Perturbation Training Improves Knee Kinematics and Reduces Muscle Co-contraction After Complete Unilateral Anterior Cruciate Ligament Rupture

Methods

Results

Main effects were identified from the analysis of both peak knee flexion angle and knee flexion excursion. For peak knee flexion angle, main effects included platform condition (P<.001) and time × group × condition (P=.028). For knee flexion excursion, a main effect of condition was found (P<.001).

There were main effects for VL-LH and VL-MG co-contraction indexes during each platform condition. During the locked condition, there was a main effect of time for both VL-LH (P<.001) and VL-MG (P<.001). There was a main effect of time during the lateral condition for both VL-LH (P<.001) and VL-MG (P<.001). There also were main effects during the lateral condition for VL-LH co-contraction for time × group (P=.045), phase of gait (P=.05), and group (P=.07). During the anterior condition, there were main effects of time (P<.001), time × group (P=.069), phase of gait (P=.019), and time × phase of gait (P=.043) for VL-LH. There also was a main effect of time for VL-MG (P<.001) during the anterior condition.

Before Training

Effects of different conditions.

There was less knee flexion excursion and lower peak knee flexion in the anterior and lateral conditions than in the locked condition for both groups (Fig. 1[A-C]).

Figure 1.

Sagittal-plane knee kinematics in the locked (Fig. 1[A]), lateral (Fig. 1[B]), and anterior (Fig. 1[C]) conditions before and after perturbation-enhanced rehabilitation. The solid black line represents control subjects; the dashed black line represents potential copers. The gray region represents the standard deviation of the control subjects. Time is normalized to 100% of stance, with 0%=heel-strike and 100%=toe-off. Statistically significant differences between groups for peak knee flexion at a level of P<.05 are identified by a single asterisk (*); pretraining-posttraining differences for peak knee flexion in potential copers at a level of P<.05 is identified by a double asterisk (**).

Kinematics.

The potential copers had lower peak knee flexion angles (X̄=19.81°, SD=4.86°, P=.016) than the control subjects (X̄=24.28°, SD=5.37°, P=.016) in the locked condition (Tab. 2, Fig. 1[A]). There were no differences between groups for peak knee flexion angle in the anterior condition (Tab. 2, Fig. 1[C]) (P=.234) or the lateral condition (Tab. 2, Fig. 1[B]) (P=.067). There were no differences in knee flexion excursion between groups for the locked condition (P=.143) (Tab. 2, Fig. 1[A]), the lateral condition (P=.212) (Tab. 2, Fig. 1[B]), or the anterior condition (P=.429) (Tab. 2, Fig. 1[C]).

View this table:

Table 2.

Knee Flexion Angle and Excursion (in Degrees) by Group and Condition Before and After Training^a

Co-contraction.

In the locked condition, group differences were found only in the VL-LH co-contraction index (Tab. 3, Fig. 2[A]). The VL-LH co-contraction index was higher in the potential copers than in the control subjects for both the preparatory interval (potential copers: X̄=28.83, SD=14.55, P=.062; control subjects: X̄=20.44, SD=10.39, P=.062) and the weight-acceptance interval (potential copers: X̄=41.69, SD=17.34, P=.034; control subjects: X̄=30.40, SD=11.87, P=.034). There were no differences between groups for VL-MG co-contraction during either the preparatory interval (P=.113) or the weight-acceptance interval (P=.341).

Figure 2.

Co-contraction for potential copers (PC) and control subjects (C) in the locked (Fig. 2[A]), lateral (Fig. 2[B]), and anterior (Fig. 2[C]) conditions. Pretraining values are represented by the solid black bars; posttraining values are represented by the black-and-white spotted bars. Statistically significant differences (P<.10) between groups are denoted by a single asterisk (*); differences within groups after training are denoted by a double asterisk (**). VL-LH=vastus lateralis-lateral hamstring muscle co-contraction; VL-MG=vastus lateralis-medial gastrocnemius muscle co-contraction.

View this table:

Table 3.

