BackgroundThe gastrocnemius (GA) is the lone bi-articular muscle of the leg, crossing both the knee and ankle. As with any bi-articular muscle, both joints affect its length/tension curve. The role of the GA as a plantarflexor is firmly established; however, no current research has investigated how changes in knee and ankle joint positions on its ability to generate a plantarflexion (PF) torque. This paper reports on the PF force generated by the GA at specific knee and ankle joint combinations.MethodsThe right GA of 26 participants was electrically stimulated via surface electrodes following a standardized protocol at 24 knee and ankle joint combinations. Three stimulations were applied at each of the 24 positions. Data were recorded on three dependent measures: the passive moment, which was the PF moment created by the tissue without stimulation, the maximum moment, which was the highest PF moment during the stimulation and included the passive moment, and the stimulated moment, which reflected the PF moment during stimulation minus the passive moment.ResultsA straight knee and dorsiflexed ankle create the position in which the GA generates the greatest PF moment, but it is also the position of greatest length. This finding is in contrast to conclusions from previous research with bi-articular muscles, which has consistently shown that the greatest length is not a muscle’s optimal length. The full ranges of motion for the knee and ankle apparently do not elongate the GA beyond its optimal length for producing a PF moment. Clinicians commonly evaluate GA status with the patient seated and the foot subject to gravity.ConclusionsThe present results indicate that manual testing of the GA in isolation should be performed, whenever possible, with the knee extended and the ankle dorsiflexed to potentially elicit the maximum PF torque from the GA.

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Bi-articular muscles cross more than one joint and contribute to motion at both joints, and the extremities of the human body contain several such muscles. Actions produced by all muscles are determined, to a large extent, by joint moment arms and muscle length. These are transient factors which change as joint angles are altered. Measuring muscle moments while manipulating both joints will produce a better understanding of the actions of bi-articular muscles. This review summarizes investigations which have explored the actions of the biceps brachii as shoulder and elbow joints are moved into various angle combinations. Clinical implications of the findings are discussed.

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