Clinical Study The Effects of High-Intensity versus Low-Intensity Resistance Training on Leg Extensor Power and Recovery of Knee Function after ACL-Reconstruction Theresa Bieler, 1,2 Nanna Aue Sobol, 1,2 Lars L. Andersen, 3 Peter Kiel, 2 Peter Løfholm, 2 Per Aagaard, 4 S. Peter Magnusson, 1,2 Michael R. Krogsgaard, 5 and Nina Beyer 1,2 1 Musculoskeletal Rehabilitation Research Unit, Bispebjerg and Frederiksberg Hospitals, University of Copenhagen, Bispebjerg Bakke 23, 2400 NV Copenhagen, Denmark 2 Institute of Sports Medicine, Copenhagen, Bispebjerg and Frederiksberg Hospitals, University of Copenhagen, Bispebjerg Bakke 23, 2400 NV Copenhagen, Denmark 3 National Research Centre for the Working Environment, Lersø Parkall´ e 105, 2100 Copenhagen, Denmark 4 Institute of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), University of Southern Denmark, Campusvej 55, 5230 Odense, Denmark 5 Section for Sports Traumatology, Department of Orthopedic Surgery, Bispebjerg and Frederiksberg Hospitals, University of Copenhagen, Bispebjerg Bakke 23, 2400 NV Copenhagen, Denmark Correspondence should be addressed to Teresa Bieler; theresa.bieler@live.dk Received 21 January 2014; Revised 24 March 2014; Accepted 28 March 2014; Published 27 April 2014 Academic Editor: Nicola A. Mafuletti Copyright © 2014 Teresa Bieler et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Objective. Persistent weakness is a common problem afer anterior cruciate ligament- (ACL-) reconstruction. Tis study investigated the efects of high-intensity (HRT) versus low-intensity (LRT) resistance training on leg extensor power and recovery of knee function afer ACL-reconstruction. Methods. 31 males and 19 females were randomized to HRT ( = 24) or LRT ( = 26) from week 8–20 afer ACL-reconstruction. Leg extensor power, joint laxity, and self-reported knee function were measured before and 7, 14, and 20 weeks afer surgery. Hop tests were assessed before and afer 20 weeks. Results. Power in the injured leg was 90% (95% CI 86–94%) of the noninjured leg, decreasing to 64% (95% CI 60–69%) 7 weeks afer surgery. During the resistance training phase there was a signifcant group by time interaction for power ( = 0.020). Power was regained more with HRT compared to LRT at week 14 (84% versus 73% of noninjured leg, resp.;  = 0.027) and at week 20 (98% versus 83% of noninjured leg, resp.;  = 0.006) without adverse efects on joint laxity. No other between-group diferences were found. Conclusion. High-intensity resistance training during rehabilitation afer ACL-reconstruction can improve muscle power without adverse efects on joint laxity. 1. Introduction Anterior cruciate ligament (ACL) injuries of the knee are amongst the most common major injuries in sport [1]. People with a high preinjury level of sports participation are ofen recommended to undergo an ACL-reconstruction [2] and these people are also likely to choose ACL-reconstruction [3]. Te goal of a rehabilitation program afer an ACL- reconstruction is to regain mobility and muscle function and ultimately to return to sports participation [4]. However, despite postoperative rehabilitation, defcits in muscle func- tion of the operated leg persist up to several years postsurgery [5–12]. Tese defcits in muscle function are of much concern to clinicians and researchers because a regained muscle function is important for dynamic joint stability [13, 14]. Several studies have demonstrated moderate-to- strong associations (r = 0.34–0.74) between thigh muscle strength (primarily quadriceps strength) and knee function (assessed as hop tests) afer ACL-reconstruction [15–18]. In addition, it has been shown that inadequate quadriceps Hindawi Publishing Corporation BioMed Research International Volume 2014, Article ID 278512, 11 pages http://dx.doi.org/10.1155/2014/278512