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School of Medical Rehabilitation, University of Manitoba, Winnipeg, Manitoba, Canada R3E 0W3
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Webber, Sandra, and Dean Kriellaars. Neuromuscular
factors contributing to in vivo eccentric moment generation.
J. Appl. Physiol. 83(1): 40-45, 1997.
Muscle series elasticity and its contribution to eccentric
moment generation was examined in humans. While subjects [male,
n = 30; age 26.3 ± 4.8 (SD) yr; body mass 78.8 ± 13.1 kg] performed an isometric contraction
of the knee extensors at 60° of knee flexion, a quick stretch was imposed with a 12°-step displacement at 100°/s. The test was
performed at 10 isometric activation levels ranging from 1.7 to 95.2%
of maximal voluntary contraction (MVC). A strong linear relationship was observed between the peak imposed eccentric moment derived from
quick stretch and the isometric activation level
(y = 1.44x + 7.08; r = 0.99). This increase in the
eccentric moment is consistent with an actomyosin-dependent elasticity
located in series with the contractile element of muscle. By
extrapolating the linear relationship to 100% MVC, the predicted
maximum eccentric moment was found to be 151% MVC, consistent with in
vitro data. A maximal voluntary, knee extensor strength test was also
performed (5-95°, 3 repetitions, ±50, 100, 150, 200, and
250°/s). The predicted maximum eccentric moment was 206% of the
angle- and velocity-matched, maximal voluntary eccentric moments. This
was attributed to a potent neural regulatory mechanism that limits the
recruitment and/or discharge of motor units during maximal
voluntary eccentric contractions.
actomyosin; cross bridge; series elasticity; strength
INTRODUCTION ACCORDING TO THE IN VITRO force-velocity relationship
depicted for maximally activated muscle, force generated during muscle lengthening substantially exceeds maximum isometric force, which in
turn exceeds force produced during muscle shortening (7-9, 12, 14,
24). Moment-angular velocity relationships derived from human studies
demonstrate that the isometric, maximal voluntary contraction (MVC)
exceeds peak and angle-specific concentric moments that decrease with
increasing velocity of shortening (6, 17, 32, 33). However, the ratio
of eccentric to isometric moments is not consistent with that observed
for in vitro force-velocity data. Peak forces generated during muscle
lengthening in vitro have been shown to reach 1.5-1.9 maximum
isometric force (8, 11, 12, 24), whereas human studies
have generally found that maximal voluntary eccentric moments are not
statistically greater than MVC (6, 23, 25, 32-34). Studies have
employed electrical stimulation (6, 34) and quick stretch (10, 28) in
attempts to achieve eccentric magnitudes similar to in vitro data (2,
4, 18, 35). The eccentric-to-isometric ratios achieved in these studies
have not reached the lower range of that observed in vitro.
The neuromuscular factors that contribute to the generation of
resultant joint moment during eccentric muscle activity are not well
understood. Investigators have postulated that differences in
actomyosin cross-bridge cycling and engagement processes might allow
for greater moment production during eccentric contractions compared
with concentric or isometric contractions (10, 12, 24). The presence of
a series elastic component related to cross-bridge engagement may be
the most significant factor responsible for the enhanced moment
generation observed during lengthening contractions (2, 9, 10, 15, 18,
35).
Understanding the neuromuscular factors that contribute to the ability
to control segmental rotation (or strength) is important. This study
was designed to examine the relationship between maximal voluntary
eccentric moments and eccentric moments generated by a quick stretch
imposed during different levels of isometric contraction.
MATERIALS AND METHODS
k;
A), angular velocity
(
leg;
B), angular acceleration
(
leg;
C), and resultant joint moment about the knee (Mk;
D) are shown for a step test.
Subject performed test with 10 isometric levels (initial isometric)
ranging from 15.4 to 95.2% maximal voluntary contraction (MVC). During
step perturbation, Mk data
revealed an early, inertial-dependent component (t 
40 ms) followed by peak imposed
eccentric moment (t 120 ms). a,
Isovelocity region
(±300°/s2).
