Abstract The general objectives of the work reported in this article were to describe and validate a method for directly measuring strain in the posterolateral bundle (PLB) of the anterior curciate ligament (ACL). The method is a procedure for gaining surgical access to the posterior fibers of the PLB through a portal incised in the joint capsule and then suturing a liquid metal strain gage (LMSG) on to these fibres. Because the incision possibly alters the load-displacement mechanics of the joint, validation included performing experiments to test the hypothesis that the incision did not significantly affect load-displacement relations. To illustrate the utility of the method, strains in both the anteromedial bundle (AMB) and PLB were measured and compared over the full range of flexion. Validation experiments revealed that the capsular incision as well as other incisions had no measurable effect on the load-displacement mechanics of the joint. Also, the PLB strain was significantly different from the AMB strain during passive flexion with a reciprocating function in load sharing evident between the two bundles.