Effects of resistance training during bed rest on the viscoelastic properties of tendon structures in the lower limb

The purpose of this study was to investigate the effects of resistance training on the tendon properties in knee extensors during 20 days of bed rest. Sixteen men were assigned to the resistance training group (BR-Tr) or the non-training, control group (BR-Con). Leg-press exercises were performed as five sets of 10 repetitions at 90% of maximum load daily for 20 days during bed rest. Before and after bed rest, the elongation of the tendon structures of the vastus lateralis muscle during isometric knee extension was determined using ultrasonography, while subjects performed ramp isometric contraction up to the voluntary maximum, followed by a ramp relaxation. The relationship between estimated muscle force (F_m) and tendon elongation (L) was fitted to a linear regression curve, the slope of which was defined as stiffness. The hysteresis was calculated as the ratio of the area within the F_m-L loop to the area beneath the load portion of the curve. The stiffness decreased significantly after bed rest for BR-Con, but not for BR-Tr. Similarly, the hysteresis increased significantly after bed rest for BR-Con, but not for BR-Tr. These results suggested that the bed rest caused the stiffness of tendon structures to decrease and their hysteresis to increase, and that leg-press training prevents the deconditioning of the tendon structures in knee extensors.