Each subject was required to perform the step test with 10 levels of isometric activation ranging from 5 to 95% of MVC. The 10 perturbations were performed in 2 sets of 5 repetitions (interleaved levels), with a 2-min rest between sets and a 4-s pause between perturbations. The subjects were instructed to produce a constant isometric contraction near the target level. The subject viewed the dynamometer monitor, which displayed the force transducer values numerically and provided a real-time continuous graphical display of the force values. This feedback was used to assist subjects in attaining relatively constant isometric Mk near the specified target levels. The subjects were instructed to relax after the perturbation was completed and while the actuator arm returned the leg to the starting position. With these instructions, the subjects produced isometric contractions that were steady but that were systematically lower than the specified target levels. The Mk data for each imposed eccentric contraction consisted of two peaks (Fig. 1). The first peak occurred consistently 40 ms after onset of the perturbation. This is an acceleration-dependent inertial moment equal to the product of the angular acceleration and the moment of inertia of the leg, foot, and resistance pad (13). The peak imposed eccentric Mk (second peak) was determined for each of the 10 initial isometric levels during the constant angular velocity or isovelocity phase (Fig. 1). The magnitude of the initial isometric Mk was determined by averaging the moment data in a 220-ms window just before onset of the perturbation. Linear regression was performed between the initial isometric Mk (independent) and the peak imposed eccentric Mk (dependent). The musculotendinous tissues about the knee contribute a passive component to Mk. The contribution of the passive component was estimated by applying the step test perturbation (5 repetitions) while the subject was relaxed. The mean passive component was then subtracted from the Mk to partially account for differences in the passive resistance of tissues among subjects. In this range of motion, the magnitude of the passive component was small [2.76 ± 2.71 (SD) N · m]. Very good test-retest reliability (n = 5) was observed for the step test by using the slope parameter derived from linear regression with an intraclass correlation coefficient of 0.91. Stretch-evoked reflex contribution. The contribution of a stretch-evoked reflex contribution from the quadriceps to Mk was assessed in 10 additional subjects. Passive displacements (3 repetitions, 0-90° flexion, 135°/s) were imposed with differential electromyographic (EMG) recording of the quadriceps muscle group [Adult Medtronic Cleantrace electrodes, 2- to 4-cm spacing between active electrodes located midthigh, 44 M
at 60-Hz
preamplifier input impedance, >90-db common mode rejection
ratio, 10-1,000 Hz (±3-db band-pass
filtered)]. The band-pass-filtered EMG, angle, angular velocity,
and dynamometer moment were digitized (12 bit) at 2,000 Hz. A
stretch-evoked increase in the quadriceps EMG was not observed. In two
additional subjects, quadriceps EMG was recorded during the step test
with 10 isometric activation levels. A stretch-evoked response was not
observed in the EMG at any of the isometric levels.
Strength test.
Subjects performed three maximal voluntary concentric and eccentric
knee extensor contractions at five speeds (50, 100, 150, 200 and
250°/s) through a 90° range of motion (5-95° knee
flexion). A 4-s pause was provided between successive concentric and
eccentric contractions at each velocity. A 2-min rest was given between test speeds. Order of testing was blocked (50, 100, 150, 200, and 250 or 150, 200, 250, 50, and 100°/s). A familiarization bout was
provided, consisting of three low-level, submaximal repetitions of
concentric and eccentric contractions at each test speed.
Analysis of data obtained from the strength test was restricted to
isovelocity regions. Under normal operation, the dynamometer actuator
arm's speed fluctuates about the preset value throughout the range of
motion. The isovelocity region of each repetition recorded in this
study was determined by using an angular acceleration threshold of
±300°/s2. At constant
angular velocity, the sum of all moments acting about the dynamometer
axis of rotation equals zero. When the dynamometer actuator arm is
undergoing acceleration, the moments sum to the product of the angular
acceleration and the moment of inertia of the attached segments (13).
In this study, the maximum error in determining the magnitude of
Mk associated with a
±300°/s2 angular
acceleration threshold corresponded to ±3.0
N · m.
Absolute peak Mk (largest of the
peak moments from 3 repetitions), mean peak
Mk (average of the peak moments
from 3 repetitions), and mean angle-specific
Mk (average of moments recorded at
70° over 3 repetitions) were determined for eccentric and
concentric contractions for all speeds. The angle of occurrence of peak
Mk was also recorded. Acceptable
test-retest reliability (n = 5) was
obtained, with intraclass correlation coefficients ranging from 0.7 to
0.92 for the strength parameters.