Co-contraction Values During Preparatory and Weight-Acceptance Phases of Gait Before and After Training for Potential Copers and Control Subjects^a

In the lateral condition, preparatory muscle co-contraction indexes were consistently larger in the potential copers (VL-LH: X̄=29.17, SD=14.59, P=.095; VL-MG: X̄=16.35, SD=7.27, P=.018) than the control subjects (VL-LH: X̄=21.38, SD=11.69, P=.095; VL-MG: X̄=11.13, SD=4.23, P=.018) (Tab. 3, Fig. 2[B]). In addition, the VL-LH co-contraction index was higher in potential copers (X̄=51.95, SD=22.75, P=.008) than in the control subjects (X̄=34.70, SD=11.15, P=.008) during the weight-acceptance interval. There was no difference between groups in the VL-MG co-contraction index (P=.481) during the weight-acceptance interval.

In the anterior condition, potential copers had a higher VL-MG co-contraction index in the preparatory interval (X̄=15.70, SD=6.85, P=.044) compared with the control subjects (X̄=11.27, SD=5.08, P=.044) (Tab. 3, Fig. 2[C]). Preparatory VL-LH co-contraction was not different between potential copers and control subjects (P=.151). The co-contraction index was larger in potential copers (X̄=53.45, SD=25.00, P=.022) than in the control subjects (X̄=36.93, SD=13.25, P=.022) during the weight acceptance interval for VL-LH but not for VL-MG (P=.223).

Discussion and Conclusions

The results of this study indicate that neuromuscular changes that occur in potential copers after perturbation training may explain the improved functional outcomes previously demonstrated by Fitzgerald et al.¹⁰ As we hypothesized, prior to training, potential copers stiffened their knees with higher muscle co-contraction and slightly lower peak knee flexion angles, indicating an undeveloped knee stabilization strategy. Our hypothesis that platform translation would be destabilizing to both groups also was correct. Both groups demonstrated altered knee kinematics that included lower peak knee flexion angles and truncated knee flexion excursion during the conditions with platform movement compared with the locked condition. After perturbation training, the knee flexion angles increased and muscle co-contraction was generally lower in the potential copers, making their movement patterns similar to those of the control subjects. The effect of specialized training was to change the knee stabilization strategy from a joint-stiffening pattern to a pattern that may allow the potential copers to dynamically stabilize their knee in response to unexpected perturbations and perhaps preserve joint integrity over time.

Before the training, the potential copers stiffened their knees and co-contracted their muscles, which indicates an immature stabilization strategy. “Freezing degrees of freedom” has been described as a primitive strategy when mastering a new skill.²³ Potential copers must become skilled at stabilizing their knees in the absence of the passive restraint and afferent feedback provided by an intact ACL. Prior to training, they used a joint-stiffening strategy even while walking across a stationary platform when there was minimal threat to knee stability. The use of a joint-stiffening strategy in this case may be a response to potential instability brought on by an unopposed quadriceps femoris muscle contraction that can cause anterior tibial translation,^24,25 because a strong eccentric quadriceps femoris muscle contraction is present during weight acceptance of walking. Before training, there was a strong trend for the potential copers to flex less than control subjects in response to the lateral translation. Although the difference in knee angle was small, as we have previously demonstrated in potential copers,¹³ it is consistent with an immature compensation strategy for ACL injury because people who can fully cope with the injury have peak knee flexion angles that are no different from those of people without injuries in a variety of tasks.^4,5

During the anterior and lateral conditions, both potential copers and control subjects had less knee flexion than during locked conditions both before and after training, indicating that the activity was challenging to both groups. Similar results were obtained by Ferber et al,²¹ who also demonstrated that people without injuries flexed the knee joint less in trials with anterior support surface translation than in trials when the platform was stationary. Despite comparable changes in knee motion, higher muscle co-contraction in potential copers reveals that they respond to a translation of the support surface differently than people without injuries. The change in muscle activity sheds light on the stabilization strategy used by individuals who are in the process of learning to stabilize the knee.

In the locked condition and in response to both anterior and lateral platform translations, the higher muscle co-contraction that was observed in the potential copers may have contributed to the observed joint stiffening. Higher VL-LH co-contraction prior to initial contact might be an attempt to preset the limb in a position of relative stability prior to heel-strike. The higher VL-LH co-contraction seen in the potential copers during weight acceptance was likely also related to stabilization through knee stiffening.

