Isometric and concentric performance of electrically stimulated ankle plantar flexor muscles in intact rat

The relationship between muscle force and ankle position during isometric a nd pre-loaded slow concentric contractions (angular velocity, 0.52 rad s(-1 ); range of motion, 1.22 rad) and the recovery of isometric force following concentric contractions at different velocities were determined for electr ically stimulated plantar flexor muscles in intact rats. Pre-loaded refers to the isometric contraction which immediately precedes the concentric cont raction. Ankle position was controlled by a dynamometer and force was recor ded under the sole of the foot. The peak isometric force (19.2 N) was nearl y constant at all ankle positions (range of motion, 1.57 rad). The muscle l ength and distal fibre length of gastrocnemius medialis at ankle positions between 0.79 rad and 2.01 rad were increased by 12.6 % and 20.3 %, respecti vely. During slow concentric contractions, the force progressively decrease d (23.1 +/- 2.1 %); the force decreased by only 6.3 +/- 0.9 % during sustai ned isometric contractions of similar duration (3400 ms). The recovery of i sometric force following concentric contractions with similar stimulation f requencies (80 Hz) was velocity dependent (i.e, more rapid at higher veloci ties). it is concluded that pre-loaded slow concentric contractions of the plantar flexor muscles in intact rats do not follow the same relationship a s that of isometric force and ankle position. Our results in intact rats sh ow that the force output of electrically stimulated ankle plantar flexor mu scles measured under the sole of the foot can be used to study the physiolo gical properties of skeletal muscle working in situ.