AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting
of the American Society of Biomechanics
Clemson University, South Carolina
September 24-27, 1997
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| AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting |
Acute anterior cruciate ligament disruption is a common and potentially devastating injury. It is estimated that in any given year, one in 3,000 people in the general population will suffer an anterior cruciate ligament (ACL) tear (Smith et al, 1993). An estimated 70% of ACL injuries are sports related. Unfortunately, other than in skiing, research concerning ACL injury has tended to focus on surgical repair and rehabilitation rather than actual mechanisms of injury. The goal of this study was to analyze the biomechanical effects of various athletic maneuvers commonly associated with ACL injury.
The majority of the non-skiing mechanisms of ACL injuries are non-contact situations where the injured player was not hit or touched by another player (Boden et al, 1996). In addition, these non-contact situations occur near foot strike when the quadriceps are eccentrically contracting to resist flexion. They are also characterized by the following: deceleration, change of direction as in cutting or landing, and a varus/valgus moment about the knee or an internal/external rotation of the leg (Boden et al, 1996). In the analysis of human movement, only inertia, which is an object's resistance to any change in motion, and muscle, which can exert a tensile (pulling) force could contribute to these non-contact injuries (Enoka, 1994). Given the non-contact situation of cutting or landing from a jump, inertial forces would cause an anterior force on the femur and a posterior force on the tibia which would stress the posterior cruciate ligament (PCL). Therefore, it seems reasonable to assume that the mechanism of non-contact injury to the ACL involves internal forces that are generated by the leg muscles of the athlete.
The quadriceps have been implicated for their role in pulling the tibia in the anterior direction and stressing the ACL at low knee flexion angles. In contrast, the role of the hamstrings in stabilizing the ACL is similarly well documented (Ciccotti et al, 1994). Research documenting quadriceps and hamstring activity during cutting, stopping, and landing, however, has been minimal. Previous works studying cutting have primarily dealt with the strategies and kinematics of the movement (Andrews et al, 1977, Cross et al, 1989). Therefore, while it is known that full activation of quadriceps muscles can potentially generate sufficient anterior shear force to rupture the ACL at low angles of knee flexion, the balance of quadriceps and hamstring muscle activity during common athletic motions is unknown. The purpose of this study was to qualitatively characterize the muscle activation of the quadriceps and hamstrings, as well as knee flexion angle during the eccentric motion of athletic maneuvers most involved with ACL injury: sidestep cutting, cross-cutting, stopping, and landing.
Fifteen healthy collegiate and recreational athletes were tested. The subjects average age was 22.2 (± 1.70) years. Surface electrodes were placed over the following muscles on the subject's dominant side: vastus lateralis, vastus medialis oblique, rectus femoris, biceps femoris and medial hamstrings. Retro-reflective markers were placed on the hip (greater trochanter of the femur), knee (lateral condyle of the femur) and ankle (lateral malleolus). During the testing session, the subject first performed a series of isometric maximum voluntary contractions (MVCs). EMG (600 Hz) and two-dimensional kinematic data (60 Hz) were collected while the subjects performed four athletic maneuvers: sidestep cutting, cross-cutting, stopping, and landing. Sidestep cutting and cross-cutting involved running along an eight meter runway, planting with the test limb and cutting to the contralateral and ipsilateral sides, respectively. Stopping was performed with the subject running along an eight meter runway and decelerating with the test limb. Landing was performed with the subjects jumping down from a height of 0.5 meters, landing on both legs, and then pivoting to the contralateral side.
The rectified EMG signals recorded during the four maneuvers were integrated to match the frame speed of the camera. For the MVCs, the rectified EMG signals were integrated every second, and the highest second of muscle activation (representing 100% EMG activity) was used to normalize the dynamic contractions recorded during the four maneuvers. Knee angles were analyzed using a Peak Performance motion measurement system. Maneuvers were analyzed during eccentric motion (five frames before heel strike until the subject moved outside of the 2D reference frame).
Representative examples of EMG and the corresponding knee flexion angle during the eccentric phase for the sidestep cut and cross-cut are presented in Figure 1. Qualitatively, all maneuvers demonstrated increasing activity in the quadriceps at heel strike. Meanwhile, all maneuvers except landing were characterized by increasing hamstring activity at and following heel strike. To help explain the eccentric part of the motion, three points of interest were examined: peak quadriceps activation, minimum hamstrings activation, and the maximum activation difference between the two muscle groups. The quadriceps activation peaked at mid eccentric motion while the minimum hamstrings activation occurred just after heel strike. The maximum difference between quadriceps and hamstring muscle activation occurred after the minimum hamstring activation, but prior to the peak quadriceps activation. Heel strike occurred at an average of 22° of knee flexion for all maneuvers.
Figure 1. Representative example of EMG (%MVC) and the corresponding knee flexion angle at a given time frame for one subject for a) side step cut and b) cross-cut. * represents heel strike
The results of this study indicate quadriceps activation begins just before heel strike and peaks in mid eccentric motion for these movements. This may be related to non-contact injuries. In these maneuvers, the level of quadriceps activation frequently exceeded that seen in a maximum isometric contraction. Furthermore, there was submaximal activity in the hamstrings at and following heel strike. Coupled with this partial hamstring relaxation, forces generated by the quadriceps muscles at the knee could produce significant anterior force to tear the ACL. While the forces which athletes encounter are usually well controlled, unexpected conditions or uncoordinated action such as a slip or fall may result in an ACL injury.
There have been several studies which have shown that the quadriceps pull the tibia in the anterior direction and significantly stress the ACL at low knee flexion angles (Markolf et al, 1990, Smidt, 1973). Moreover, in a study examining the mechanisms of non-contact ACL injuries, Boden reported that the average angle of knee flexion at the time of injury was 21° (Boden et al, 1996). In this study, the average knee flexion angle at heel strike was 22° for each of the four maneuvers suggesting that the quadriceps may be exerting a force strong enough to pull the tibia in the anterior direction, straining the ACL. The hamstrings provide dynamic stability to the knee by resisting both mediolateral and anterior translational forces on the tibia (Norkin and Levangie, 1983). In these four dynamic events, the level of quadriceps activation during the eccentric phase frequently exceeded that seen in a maximum isometric contraction. Furthermore, there was submaximal activity in the hamstrings at and following heel strike. This muscle imbalance is more clearly illustrated when looking at the maximum difference between quadriceps and hamstring activity which ranged from 64 - 87% MVC in the four maneuvers. The results of the study suggest that the levels of hamstring activity might not be sufficient enough to prevent anterior tibial displacement. A preventive strategy involving strength programs may reduce the number of injuries in contact sports. Strengthening programs and different techniques of performing these maneuvers may be developed to help reduce the incidence of ACL injuries.
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