Medial and Lateral Hamstring Muscle Activity Differs for ACL Graft Types

OBJECTIVES: The hamstrings are anterior cruciate ligament (ACL) agonists, with the semitendinosus and gracilis (ST/G) assisting the ACL by limiting excessive external rotation. Use of ST/G autografts is common in ACL reconstruction (ACLR). Regrowth of these tendons over the 1st post-operative year has also been reported. Little is known, however, regarding hamstring muscle activity over this period. The purpose of this study was to examine medial (MH) and lateral hamstring (LH) muscle activity during gait in athletes with different graft types one year after ACLR. METHODS: Athletes who participated in cutting and pivoting sports before ACLR were tested one year after ACLR; 18 with ST/G (HS) autografts, 5 with bone-patella-tendon-bone (BPTB) autografts, and 12 with allografts (Allo). EMG electrodes were secured on the MH and LH. Athletes performed maximal voluntary isometric contractions for EMG normalization purposes followed by walking at a self-selected speed. Filtered EMG data were normalized to maximum muscle activity and mean activity was calculated for weight acceptance and the mid-to-terminal phases of swing. A repeated measures ANOVA was used to assess differences in MH and LH muscle activity between limbs in athletes with different graft types. Post-hoc one way ANOVAs and t-tests were used to further elucidate differences (p≤0.05). RESULTS: During weight acceptance, there was a limb x graft (p<0.01) and a muscle x graft interaction (p<0.01) (Fig. 1). The HS group had significantly higher involved limb MH activity (19.1± 1.5%) than those in the Allo (8.9± 1.8%, p<0.01) or BPTB (10.4±2.8%, p=0.03) groups. The HS group also had greater involved limb MH activity compared to the uninvolved MH (13.4± 1.7%, p<0.01) and greater involved limb LH (15.4± 1.7%) compared to the uninvolved LH (11.3± 1.6%, p<0.01). Furthermore, the HS group had greater involved limb MH activity when compared to the involved limb LH activity (p=0.03). There were no between muscle or limb differences for the BPTB or Allo groups. During the mid-to-terminal phases of swing, there was a muscle group x graft interaction (p<0.01). The HS group had significantly greater involved limb MH activity (14.8± 1.2%) than the Allo (7.5± 1.4%, p<0.01) and BPTB (7.7± 2.2%, p=0.02) groups. The HS group also had significantly greater MH activity on the involved limb compared to the uninvolved limb (11.0± 1.1%, p=0.04) as well as significantly greater MH compared to LH muscle activity on the involved limb (11.1± 1.3%, p=0.01). CONCLUSION: During both stance and swing phases, athletes in the HS group had greater MH activity on the involved limbs. The increased activity could be in attempt to produce the necessary force to control the limb with atrophied hamstrings. HS athletes’ involved limb MH activity was the highest among the muscles examined in this study, and the HS group was the only to have significantly differences between MH and LH activity regardless of limb. Use of a ST/G autograft is known to decreased knee flexion strength, and with these results potentially indicates that continued hamstring strengthening beyond discharge from rehabilitation could be beneficial.