QUADRICEPS STRENGTH AND THE ACL DEFICIENT NON-COPER
Most individuals who sustain anterior cruciate ligament (ACL) injury experience knee instability ( non-copers ), and have marked quadriceps muscle weakness. There are some individuals, however, with good dynamic knee stability early after ACL injury
( potential copers ) who are able to asymptomatically resume pre-injury activities for a limited period of time. The University of Delaware has developed a screening tool that prospectively classifies patients with different levels of dynamic knee stability early after
ACL injury. The first goal of this work was to characterize an entire population of highly active patients with acute, isolated ACL rupture using the results of this screening algorithm. Analysis of 300 consecutive subjects indicated potential copers have greater quadriceps strength on bilateral limb comparison (X=88%) than non-copers (X=84%).
Quadriceps weakness in non-copers is clinically significant, as quadriceps strength contributed substantially to their functional performance measured during hop testing.
The amount of passive anterior knee laxity present after ACL rupture does not contribute to knee instability after injury. A 10 year review of screening results indicated there are specific patient groups who are more likely to experience knee instability after ACL injury, including female athletes and individuals who sustain injury via non-contact mechanisms. Overall, the average individual who sustains an ACL injury is young and likely to experience knee instability. xiii

The second goal of this work was to garner insight to the movement strategies of the non-coper, and determine their responsiveness to physical therapy intervention designed to improve quadriceps strength. Gait analysis of 21 subjects classified as non-copers during the mid-stance (peak knee flexion to peak knee extension) portion of stance we identified a control strategy novel to this phase of the gait cycle that includes a shift in the support moment from the knee to the ankle and variable combinations of muscle adaptations that produce joint stiffness. Preliminary results from a clinical trial indicate a stable knee may be more permissive of quadriceps strengthening, as individuals who participated in neuromuscular training and quadriceps strengthening exercises had greater improvements in strength than those who performed only strengthening exercises. Both improved knee stability and quadriceps strength contributed to the normalization of movement patterns after the clinical intervention.
These results indicate there is a complementary relationship between strength and stability after ACL injury. xiv