title: Hip external rotation strength predicts hop performance after anterior cruciate ligament reconstruction.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
  
  
  METHODS
  Prognostic Study, Level II.
authors.affiliation: Department of Rehabilitation Sciences, University of Kentucky, Lexington, KY, 40536-0200, USA.
authors.forename: Paul W
authors.identifier: http://orcid.org/0000-0002-2366-9140
authors.initials: PW
authors.lastname: Kline
authors.affiliation: Center for Sports Medicine, University of Pittsburgh Medical Center, Pittsburgh, PA, 15203, USA.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Burnham
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine, University of Kentucky, Lexington, KY, 40536-0200, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Yonz
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine, University of Kentucky, Lexington, KY, 40536-0200, USA.
authors.forename: Darren
authors.initials: D
authors.lastname: Johnson
authors.affiliation: Department of Orthopedic Surgery and Sports Medicine, University of Kentucky, Lexington, KY, 40536-0200, USA.
authors.forename: Mary Lloyd
authors.initials: ML
authors.lastname: Ireland
authors.affiliation: Department of Rehabilitation Sciences, University of Kentucky, Lexington, KY, 40536-0200, USA. b.noehren@uky.edu.|Center for Sports Medicine, University of Pittsburgh Medical Center, Pittsburgh, PA, 15203, USA. b.noehren@uky.edu.|Division of Physical Therapy, University of Kentucky, 900 S Limestone, Room 204D, Lexington, KY, 40536-0200, USA. b.noehren@uky.edu.
authors.forename: Brian
authors.initials: B
authors.lastname: Noehren
mesh_terms: D000293:Adolescent; D000328:Adult; D000070598:Anterior Cruciate Ligament Injuries / Q000503:physiopathology / Q000534:rehabilitation / Q000601:surgery*; D059549:Anterior Cruciate Ligament Reconstruction / Q000534:rehabilitation*; D016022:Case-Control Studies; D003430:Cross-Sectional Studies; D005260:Female; D006621:Hip Joint / Q000502:physiology*; D006801:Humans; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D053580:Muscle Strength; D052097:Quadriceps Muscle / Q000502:physiology; D000068958:Return to Sport / Q000502:physiology*; D012399:Rotation; D016896:Treatment Outcome; D055815:Young Adult
keywords: ACL; Hip strength; Quadriceps; Rehabilitation