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A practicable EMG-force model is evaluated for muscles about the knee. The model included envelope signal processing and a gain-dependency of knee angle and angular velocity. Six healthy subjects participated in the experiments. For calibration, maximal isokinetic contractions about the knee were performed on a dynamometer with recordings of knee joint movement, net moment and EMG of thigh muscles. The model parameters were fitted on these calibration experiments. For validation, estimated moments from the EMG levels were compared to the actual exerted moments of two independent isokinetic contractions. Averaged RMS values of the difference ranged from 11 to 20% of the actual exerted moment. For isokinetic concentric contractions, the present model is suitable as a method to estimate muscle moments.

Objective. To investigate whether a simple electromyography–force (EMG-force) model can be used to measure different levels of co-contraction about the knee for healthy subjects and patients with an anterior cruciate ligament deficiency. Design. To evaluate an EMG-to-force processing model, two groups of subjects, with and without deficiency of the anterior cruciate ligament, participated in experiments in which surface EMG, kinematics and kinetics about the knee were recorded during isokinetic and functional movements. Background. Clinical and biomechanical evidence supports the hypothesis that higher level of co-contraction of quadriceps and hamstrings provide an active stabilization of the knee to compensate for the lost anterior cruciate ligament. To quantify the level of co-contraction, the contribution of both agonist and antagonist muscles to the net joint moment must be known. Methods. Surface EMG levels were calibrated to moment by means of a limited number of isokinetic contractions about the knee. With these calibration values, an estimate of the muscle moments during a vertical jump were obtained and compared with the net joint moment, calculated with inverse dynamics. Also co-contraction indices were determined. Results. The EMG–force model provided a fair estimate of the net joint moment. The co-contraction index in anterior cruciate ligament deficient subjects was significantly higher (mean 0.54 (SD, 0.04)) compared to healthy subjects (mean 0.25 (SD, 0.07)). Conclusion. Although the EMG-to-force processing model is not perfectly accurate, it is appropriate within a clinical context. Relevance Previous research supports the hypothesis that subjects with an anterior cruciate ligament deficiency compensate the loss of passive stability by developing higher co-activation levels of the knee muscles, i.e. active stabilization. Quantifying co-contraction may serve as a valuable parameter to evaluate clinical interventions and rehabilitation processes. The EMG–force model presented in this study appears to be a useful instrument for this purpose.

In this study, two different strategies of rising from a chair were compared, using integrated biomechanical and electromyographic analyses. Nine healthy subjects were instructed to rise using two different strategies: natural sit-to-stand transfer (NSTS) and a sit-to-stand transfer with full flexion of the trunk (FSTS). Sagittal kinematics and ground reaction forces were registered. Muscle activity of nine muscles of the right leg were recorded by means of surface EMG. All signals were synchronized at seat-off. The results show that no differences occur between the kinematics of knee and ankle, whereas the hip flexion is, as expected, higher during FSTS. The higher moment about the knee during NSTS is shifted to proportionally higher moments about the hip and ankle during FSTS. It is mainly the differences in the EMG-levels of the biarticular hip and knee muscles which might explain the differences in net moment. These results are in accordance with a theory about a particular role of biarticular muscles. On the other hand, the shift from knee to ankle cannot be associated with a particular increase in activity of the biarticular m.gastrocnemius. It is hypothesized that about the ankle, control of stability is preferred over movement control. An important conclusion for rehabilitation medicine is that a lower net moment about the knee in FSTS does not automatically imply that this reduces the load on the knee extensors.

M.M. Eken1,2,∗, H. Houdijk2,3, C.A.M. Doorenbosch1,4, H. Dekkers2, J.G. Becher1, A.J. Dallmeijer1 1 Department of Rehabilitation Medicine, MOVE Research Institute Amsterdam, VU University Medical Center, Amsterdam, The Netherlands 2 Heliomare Rehabilitation, Research and Development, Wijk aan Zee, The Netherlands 3 MOVE Research Institute Amsterdam, Faculty of Human Movement Science, VU University, Amsterdam, The Netherlands 4 Academy of Human Kinetic Technology, University of Applied Sciences, The Hague, The Netherlands