Objectives: The purpose of this study was to investigate the effects of 20 days' bed rest on the viscoelastic properties of human tendon structures in knee extensor and plantar flexor muscles in vivo. Methods: Eight healthy men (age: 24 ± 4 years, height: 172 ± 9 m, body mass: 69 ± 13 kg) carried out a 6° head-down bed rest for 20 days. Before and after bed rest, elongation (L) of the tendon and aponeurosis of vastus lateralis (VL) and medial gastrocnemius muscles (MG) during isometric knee extension and plantar flexion, respectively, were determined using real-time ultrasonic apparatus, while the subjects performed ramp isometric contraction up to the voluntary maximum, followed by ramp relaxation. The relationship between estimated muscle force (Fm) and tendon elongation (L) was fitted to a linear regression, the slope of which was defined as stiffness. The hysteresis was calculated as the ratio of the area within the Fm-L loop to the area beneath the load portion of the curve. Results: L values above 100 N were significantly greater after bed rest for VL, while there were no significant differences in L values between before and after for MG. The stiffness decreased after bed rest for VL (70.3 ± 27.4 v 50.1 ± 24.8 N/mm, before and after bed rest, respectively; p = 0.003) and MG (29.4 ± 7.5 v 25.6 ± 7.8 N/mm, before and after bed rest, respectively; p = 0.054). In addition, hysteresis increased after bed rest for VL (16.5 ± 7.1% v 28.2 ± 12.9%, before and after bed rest, respectively; p = 0.017), but not for MG (17.4 ± 4.4% v 17.7 ± 6.1%, before and after bed rest, respectively; p = 0.925). Conclusions: These results suggested that bed rest decreased the stiffness of human tendon structures and increased their hysteresis, and that these changes were found in knee extensors, but not the plantar flexors.
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