In‐situ measurements of tensile forces in the tibialis anterior tendon of the rat in concentric, isometric, and resisted co‐contractions

Tensile‐force transmitted by the tibialis anterior (TA) tendon of 11 anesthetized adult male Wistar rats (body‐mass: 360.6 ± 66.3 g) was measured in‐situ within the intact biomechanical system of the hind‐limb using a novel miniature in‐line load‐cell. The aim was to demonstrate the dependence of the loading‐profile experienced by the muscle, on stimulation‐frequency and the resistance to shortening in a group of control‐animals. Data from these acute‐experiments shows the type of loading achievable by means of implantable electrical stimulators activating agonists or agonist/antagonist groups of muscles during programmed resistance‐training in freely moving healthy subjects. Force‐responses to electrical stimulation of the common peroneal nerve for single pulses and short bursts were measured in unloaded and isometric contractions. A less time‐consuming approach to measure the force‐frequency relationship was investigated by applying single bursts containing a series of escalating stimulus‐frequencies. We also measured the range of loading attainable by programmed co‐contraction of the TA‐muscle with the plantar‐flexor muscles for various combinations of stimulation‐frequencies. The maximal average peak‐force of single twitches was 179% higher for isometric than for unloaded twitches. Average maximal isometric tetanic‐force per gramme muscle‐mass was 16.5 ± 3.0 N g(−1), which agrees well with other studies. The standard and time‐saving approaches to measure the force‐frequency relationship gave similar results. Plantar‐flexor co‐activation produced greatly increased tension in the TA‐tendon, similar to isometric contractions. Our results suggest that unloaded contractions may not be adequate for studies of resistance‐training. Plantar‐flexor co‐contractions produced considerably higher force‐levels that may be better suited to investigate the physiology and cell‐biology of resistance‐training in rodents.