The fiber-type-specific effect of inactivity and intermittent weight-bearing on the gastrocnemius muscle of 30-month-old rats.
OBJECTIVE
To characterize single skeletal muscle fiber contractile properties from the gastrocnemius muscle that occur during inactivity and intermittent weight-bearing in 30-month-old animals.


METHODS
Randomized control trial.


METHODS
A controlled laboratory environment.


METHODS
Eighteen 30-month-old male Fisher 344 Brown Norway F1 Hybrid rats were randomly assigned to control (C), hindlimb unweighted (HU), and hindlimb unweighted with intermittent weight-bearing (HU-X) groups.


METHODS
The HU and HU-X rats were suspended for 7 days. The HU-X animals were unsuspended for four 15-minute bouts of weight-bearing.


METHODS
Single skeletal muscle fiber contractile properties (diameter, peak active force [P0], peak specific tension [P0/CSA], and maximal shortening velocity [V0] by fiber type) were determined from the deep portion of the lateral head of the gastrocnemius muscle (RG).


RESULTS
In comparison to C animals, the ratio of gastrocnemius weight to body weight decreased by 18% and 14% following HU and HU-X, respectively. Diameter and P0 of type I fibers from the RG were reduced after HU. Attenuation of the decline in diameter and P0 was observed in type I fibers from the RG with HU-X. P0 was reduced in type IIa fibers and type I-IIa fibers with HU. Attenuation of the decline in P0 by intermittent weight-bearing in type IIa fibers and type I-IIa fibers did not occur.


CONCLUSIONS
Inactivity altered the contractile properties of single skeletal muscle fibers from the gastrocnemius muscle of 30-month-old animals. The inactivity-induced alterations were present in the three fiber types. Therapeutic intervention of weight-bearing attenuated the inactivity-induced changes in the type I fibers from the gastrocnemius, but not the other fiber types.
Sandmann
M E
ME
Department of Physical Medicine and Rehabilitation, School of Medicine, University of Minnesota, Minneapolis 55455, USA.
Shoeman
J A
JA
Thompson
L V
LV
D000222:Adaptation, Physiological; D000375:Aging / Q000502:physiology*; D000818:Animals; D001835:Body Weight; D019417:Hindlimb Suspension; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011897:Random Allocation; D051381:Rats; D011916:Rats, Inbred F344; D012146:Rest / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology*