Physiological Reports / Insitu measurements of tensile forces in the tibialis anterior tendon of the rat in concentric, isometric, and resisted cocontractions

Tensileforce transmitted by the tibialis anterior (TA) tendon of 11 anesthetized adult male Wistar rats (bodymass: 360.6 66.3 g) was measured insitu within the intact biomechanical system of the hindlimb using a novel miniature inline loadcell. The aim was to demonstrate the dependence of the loadingprofile experienced by the muscle, on stimulationfrequency and the resistance to shortening in a group of controlanimals. Data from these acuteexperiments shows the type of loading achievable by means of implantable electrical stimulators activating agonists or agonist/antagonist groups of muscles during programmed resistancetraining in freely moving healthy subjects. Forceresponses to electrical stimulation of the common peroneal nerve for single pulses and short bursts were measured in unloaded and isometric contractions. A less timeconsuming approach to measure the forcefrequency relationship was investigated by applying single bursts containing a series of escalating stimulusfrequencies. We also measured the range of loading attainable by programmed cocontraction of the TAmuscle with the plantarflexor muscles for various combinations of stimulationfrequencies. The maximal average peakforce of single twitches was 179% higher for isometric than for unloaded twitches. Average maximal isometric tetanicforce per gramme musclemass was 16.5 3.0 N g1, which agrees well with other studies. The standard and timesaving approaches to measure the forcefrequency relationship gave similar results. Plantarflexor coactivation produced greatly increased tension in the TAtendon, similar to isometric contractions. Our results suggest that unloaded contractions may not be adequate for studies of resistancetraining. Plantarflexor cocontractions produced considerably higher forcelevels that may be better suited to investigate the physiology and cellbiology of resistancetraining in rodents.(VLID)483993