Effects of jump training on passive mechanical stress and stiffness in rabbit skeletal muscle: role of collagen

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Abstract
Aims:  This study evaluated the effect of a 15‐week jump training period on mechanical parameters and collagen concentration of different muscle types in rabbits, at 50, 90 and 140 days of age.Methods:  Trained (T) animals were made to jump over obstacles in order to get food and water. The height of the obstacle was increased according to the animal's age. Control (C) animals were sedentary. Mechanical parameters (force, stress, stiffness) and collagen concentration were measured in Extensor Digitorum Longus (EDL), Rectus Femoris (RF), Semimembranosus Proprius (SMP) and Psoas Major (PSOAS).Results:  Both EDL and RF collagen concentrations and passive mechanical parameters increased between 50 and 140 days of age (P < 0.001), whereas SMP and PSOAS collagen concentrations decreased (P < 0.001). Soluble collagen concentration decreased similarly with age in all muscles and groups. Exercise training at 140 days induced a significant increase in force (EDL 20.4% and RF 15.3%, 0.01 < P < 0.05), stress (EDL 26.8% and RF 22.6%, P < 0.001), and stiffness (EDL 13.2% and RF 16%, 0.001 < P < 0.05) with a greater collagen concentration (EDL 16.1% and RF 19.1%, 0.001 < P < 0.05). Finally, stiffness and stress are well correlated with collagen concentration in EDL and RF muscles at 140 days (0.74 < r < 0.84, P < 0.05).Conclusion:  These results suggest that jump training can increase muscle capabilities of strength and stiffness in fast‐twitch mixed muscles with pennate architecture (EDL, RF). This increase is essentially mediated by a rise in cross‐linked collagen fraction, coupled with a relative decrease in soluble collagen. The collagen covalent cross‐links improve resistance and stability in force transmission processes during stretching.