080 ms) are, however, similar to those reported by Kamen (1983) in the same muscle group. A possible explanation might be the compliance associated with the different parts of the measurement system. Lengthening of the fixation straps holding the foot to the soleplate is difficult to prevent under the high external moment of force (1855230 Nm) generated. Another factor which may have influenced the force-angle records is the difference between the ankle angle given by the apparatus and the actual position of the foot at high force levels. When the plantarflexion forces are high, the heel leaves the footplate so that it is no longer parallel to the soleplate. In fact, using photographic measurements, we found a difference of 5” between the recorded and actual ankle position at force levels equal to and greater than 115 Nm (n = 2). This implies a complex shifting of all the recorded force curves (once the force exceeds 115 Nm) about 5” toward more plantarflexion and may have influenced the determination of the force and time parameters. Calculations which include this apparent plantarflexion shift, however, indicate that the differences (not
Meastirements
of the plantarflexors
statistically significant) observed between the static and dynamic force-time parameters become even smaller. Compliance of the joints of the foot is a second factor. Transmission of force occurs across many joints and deformation of the longitudinal arch of the foot absorbs energy (Tardieu et al., 1981). Compliance of the internal series-elastic components of the muscular-tendinous complex, which varies in different muscles (Wells, 1965), is also important. The force rises more slowly with time when compliance is present in the system. Finally, one muscle of the plantarflexor group, the soleus, has a very high percentage of slow fibers (Johnson et al., 1973; Gollnick et al., 1974; Edgerton er al., 1975; Fugl-Meyer et al., 1975; Elder et al., 1982). Since the mechanical contribution of the soleus is estimated to be at least 40”/6 of the plantarflexor group (Murray et al., 1976; Alexander and Vernon, 1976; Ripperger et al., 1980), the long contraction time in this muscle should contribute to the long force rise time measured. As observed in the isokinetic force-angle curves recorded without pre-loading (Fig. 7), the initial period of force development results in a peak force value which is, in fact, an equilibrium point where the rate of force development is equal to the rate of force lost due to the change of angle position. Consequently, this ‘peak’ value is not a valid measurement of the maximum force capacity. The duration of the contractile rise period is related to the nature and speed of the effort deployed by the subject and underlines the importance of the instructions given prior to the effort. The force recorded during this period of contractile tension rise thus represents a maximal effort (when starting from a minimal force level) but does not reflect the maximal strength capacity of the ankle plantarflexors in the early part of the isokinetic movement. Strength capacity in the early part of the movement will be greater in dynamic contractions preceded by pre-loading. Moreover, considering that maximum static tension is attained 2-3 s after initiation of the effort, it is possible that the true maximum is not reached in a dynamic contraction with a duration of 1.3 s. The use of static pre-loading before the beginning of the isokinetic movement removes the effect of the period of contractile tension rise (force-time curve) and consequently improves the estimate of maximal dynamic capacity (Gransberg and Knutsson, 1983), in the early part of the movement. Use of pre-loading IS thus especially important when evaluating the force produced over a limited angular displacement at high movement velocities. Although static pre-loading may improve the measurement of maximal dynamic capacity in the early part of an isokinetic movement, our findings suggest that opposite effects may occur in the last part of the movement. Thus, in the dynamic contractions preceded by static pre-loading, the forces produced from 7 to 30” of plantarflexion were less than for the two dynamic tests performed without pre-
95
in man
loading. Clarke (1968) also found that the maximal static handgrip force produced from resting tension was significantly greater than that developed after static pre-loading. Since maximal pre-loading involves a maximal voluntary static contraction of about 2 s duration which is then followed by a dynamic contraction of an additional duration of about 1.3 s (at 3O”is). the total duration of the effort is about 3.3 s. It may be that the dropout of fast-twitch fatiguable motor units (Grimby and Hannerz, 1977), and fatigue-related factors (Caldwell, 1964; Bonde-Petersen et al., 1975) may contribute to a decreased force capacity in the last part of the movement. Acknowledgements-The authors would like to thank Mr Vincent Piette and Mr Stephen Desjardins for their collaboration. This work was supported by a grant from the Institut de recherche en sank et en skurith du travail du Quebec (IRSST).
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