Higher VL-MG co-contraction values in preparation for heel-strike were observed in the potential copers before training only in the trials with platform movement. Co-contraction of the VL-MG may serve a direct role at the knee, similar to quadriceps femoris-hamstring muscle co-contraction. In preparation for a known disturbance, a biarticular muscle that acts at the knee and ankle may be in a unique position to provide stability as the foot contacts the moving platform.

After perturbation training, the potential copers flexed more during weight acceptance, and the degree of co-contraction was reduced; they became more similar to the control subjects. This change was particularly pronounced when the platform was stationary, where group differences prior to training were eliminated after training, and when the platform translated laterally, where potential copers had a pretraining-posttraining change in peak knee flexion angles. Although in both cases the magnitude of the change was small (1.5° for the stationary condition and 1.7° for the lateral condition), we believe the changes have clinical significance. The knee kinematics of potential copers are not typically very different from those of people without injuries, so the magnitude of change was not expected to be extreme in the potential copers. An increase in knee flexion indicates the adoption of a movement pattern that is consistent with clinical findings of improved dynamic knee stability after training.¹⁰ Importantly, the results of our study suggest that muscle co-contraction is lessened after patients participate in a specialized training program that requires them to produce specific but varied muscle responses when reacting to random forces. Our findings are consistent with the work of Nichols,²⁶ who showed that force-feedback reflexively altered muscle-firing patterns to control both joint forces and torques in a decerebrate cat model. Recent work from the same group suggests that force feedback is organized to regulate coupling between joints.²⁷ Lower co-contraction reflects a change to a more selective pattern that helps explain increased knee flexion after training as well as the posttraining improvements in function found by other investigators.^10–12 Perturbation training provides the stimulus for reorganizing muscle responses that may ultimately lead to improved function.

We believe a key principle underlying perturbation training is that patients with ACL deficiencies should be exposed to carefully controlled forces that destabilize the knee joint enough to elicit appropriate responses without putting the knee joint at risk for further injury. Gradually adding more challenging forces allows them to learn more appropriate muscle responses to unexpected forces that release the tight control of the knee that manifests in reduced knee flexion. This premise is supported by our data. When learning and skill acquisition take place, rigid control over the degrees of freedom is released in 2 stages. In the first stage, restrictions are gradually lifted, and the degrees of freedom become incorporated into larger functional units (ie, groups of muscles are constrained to act as functional units).²³ In the second stage, the organization becomes more economical, enhancing the efficiency of muscular forces.²³ Compared with noncopers, who restrict joint motion to a larger extent, potential copers have only minor kinematic deviations during gait, suggesting that potential copers are in the first stage of learning to stabilize the ACL-deficient knee. Resolution of rigid muscle firing patterns in conjunction with normal gait kinematics after training is consistent with Bernstein's second stage of skill acquisition.²³ The posttraining adaptations observed in the potential copers in our study are thus indicative of a more mature and refined stabilization strategy.

Elimination of the joint-stiffening strategy used by people with ACL deficiencies is important. High co-contraction may help to stabilize the joint through joint stiffening; however, this strategy could be detrimental over time. Shear forces, such as those associated with episodes of giving way, and compression that may be associated with excessive muscle co-contraction, coupled with reduced shock absorption that accompanies limited knee flexion and decreased quadriceps femoris muscle force, can contribute to the biochemical and metabolic changes that characterize degeneration of articular cartilage.²⁸ The development of posttraumatic, unilateral knee osteoarthritis may be accelerated by the manner in which the potential copers attempted to stabilize the knee prior to training.^1,5,6

Evidence supporting the use of perturbation-enhanced rehabilitation protocols for the management of patients with ACL deficiency and classified as potential copers is strong.¹⁰ The response of patients with ACL deficiency and classified as noncopers who participate in perturbation-enhanced rehabilitation is unknown and warrants investigation. Nonoperative management of patients with ACL ruptures has resulted in limited success in returning individuals to physically active lifestyles without episodes of giving way.^1,29,30 Fitzgerald et al,¹⁰ in a randomized trial of potential copers, found that perturbation training resulted in 93% of those subjects who completed the training successfully returning to high-level activity without episodes of giving way. Only 50% of those subjects in the traditional rehabilitation group returned to high-level activities.¹⁰ The results of our study reveal a possible mechanism underlying the high success rate for return to sporting activities of potential copers who participate in perturbation training.