Journal of Electromyography and Kinesiology 14 (2004) 307–315 www.elsevier.com/locate/jelekin Are hamstrings activated to counteract shear forces during isometric knee extension efforts in healthy subjects? Idsart Kingma  , Sietske Aalbersberg, Jaap H. van Diee ¨n Institute for Fundamental and Clinical Human Movement Sciences, Faculty of Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands Received 2 July 2003; received in revised form 15 December 2003; accepted 6 January 2004 Abstract The hamstring muscles have the potential to counteract anterior shear forces at the knee joint by co-contracting during knee extension efforts. Such a muscle recruitment pattern might protect the anterior cruciate ligament (ACL) by reducing its strain. In this study we investigated to what extent co-activation of the knee flexors during extension efforts is compatible with the hypoth- esis that this co-activation serves to counteract anterior tibial shear forces during isometric knee extension efforts in healthy sub- jects. To this aim, it is investigated whether co-activation varies with the required knee extension moment, with the knee joint angle, and with the position of the external flexing force relative to the knee joint. With unaltered moment and muscle activation, distal positioning of the flexing force on the tibia causes higher resultant (muscular plus external) forward shear forces at the knee as compared to proximal positioning. In ten subjects, knee flexor and extensor EMG was measured during a quasi-isometric positioning task for a range (5–50 degrees) of knee flexion angles. It was found that the co-activation of the knee flexors increased with the extension moment, but this increase was less than proportional (p< 0:001). The extension moment increased 2.7 to 3.4 times, whereas the activation of Biceps Femoris and Semitendinosus increased only a factor 1.3 to 2.0 (joint angle dependent). Furthermore, a strong increase in co-activation was seen near full extension of the knee joint. The position of the external extension load on the tibia did not affect the level of co-contraction. It is argued that these results do not suggest a recruitment pattern that is directed at reduction of anterior shear forces in the knee joint during sub-maximal isometric knee extension efforts in healthy subjects. # 2004 Elsevier Ltd. All rights reserved. Keywords: EMG; Knee; ACL; Hamstrings; Shear forces 1. Introduction During knee extension efforts, quadriceps activation causes the tibia to be pulled forward relative to the femur. Even in full extension, the patellar tendon has a considerable angle with the long axis of the tibia, caus- ing knee extension efforts to be accompanied by a sub- stantial shear force on the tibia [10]. In healthy subjects, the anterior cruciate ligament is assumed to play an important role in counteracting forward shear forces on the tibia. Full activation of the quadriceps is estimated to cause a force in the anterior cruciate liga- ment (ACL) of over 500 N [14]. This force depends on the knee angle with a maximum being reached at 15 v of knee flexion [14]. Co-contraction of the hamstring muscles can potentially assist in counteracting forward tibial shear because the hamstrings’ force has a back- ward shear component. However, a disadvantage of this strategy is that, in order to maintain the same extension moment, the quadriceps force also has to be increased to counteract the flexion moment of the ham- strings. In a simulation study, Liu and Maitland [12] estimated that cancelling out forward tibial shear by hamstrings co-contraction would result in about a doubling of the required quadriceps force in gait. Due to the relatively small knee extension moments required in gait, protective co-contraction may not be needed in healthy subjects. However, when large extension moments are required, for instance up to 100 Nm in the forward lunge movement, substantial co-contrac-  Corresponding author. Tel.: +31-20-4448492; fax: +31-20- 4448529. E-mail address: i_kingma@fbw.vu.nl (I. Kingma). 1050-6411/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2004.01.003