Some individuals can stabilize their knees following anterior cruciate ligament rupture even during activities involving cutting and pivoting (copers), others have instability with daily activities (non-copers). Movement and muscle activation patterns of 11 copers, ten non-copers and ten uninjured subjects were studied during walking and jogging. Results indicate that distinct gait adaptations appeared primarily in the non-copers. Copers used joint ranges of motion, moments and muscle activation patterns similar to uninjured subjects. Non-copers reduced their knee motion, and external knee flexion moments that correlated well with quadriceps strength. Non-copers also achieved peak hamstring activity later in the weight acceptance phase and used a strategy involving more generalized co-contraction. Both copers and non-copers had high levels of quadriceps femoris muscle activity. The reduced knee moment in the involved limbs of the non-copers did not represent "quadriceps avoidance" but rather represented a strategy of general co-contraction with a greater relative contribution from the hamstring muscles.

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The presence of medial laxity in patients with knee OA is likely contributing to the altered gait patterns observed in those with medial knee OA. Greater medial co-contraction and knee adduction moments bodes poorly for the long-term integrity of the articular cartilage, suggesting that medial joint laxity should be a focus of interventions aimed at slowing the progression of disease in individuals with medial compartment knee OA.

Single-leg hops are used clinically to assess knee function in patients following anterior cruciate ligament (ACL) rupture and reconstruction. Researchers study ACL-deficient individuals in order to identify movement strategies in the absence of a major knee stabilizer, thereby providing information to clinicians regarding treatment options. Single-leg hops represent an activity which places higher demands on the knee than walking or jogging. Hops are thought by some to represent demands that are more comparable to those found during high level sports. Therefore hopping might provide more information about knee stability during dynamic activities than less strenuous activities. This paper reflects one component of a larger study involving comparisons of joint motions and muscle activity patterns in uninjured individuals (n=10) and two groups of athletes who had complete ACL ruptures; one group had substantial knee instability (noncopers, n=10), and the other had no signs of knee instability (copers, n=11). In this paper we report the findings from the single-leg hop activity. The results indicate that coper subjects move in a manner nearly identical to uninjured persons. Kinetic data suggest that copers stabilize their knees with greater contributions from the ankle extensor muscles. Muscle activity data demonstrate that there is no reduction in quadriceps femoris activity in the coper subjects. In the group of ten subjects with knee instability (noncopers) who participated in the overall study involving walking, jogging, hopping, and a step activity only four were willing to hop. Work in our laboratory has established that when high level athletes rupture their ACL, the majority of them cannot return to high level sports. The small number of noncopers in this study who were willing to hop supports our previous findings. Those noncopers who did hop displayed reduced knee range of motion and external knee flexion moments, a movement strategy remarkably similar to that found during other activities. Neither the copers nor the noncopers showed evidence that quadriceps activation was diminished.

Purpose: Quadriceps weakness is common in patients with knee osteoarthritis (OA), and has been attributed to failure of voluntary activation. Methodological differences may have contributed to previous reports of extensive failure of voluntary activation in patients with osteoarthritis. The purpose of this study was to determine the extent of quadriceps muscle weakness and activation failure in middle aged patients with symptomatic medial knee osteoarthritis using maximum voluntary isometric contractions (MVIC) and a burst superimposition technique.Methods: Measurements of quadriceps MVIC and extent of voluntary activation were made in 12 subjects with knee OA and 12 similarly aged uninjured subjects. Voluntary activation was tested by superimposing a train of electrical stimulation on a maximal effort volitional contraction of the quadriceps muscle.Results: The group of subjects with knee OA had significantly less quadriceps strength relative to body mass index (BMI) than the group of control subjects (p = 0.010). No difference in voluntary activation was observed (p = 0.233), however, 50Yn of the OA group, and only 25% of the control group failed to fully activate the quadriceps.Discussion: The finding of quadriceps weakness is consistent with past literature. Providing adequate instruction, feedback, and several attempts to maximally contract the muscle likely yielded greater volitional activation (thus less activation failure) than had been reported previously. This finding has implications for the rehabilitation of weakened quadriceps in patients with knee osteoarthritis.