A number of reports have indicated that eccentric knee flexor coactivation stiffens the knee during voluntary extension, thereby attenuating strain to joint ligaments (5,20,32,42). This function is considered crucial to the dynamic stability of the knee joint(5,42). In this study, mean eccentric flexor torque and EMG (gastrocnemius) in the post-surgical limb were, respectively, 13% and 25% less than the contralateral uninjured extremity (P < 0.05), for the fastest speed tested (60°·s-1). This finding suggests that an association exists between EMG eccentric flexor torque and flexor EMG; however, the magnitude of torque may not be directly estimated from EMG data.
Although the hamstrings and gastrocnemius are both knee flexors, in this study only the gastrocnemius muscle of the ACL limb exhibited a significant EMG decrease compared with the UNI limb. Since the role of the hamstrings in unloading the cruciate ligaments in considered to be distinct from that of the gastrocnemius (37), a direct comparison to studies of hamstring activity is limited. Little research has been conducted on the role of the gastrocnemius as a knee joint stabilizer. O'Connor(37) and Collins (16) produced models indicating that the gastrocnemius, as well as the hamstrings, can restrain extensor torque and may unload cruciate ligaments. Their calculations assumed isometric (37) or closed chain activity(16), and direct application of the quantitative results requires further study. The finding of significant ACL/uninjured limb differences in gastrocnemius but not hamstring musculature may be related to speed of motion. Hagood et al. (21) reported that as knee extension velocity increased to 240°·s-1 there was a substantial EMG rise in the antagonist musculature, suggesting an increase in joint stiffness and reduction in laxity. It has also been shown that eccentric hamstring EMG activity during rapid knee extension is significantly less in the limbs of subjects who had undergone ACL reconstructive surgery compared with the uninjured contralateral counterpart (38). The significant difference was more pronounced as speed of motion was increased from 100° to 300°·s-1. It was suggested that the variation in eccentric hamstring coactivation may indicate a reduction in the restraining role these muscles play in ACL dysfunctional limbs. Since the speeds of motion were considerably higher than in the present study, it is possible that 60°·s-1 was not fast enough to evoke differences in hamstring activation. However, the increase to 60°·s-1 may have been sufficient to elicit gastrocnemius differences between the ACL and uninjured counterpart.
It has been postulated that regulation of agonist-antagonist muscle coactivation may be influenced by afferent discharge from ligament mechanoreceptors (20,23,24,40,41) and that ligament disruption and post-injury surgical reconstructions may disrupt knee mechanoreceptors and affect consequent extremity function(6,20,27,30,31). It is possible that trauma or surgical intervention may have denervated ligamentous tissue in the surgical knee resulting in reduced flexor responses at faster speeds of motion. In order to directly measure eccentric flexor torque in this study, the extensor moment was applied by a weighted lever arm against which the subjects were required to resist. Therefore, there are limitations in comparisons of the present data with those studies in which the extensor moment was applied by the action of the quadriceps muscles, an action that has been shown to load the ACL (20,37,39).
Movement speed appeared to have an effect on the eccentric muscular responses in the contralateral limbs of both groups of subjects. The steady rise in eccentric torque with speed found in this study is congruous with the early work of Katz (26), who, using prepared muscle specimens, calculated that force in lengthening contractions increase with velocity of stretch. Later work with human muscle(2,4) demonstrated that eccentric contractions produced greater force as lengthening velocity increased up to high velocities where force leveled off. Abbott and Aubert (1) reported that when stretch at constant speed is imposed on a muscle the rate of tension rise increases with the speed of stretch, but the tension has an upper limitation value independent of speed. More recent reports on human eccentric force-velocity relationships using isokinetic exercise, however, have yielded variable results. In these studies, whether peak torque rose or remained stable across increasing speeds appeared to be dependent upon training condition (24), subject gender (15), and joint position (46). Since average torque across the joint range rather than peak torque was used in this study, direct comparisons to those measuring peak torque are limited.
It has been suggested that the leveling off of peak torque with increasing eccentric speed found in some studies may be due to a reduction in neural drive, which may protect the neurologically intact muscle from injury at high velocities of lengthening (46,47). It is possible that the velocities in this study were not fast enough to invoke inhibitory stimuli and that torque continued to increase with speed of lengthening. This rise was attenuated in the surgical limb of the INJ group as only the uninjured limb of the INJ group and the contralateral limbs of the NOR group demonstrated a significant increase in torque from 45° to 60°·s-1. This attenuation may similarly be due to afferent receptor dysfunction resulting from trauma or surgery(30), thereby reducing flexor responses at faster speeds of motion.
The ratio of eccentric flexor torque to concentric extensor torque increased with speed for both groups and approximated equality at 60°·s-1 for the ACL and UNI limbs of the INJ group. This suggests that as speed increases the restraining action of the eccentrically contracting hamstrings may have an increasing capacity to counter the tibial anterior shear moment created by forcefully contracting knee extensors. In a recent computer modeling study of knee function, O'Connor(37) indicated that the coactivating knee flexors can generate sufficient isometric resistance to unload the cruciate ligaments over a specified range of motion. The present data suggest that knee flexor muscles may function similarly under conditions of dynamic loading as well. It is interesting to note that the eccentric flexor/concentric extensor ratio for both the ACL and UNI limbs of the INJ group significantly (P < 0.05) exceeded that of the left and right NOR group contralateral limbs for the 45° and 60°·s-1 conditions (Fig. 3). The ACL subjects may have accommodated their injuries and surgeries by more highly developing the flexor eccentric function relative to the antagonist quadriceps, thus potentially reducing anterior tibial shear. To maintain contralateral symmetry, this eccentric development may have been cross transferred to the uninjured extremity(19,33).
The finding that knee flexor eccentric actions generated less EMG activation per unit force than concentric actions is consistent with other studies (12,32,45). It has been postulated that eccentric contractions reflect reduced energy costs compared with concentric contractions since for a given load fewer muscle fibers are recruited for voluntary lengthening actions than with shortening actions(11,44). The results of this study suggest that speed of movement may influence the relative energy cost of eccentric muscle action as the eccentric EMG/torque ratio as a percentage of the concentric EMG/torque ratio tended to decrease across speeds.
The results suggest that ACL dysfunction may result in reduced eccentric flexor torque at rapid movement speeds, eccentric flexor torque increases with speed of movement and may have the capacity to counter forceful extensor concentric torque, and eccentric muscle actions produce less muscle activation per unit force than concentric actions which may reflect reduced energy cost(11,44,45).
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ECCENTRIC EXERCISE; ANTERIOR CRUCIATE LIGAMENT; ELECTROMYOGRAPHY