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Abstract
The purpose of this study was to investigate whether tendon vibration could prevent soleus muscle atrophy during hindlimb unloading (HU). Mechanical vibrations with a constant low amplitude (0.3 mm) were applied (192 s/day)
with constant frequency (120 Hz) to the Achilles tendon of the
unloaded muscle during the 14-day HU period. Significant
reductions in muscle mass ( 41%), fiber size, maximal twitch
( 54%), and tetanic tensions ( 73%) as well as changes in
fiber type and electrophoretic profiles and twitch-time parameters ( 31% in the contraction time and 30% in the half relaxation time) were found after 14 days of HU when compared with the control soleus. Tendon vibration applied during HU significantly attenuated, but did not prevent, 1) the loss of muscle mass (17 vs. 41%); 2) the decrease in the fiber cross-sectional area of type IIA ( 28 vs.
50%) and type IIC ( 29 vs. 56%) fibers; and 3) the decrease in maximal twitch ( 3 vs. 54%) and maximal tetanic tensions ( 29 vs. 73%) and the half relaxation time (1 vs. 30%). Changes in the contraction time and in histological and electrophoretical parameters associated with HU were not counteracted. These findings suggest that tendon vibration can be used as a paradigm to counteract the atrophic process observed after HU.hindlimb unloading; muscular atrophy; countermeasure IT IS WELL KNOWN that simulated weightlessness when the hindlimb-unloading animal model is used (29, 30) produces structural and functional deficits in skeletal muscles comparable to those described after a period of real microgravity during spaceflight (12, 39). The magnitudes of these alterations (muscular atrophy,decreases in muscle strength and twitch contraction time, modifications in fiber type and electrophoretic profiles) are specific to the muscle and fiber type. These changes are greatest in the ankle extensors, such as the soleus muscle, that normally perform antigravity postural functions (12, 39). Several different training programs have been tested for their ability to reverse or prevent muscle atrophy during unloading (11, 19, 23, 36, 40). However, no exercise countermeasure has completely prevented soleus muscle atrophy during unloading.
Muscle spindle primary endings (type Ia fibers) are sensitive to tendon vibration (6, 7, 27, 28, 35). Moreover, when vibrations are applied during passive stretching of the soleus muscle, the response of the primary sensory endings is increased. In contrast, this response decreases when vibrations are applied during muscle shortening (6). We hypothesized that stimulation of the muscle spindle primary endings via combined vibration and passive stretching movements of the rat soleus muscle during hindlimb unloading transitorily provides a reflex increase in electromyographic (EMG) and contractile activity. The objectives of this study were thus to 1) test the effects of tendon vibration on the morphological, mechanical, and histological properties of the unloaded rat soleus muscle; and 2) demonstrate the practicality of using tendon vibration as a countermeasure for preventing atrophy and loss of muscle
function in microgravity.
Materials and Methods
Animal groups and maintenance. Twenty-four male Wistar rats weighing 240-250 g at the beginning of the experiments
were divided randomly into three groups: control (Con; n7),
hindlimb unweighted (HU; n7), and hindlimb unweighted
submitted to daily intermittent tendon vibration (HU-TV; n
10). The rats were housed individually in conventional plastic
cages and had access to pellet food and water ad libitum. The
rats were acclimatized at a 23°C room temperature and with
a 12:12-h light-dark cycle for 1 wk before they were used in
the experiments. The maintenance conditions of the animals
and the experiments received authorization from both the
Agricultural and Forest Ministry and the National Education
Ministry (Veterinary Service of Health and Animal Protection; authorization 03805). Hindlimb-unloading protocol. Both unloaded groups (HU and HU-TV) were subjected to a suspension period of 14 days. The suspension model used was the tail-suspension model introduced by Morey (29). Briefly, the tail of each rat was washed, cleaned, dried, and wrapped in an antiallergic orthopedic tape-adhesive plaster. This cast, covering less than half of the tail, was secured to an overhead swivel, mounted at the top of the cage and permitting free 360° rotation of the animals. The rats were unloaded at 30° head-down angle to mimic fluid shifts characteristic of weightlessness. Tendon vibration and movement sequence paradigm. The apparatus used to induce tendon vibration and the protocol of vibrations are illustrated in Fig. 1, A-B. Vibrations were applied to the Achilles tendon of the right hindlimb. For application of the vibrations, anesthetized rats (80 µg ketamine and 8 µg acepromazine/g body wt) of the HU-TV group were placed in a custom-built system, which allowed the maintenance of the unweighted position but kept the animal's body and the knee of the right hindlimb in a firm position while leaving the ankle free to traverse its normal range of motions (plantar flexion at 160-180° to dorsiflexion at 30-35°). Mechanical vibrations were applied perpendicularly to the Achilles tendon of the right hindlimb by means of a Teflon probe (3 mm in diameter) connected to the moving coil of an electromechanical vibrator (Bruel and Kjaer minishaker 4810). The electromagnetic device was fixed in an indepen- The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked ''advertisement'' in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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