Changes in muscle size,
architecture, and neural activation after 20 days of bed rest with and without
resistance exercise
Kawakami, Y., Akima, H., Kubo, K., Muraoka, Y., Hasegawa,
H., Kouzaki, M., Imai, M., Suzuki, Y., Gunji, A., Kanehisa, H., Fukunaga, T.
Eur. J. Appl. Physiol, 84: 7-12, 2001
Nine healthy men carried out head-down bed rest (BR) for 20
days. Five subjects (TR) performed isometric, bilateral leg extension exercise
every day, while the other four (NT) did not. Before and after BR, maximal
isometric knee extension force was measured. Neural activation was assessed by
a supramaximal twitch interpolated over voluntary contraction. From a series
cross-sectional MRI scans of the thigh, physiological cross-sectional areas of
the quadriceps muscles were estimated ("uncorrected" PCSA, volume /
estimated fiber length). Decrease in muscle force after BR was greater in NT
(-10.9 +- 6.9 %, P<0.05) than in TR (0.5 +- 7.9 %, not significant). Neural
activation did not differ between two groups before BR, but after BR NT showed
smaller activation levels. Pennation angles of the vastus lateralis, determined
by ultrasonography, did not show significant changes in both groups. PCSA
decreased in NT by -7.8 +- 0.8 % (P<0.05) while in TR PCSA showed only an
insignificant tendency to decrease (-3.8 +- 3.8 %). Changes in force were
related more to changes in neural activation levels than to those in PCSA. The
results suggest that reduction of muscle strength by BR is affected by a
decreased ability to activate motor units, and that the present exercise is
effective as countermeasure.