RESULTS
The peak imposed eccentric Mk and
the initial isometric Mk derived
from the step test were normalized to MVC and expressed as a percentage
(%MVC). A strong linear relationship (Fig.
2) was observed between the normalized
magnitudes of the peak imposed eccentric
Mk and magnitudes of the initial
isometric Mk
(y = 1.44x + 7.08; r = 0.99, P < 0.001). The linear equation was
used to predict the maximum imposed eccentric moment
(y) corresponding to 100% MVC
(x = 100). The predicted maximum
imposed eccentric Mk was equal to 151% MVC. A strong
linear relationship with similar coefficients (slope and intercept) was
observed for all subjects (Table 1).
Because the peak imposed eccentric
Mk occurred at 69.3° ± 1.3 SD, 70° was specified as the angle for the angle-specific Mk.
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The Mk-angular velocity
relationship is shown in Fig. 3. The
isometric MVC was significantly greater (P < 0.05, paired t-test) than absolute peak
eccentric Mk at 50°/s. MVC was
significantly greater than the mean
(P < 0.001, paired
t-test) and angle-specific Mk (P < 0.001, paired t-test) at
50°/s. Examination of the eccentric Mk data for each subject revealed
that 12 of the 30 subjects generated peak eccentric moments for one or
more eccentric contractions that were greater than MVC (on average
9.96% greater than MVC).
), mean peak moments (
), and mean
angle-specific moments at 70° (
). 
and 
, Mean MVC and
maximum imposed eccentric moment predicted from linear equation obtained from step test (Fig. 2), respectively. SE bars are shown when
they exceed symbol size.
The predicted maximum imposed eccentric
Mk (481.7 N · m
or 151% MVC) was dramatically greater than MVC (318.86 ± 12.84 N · m) at the same joint angle (70°). At the same
angular velocity (
100°/s), the predicted maximum imposed
eccentric Mk was 206% of the
angle-specific eccentric Mk
[242.49 ± 11.09 (SE) N · m].
DISCUSSION
In this study, a strong linear relationship was demonstrated between the initial isometric activation level and the peak eccentric moment resulting from a quick stretch imposed during different levels of isometric contraction. The increase in moment observed during the imposed stretch was attributed to the development of short-range stiffness arising from an elastic component (16, 19, 21, 22, 29, 30) of the activated sarcomeres consistent with that observed in vitro. With the use of the linear equation derived from the step test, the eccentric moment corresponding to 100% MVC was determined. The predicted maximum eccentric moment was substantially greater than the maximal voluntary eccentric moment at the same joint angle and velocity. This was attributed to a restricted activation of the knee extensors during maximal voluntary eccentric contractions.
If muscles were maximally activated during an isovelocity strength test, one would expect that the eccentric-to-isometric ratio would be similar to that observed in isolated muscle. Previous studies have demonstrated eccentric-to-isometric ratios near 1 (range 0.76-1.11), which are lower than in vitro data (range 1.5-1.9) (6, 23, 25, 32-34). Using the step test, we have demonstrated the maximum imposed eccentric moment was 151% MVC predicted from the linear equation, which is consistent with the force-velocity relationship established for maximally activated muscle in vitro (8, 11, 12, 24). The magnitude of increase in force elicited with stretch has been shown to be related to the velocity of stretch in isolated muscle preparations (8, 9). For one subject, the step test was performed at two additional velocities. The maximum imposed eccentric moments were observed to increase in a linear fashion with increasing velocity of the step test, reaching 167% MVC at 150°/s and 183% MVC at 200°/s.
Compared with other studies (10, 28) that have applied stretch to voluntarily activated muscle, our findings reveal substantially greater imposed eccentric moments. Gulch et al. (10) studied eccentric force behavior in intact human skeletal muscle by imposing constant angular velocity eccentric motion on maximal effort isometric elbow flexion contractions at different velocities and over different ranges of motion. Although Gulch et al. demonstrated that forced lengthening of activated muscle induced a double-peaked increment in force, they did not interpret or quantify this phenomenon. Thomson and Chapman (28) also studied imposed eccentric moment generation, demonstrating that peak imposed eccentric forearm supination moments ranged from ~108 to 126% MVC. However, Thomson and Chapman used the peak eccentric moment generated at the end of a large imposed range of motion to represent imposed eccentric moment. Given the large ranges of motion (80 and 160°) and the total time associated with this motion, voluntary control of muscle activation may have influenced the magnitudes of these moments. In this study, the likelihood of voluntary activation of the knee extensors during the step test is minimal, given that the subjects were concentrating on maintaining a constant isometric contraction; the duration of the interstimulus period reduced the possibility for anticipation; and the time to the peak imposed eccentric moment was less than normal reaction times.
