Hip external rotation strength predicts hop performance after anterior cruciate ligament reconstruction

Quadriceps strength and single-leg hop performance are commonly evaluated prior to return to sport after anterior cruciate ligament reconstruction (ACLR). However, few studies have documented potential hip strength deficits after ACLR, or ascertained the relative contribution of quadriceps and hip strength to hop performance. Patients cleared for return to sports drills after ACLR were compared to a control group. Participants' peak isometric knee extension, hip abduction, hip extension, and hip external rotation (HER) strength were measured. Participants also performed single-leg hops, timed hops, triple hops, and crossover hops. Between-limb comparisons for the ACLR to control limb and the non-operative limb were made using independent two-sample and paired sample t tests. Pearson's correlations and stepwise multiple linear regression were used to determine the relationships and predictive ability of limb strength, graft type, sex, and limb dominance to hop performance. Sixty-five subjects, 20 ACLR [11F, age 22.8 (15-45) years, 8.3 +/- 2 months post-op, mass 70.47 +/- 12.95 kg, height 1.71 +/- 0.08 m, Tegner 5.5 (3-9)] and 45 controls [22F, age 25.8 (15-45) years, mass 74.0 +/- 15.2 kg, height 1.74 +/- 0.1 m, Tegner 6 (3-7)], were tested. Knee extension (4.4 +/- 1.5 vs 5.4 +/- 1.8 N/kg, p = 0.02), HER (1.4 +/- 0.4 vs 1.7 +/- 0.5 N/kg, p = 0.04), single-leg hop (146 +/- 37 vs 182 +/- 38% limb length, p < 0.01), triple hop (417 +/- 106 vs 519 +/- 102% limb length, p < 0.01), timed hop (3.3 +/- 2.0 vs 2.3 +/- 0.6 s, p < 0.01), and crossover hop (364 +/- 107 vs 446 +/- 123% limb length, p = 0.01) were significantly impaired in the operative versus control subject limbs. Similar deficits existed between the operative and non-operative limbs. Knee extension and HER strength were significantly correlated with each of the hop tests, but only HER significantly predicted hop performance. After ACLR, patients have persistent HER strength, knee extension strength, and hop test deficits in the operative limb compared to the control and non-operative limbs, even after starting sport-specific drills. Importantly, HER strength independently predicted hop performance. Based on these findings, to resolve between-limb deficits in strength and hop performance clinicians should include HER strengthening exercises in post-operative rehabilitation. Prognostic Study, Level II.