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Underlying the ability of the hamstrings to decrease tibial anterior shear is the time of firing in comparison with the quadriceps. This timing may be aided by neural programming during a planned or expected activity. It is theorized that individuals who have better programming ability will suffer fewer anterior cruciate ligament (ACL) injuries due to joint protection through muscular stabilization. A component of this dynamic restraint is the development of muscular tension before the knee is loaded. The objective of our study was to compare the muscular activity before footstrike in ACL-deficient (ACL-D), ACL-reconstructed (ACL-R), and control (C) females during functional activities.
Active females were divided into groups based on their ACL status. The study was conducted in a neuromuscular research laboratory.
Twenty-four female subjects (ACL-D = 6, ACL-R = 12, C = 6).
Integrated electromyographic (IEMG) activity from the thigh (vastus medialis obliquus [VMO], vastus lateralis [VL], medial hamstring, and lateral hamstring) and leg (medial gastrocnemius and lateral gastrocnemius [LG]) and footswitch signals were recorded during downhill walking (15° at 0.92 m/s), running (2.08 m/s), hopping, and landing from a step (20.3 cm). IEMG activity was normalized to the mean amplitude of the sample and analyzed for area and mean amplitude for 150 milliseconds before heelstrike. Side-to-side differences were determined by t tests, and separate one-way analyses of variance (ANOVA) were used to detect differences among the 3 groups for each muscle of each activity.
IEMG area side-to-side differences for the ACL-D group appeared in the LG (involved [I] = 36.4 ± 19.7, uninvolved [U] = 60.1 ± 23.6) during landing, in the VMO (I = 11.4 ± 3.8, U = 7.2 ± 3.1) and VL (I = 13.3 ± 2.7, U = 8.9 ± 1.9) during running, and in the VMO (I = 9.2 ± 4.2, U = 19.5 ± 7.3) during downhill walking. IEMG mean amplitude side-to-side differences for the ACL-D group appeared in the LG (I = 79.7 ± 30.3, U = 122.3 ± 34.9) during downhill walking and in the VMO (I = 78.6 ± 23.2, U = 45.8 ± 18.9) during the run; IEMG mean amplitude side-to-side differences for the ACL-R group appeared in the LG (I = 74.7 ± 40.0, U = 52.8 ± 14.3) during the hop. The ACL-D group had higher IEMG means than control in the VL (ACL-D = 12.9 ± 5.8, C = 7.1 ± 3.9), but lower in the VMO (ACL-D = 9.2 ± 4.2, C = 15.7 ± 3.6).
The side-to-side differences of the ACL-D and ACL-R groups, as well as the group differences between ACL-D and control, suggest that different muscle activation strategies are used by females when performing different dynamic activities. Therefore, muscle unit differentiation may be the cause of our results. These changes appear to be reversed through surgery or the associated postoperative rehabilitation.