STUDY DESIGN: Controlled laboratory study, cross-sectional data. OBJECTIVES: To investigate isometric knee flexion and extension strength, failure of voluntary muscle activation, and antagonist cocontraction of subjects with knee osteoarthritis (OA) compared with age-matched asymptomatic control subjects. BACKGROUND: Quadriceps weakness is a common impairment in individuals with knee OA. Disuse atrophy, failure of voluntary muscle activation, and antagonist muscle cocontraction are thought to be possible mechanisms underlying this weakness; but antagonist cocontraction has not been examined during testing requiring maximum voluntary isometric contraction. METHODS: Fifty-four subjects with knee OA (mean ± SD age, 65.6 ± 7.6 years) and 27 similarly aged control subjects (age, 64.2 ± 5.1 years) were recruited for this study. Isometric knee flexion and extension strength were measured, and electromyographic data were recorded, from 7 muscles crossing the knee and used to calculate cocontraction ratios during maximal effort knee flexion and extension trials. The burst superimposition technique was used to measure failure of voluntary activation. RESULTS: Knee extension strength of subjects with knee OA (mean ± SD, 115.9 ± 6.7 Nm) was significantly lower than for those in the control group (152.3 ± 9.6 Nm). No significant between-group difference was found for failure of voluntary muscle activation, or the cocontraction ratios during maximum effort knee flexion or extension. CONCLUSION: These results demonstrate that the reduction in isometric extension strength, measured with a 90° knee flexion angle, in subjects with knee OA is not associated with increased antagonist cocontraction.

J Orthop Sports Phys Ther 2009;39(11):807-815, Epub 15 October 2009. doi:10.2519/jospt.2009.3079

KEY WORDS: burst superimposition, OA, quadriceps strength, voluntary muscle activation

This commentary presents the rationale for training programs to reduce the incidence of knee injuries. Our studies have revealed that the external knee loading patterns during sidestep cutting are what put the anterior cruciate ligament at greatest risk for injury. Compared to running, sidestep cutting involves similar levels of knee flexion loading but increased loading in varus-valgus and internal rotation of the knee, and these external loads need to be stabilized or supported by the internal structures of the knee. People use a generalized hamstrings and quadriceps co-contraction to stabilize these external loads, thereby reducing ligament loading. It is proposed that perturbation of the joint receptors reinforces the use of selective hamstrings and quadriceps co-contraction patterns superimposed on a generalized co-contraction pattern. This is not by immediate ligamentomuscular protective reflex, which is too slow to provide any adequate support, but by enhanced proprioceptive information that may be used in learning. In contrast, the immediate effect of muscle stretch reflexes would be to reduce co-contraction, a possibly negative outcome for joint stabilization. The effects of different types of training on the control of joint stability are examined. It is proposed that resistance training may not be appropriate because it enhances muscle stretch reflexes, which may reduce co-contraction, and produces no reductions in voluntary activation times and time to peak torque. However, stability and balance training is thought to suppress muscle stretch reflexes and, in turn, enhance co-contraction. Also, stability and balance training that stimulates the knee joint ligament and capsular receptors may reinforce co-contraction patterns to facilitate greater improvements in joint stabilization. Stability and balance training and plyometric training produce reductions in voluntary activation times and times to peak torque, which may decrease muscle response times so players are more able to perform rapid and unexpected sports maneuvers. Training programs that emphasize these neuromuscular mechanisms may enhance protection of the anterior cruciate ligament and reduce the incidence of injury.

J Orthop Sports Phys Ther 2001;31(11): 645-654.

Key Words: biomechanics, muscular stabilization, sidestepping