In 1990, Westing et al. (34) demonstrated that angle-specific eccentric moments increased by 21-24% above the maximal voluntary isometric level when maximally tolerated electrical stimulation was superimposed during maximal voluntary eccentric contraction. Dudley et al. (6) performed a similar experiment to that of Westing et al. (34) by using high-intensity, superficial electrical stimulation of the quadriceps and reported induced eccentric values that were 1.4 times MVC. Compared with our predicted values of a 51% increase, this smaller increase may have resulted from a level of electrical stimulation limited by pain, which did not allow complete activation of nerves supplying the underlying musculature.
Cocontraction of the knee flexors is known to occur during isovelocity strength tests of the knee extensors (27). Increased knee flexor activity would result in a decreased extensor Mk. However, it is difficult to envision that increased knee flexor activity during voluntary knee extensor eccentric contractions could solely account for the substantial difference observed between the imposed and the voluntary eccentric moments.
In this study, we performed two control experiments in which quadriceps EMG was recorded during the step test to examine the contribution of stretch-evoked reflex activation of quadriceps motor units to the imposed eccentric moment. We demonstrated that the step test stimulus velocity was below the threshold for stretch reflex activation under passive conditions, similar to the findings of Burke et al. (5). Furthermore, we did not observe a stretch reflex response with any isometric level employed in the step test.
Investigators have speculated that a neural regulatory mechanism may limit the level of muscular activation during voluntary eccentric contractions to protect the musculoskeletal system from injury (17, 31, 33). Westing et al. (31) demonstrated that quadriceps EMG levels were lower (10-30%) for maximal effort eccentric contractions compared with concentric contractions at identical velocities, which is consistent with restricted neuromuscular activation (26) and decreased energy cost (1, 3, 20) during voluntary eccentric activity. In this study, the difference between maximal voluntary and predicted maximum imposed eccentric moments was substantial (206% of angle- and velocity-matched, maximal voluntary moments), which is consistent with a potent neural regulatory mechanism that would limit motor unit activation (recruitment and/or discharge frequency) during maximal voluntary eccentric contraction. The generalizability of this finding from isovelocity strength tests for the knee extensors to other behaviors or tasks is unknown.
Subjects were requested to produce a constant isometric moment to achieve a relatively constant level of muscle activation before the step perturbation. When instructed to maintain a constant isometric contraction, subjects produced isometric contractions that were systematically lower than the target levels, especially when target levels exceeded 80% MVC, resulting in a decreased number of data points in the upper portion of this relationship. The data points >80% MVC were generally slightly below the regression line (Fig. 2), which might indicate that the imposed eccentric moments were no longer linearly related to the amount of actomyosin engaged. However, examination of individual regression results for subjects who achieved levels >80% MVC (Table 1) revealed that strong linear relationships were maintained throughout the entire range of MVC levels produced up to 95% MVC. Given the strong linear relationship observed to that level, it is difficult to envision a mechanism through which further activation of muscle fibres would not result in a further increase in the imposed eccentric moment.
In summary, we demonstrated that the magnitude of eccentric moments induced through quick stretch was linearly proportional to the level of isometric activation. The magnitude of the predicted maximum eccentric moment was shown to be 151% MVC, which is within the range reported for isolated muscle (1.5-1.9 maximum isometric force). The finding that maximal voluntary eccentric moments were substantially lower than the predicted maximum imposed eccentric moments was attributed to a neural regulatory mechanism that significantly limits motor unit activation (recruitment or discharge frequency) during voluntary eccentric contraction.
ACKNOWLEDGEMENTS
The authors extend appreciation to Heston Holtmann for programming support. We thank Keith Massey for reviewing the manuscript and providing technical assistance in some of the experiments.
FOOTNOTES
Address for reprint requests: D. Kriellaars, School of Medical Rehabilitation, Univ. of Manitoba, Winnipeg, Manitoba, Canada R3E 0W3 (E-mail: kriel{at}ccu.umanitoba.ca).
Received 9 April 1996; accepted in final form 7 March 1997.
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