Study Design Descriptive, prospective, longitudinal single-cohort study. Objective To investigate the rate of force development to 30% (RFD30), 50% (RFD50), and 90% (RFD90) of maximal voluntary isometric contraction (MVIC) as an adjunct outcome measure for determining readiness for return to sport following an anterior cruciate ligament (ACL) reconstruction. Background One criterion of full recovery following an ACL reconstruction is the ability to achieve 85% or 90% of the maximal strength of the contralateral limb. However, the time required to develop muscular strength in many types of daily and sports activities is considerably shorter than that required to achieve maximal strength. Therefore, in addition to maximal strength, neuromuscular functions such as RFD should also be considered in the definition of recovery. Methods Forty-five male professional soccer players who underwent an ACL reconstruction were recruited. Assessment with the International Knee Documentation Committee (IKDC) Subjective Knee Evaluation Form, Tegner score, and KT1000 instrumented arthrometer was performed postinjury/prereconstruction and at 6 and 12 months after ACL reconstruction. MVIC, RFD30, RFD50, and RFD90 testing was performed preinjury, as part of standard preseason assessment, and at 6 and 12 months post-ACL reconstruction. Results The average MVIC value 6 months postreconstruction was 97% of the preinjury average value. In contrast, at 6 months, the RFD30, RFD50, and RFD90 values were 80% (P = .04), 77% (P = .03), and 63% (P = .007), respectively, of the preinjury values. The mean RFD values for the reconstructed knee attained or exceeded 90% of the preinjury mean values only at the 12-month post-ACL reconstruction assessment (RFD30, P = .86; RFD50, P = .51; RFD90, P = .56). Conclusion Despite the near recovery of MVIC strength to preinjury levels, there were still significant deficits in RFD at 6 months post-ACL reconstruction. An RFD similar to the preinjury RFD was achieved at 12 months post-ACL reconstruction, following a rehabilitation program focusing on muscle power. These results suggest that, following an ACL reconstruction, RFD criteria may be a useful adjunct outcome measure for the decision to return athletes to sports. J Orthop Sports Phys Ther 2012;42(9):772–780, Epub 19 July 2012. doi:10.2519/jospt.2012.3780