Is Human Tendon Lengthening a Valid Indicator of Contractile Force Changes in the In- Series Muscle?

Distributing net joint moments produced during a given motor task to the contributor individual muscles has traditionally been challenging due to the mechanical redundancy of the musculoskeletal system. To resolve the indeterminate problem of muscular force-sharing and enhance our understanding of the mechanisms governing motor control, optimizationand EMG-driven models have often been used (e.g. Herzog, 1996; Dowling, 1997). However, the use of most such models is limited to tasks unaffected by neuromuscular fatigue. Implantation of tendon force transducers has circumvented the above problem, but this method is invasive and unable to measure individual forces in multimuscle systems (e.g., the quadriceps and the triceps surae muscles). One parameter that does, however, relate directly to the exertion of muscle force (irrespective of whether neuromuscular fatigue is present or not) and could thus be used to assess the direction of contractile force changes in individual muscles is the resultant lengthening in the inseries tendons: If a tendon lengthens during muscle contraction in a given joint configuration more or less than in another contraction in the same configuration, this would mean that the respective muscle produced in the former contraction more or less contractile force, respectively, than in the latter contraction. Ultrasonography enables human tendon lengthening to be measured in vivo non-invasively (e.g. Maganaris, 2002; Maganaris & Paul, 2002). In the present study, we used ultrasonography to assess the direction of changes in the contractile activity of the human gastrocnemius (GS) and soleus (SOL) muscles in a series of contractions of varying intensity.