Abstract
Purpose The anterior cruciate ligament (ACL) has been reported as one of the most commonly injured ligaments of the knee. A high incidence of ACL injuries are non-contact injuries that occur during high velocity, closed chain movements and quick changes in motion, such as accelerating, decelerating, cutting, and pivoting (Noyes & Barber-Westin, 2012). There is paucity in the current literature regarding quadriceps to hamstrings (Q:H) coactivation ratios during closed chain, high velocity exercises. These exercises may be useful to prevent future knee injury by increasing the dynamic stability of the knee joint and its surrounding structures. The primary purpose of this study was to determine the functional Q:H coactivation ratios during high velocity, closed chain knee movements in healthy, recreationally active adults. A secondary purpose of this research was to determine the knee flexion angles at which the maximum EMG activity occurred for each muscle examined. Previous research has focused on the Q:H coactivation ratios during open chain isokinetic knee motion, as well as low velocity, closed chain knee motion. This study investigated the following research questions: What are the Q:H coactivation ratios during closed chain, high velocity exercises including squat jump, barrier jump side to side, barrier jump front to back, scissor jump, and lateral bounding in recreationally active adults? At what angle of knee flexion does the maximum EMG activity occur of the vastus medialis (VM), vastus lateralis (VL), medial hamstrings (MH), and biceps femoris (BF)? Number of Subjects Convenience sampling was utilized to recruit 20 healthy, recreationally active college students (12 men, 8 women) between the ages of 18-30 years old within the Department of Physical Therapy and Human Performance at Florida Gulf Coast University. Materials/Methods This was a descriptive study of cross-sectional design with repeated measures in which the participants performed 8 repetitions of 5 high velocity, closed chain exercises on the selected lower extremity. Data collection was performed utilizing Noraxon© surface electromyography (EMG) measurements of the vastus medialis, vastus lateralis, medial hamstrings, and biceps femoris, in addition to Qualisys© Motion Capture System to measure the joint angles and planes of motion during the exercises. Normalized EMG amplitude levels were used to derive Q:H coactivation ratios for each of the exercises. Ratios were calculated by dividing the sum of the peak quadriceps EMG activity (VM, VL) by the sum of the peak hamstrings EMG activity (MH, BF): (VM + VL)/(MH + BF) = Q:H coactivation ratio. A one way repeated measures analysis of variance (ANOVA) to identify differences in Q:H coactivation ratios among exercises. A multivariate analysis was used to identify the effect of the jump between subjects. In addition, a one way repeated measures analysis of variance (ANOVA) was used to identify differences in peak muscle activity for each of the four muscles during all five exercises and to identify differences in peak muscle activity for each of the five exercises. A multivariate analysis was used to identify the effect of jump on peak EMG flexion angle for each EMG channel (each muscle) and to identify the effect of jump on peak muscle activity within each exercise. SPSS was used to perform all statistical analysis. Results Statistically significant differences (p<0.05) were found between the Q:H ratios of lateral bounding and the scissor jump (mean=-1.069), 95% CI [-2.135, -0.004]) and between lateral bounding and the squat jump (mean=-0.694), 95%CI [-1.288, -0.100). In addition, there was a statistically significant difference (F4,14=37.963, p<0.001) in vastus lateralis activation during lateral bounding when compared to the other four exercises. There was a statistically significant difference (F4,14=3.22, p<0.05) in peak flexion medial hamstrings activation during bounding when compared to the barrier jump front to back, barrier jump side to side, and the scissor jump. There was also a statistically significant difference (F4,14=5.728, p<0.05) in peak flexion biceps femoris activation for lateral bounding when compared to barrier jump side to side, scissor jump, and squat jump. Furthermore, there were statistically significant differences found during the barrier jump front to back (F3,15=10.561, p<0.001), barrier jump side to side (F3,15=14.810, p<0.001), lateral bounding (F3,15=3.533, p<0.05, and scissor jump (F3,15=13.216, p<0.001). Conclusion We evaluated the Q:H coactivation ratios among five high velocity, closed chain plyometric exercises, as well the knee flexion angles that coincide with peak muscle activity. Results of our study identified that the barrier jump front to back, barrier jump side to side, and scissor jump facilitated earlier activation of the hamstrings in relation to the quadriceps suggesting that these exercises provide the most stability to the posterior aspect of the knee, thus protecting the ACL. In contrast, lateral bounding facilitates earlier quadriceps activation and therefore should be used with caution in the early stages of ACL rehabilitation due to the anterior shear force placed on the ACL from the quadriceps. In conclusion, having knowledge of both the overall Q:H ratios as well as the timing of peak muscle contraction allows for better exercise prescription and progression and could also be used in injury prevention programs to decrease the likelihood of ACL injury or re-injury. Clinical Relevance This study identified exercises that facilitate hamstring activation and stabilization, as well as exercises that should be used with caution during ACL rehabilitation. Clinicians can use the results of this study to guide their exercise prescription with the ACL rehabilitation and prevention population.