Study on the Release of Muscle Passive Force by Ultrasonic Mechanical Effect and its Related Mechanism

11 Background: Excessive muscle force impedes physical movement and relaxing 12 passive muscle force substantially improves movement impairment. Ultrasound is an 13 energy carrier with the characteristics of repetitive mechanical stimulation, which may 14 be a feasible method to relieve muscle tension. 15 Methods: We performed stress relaxation experiments on soleus muscle and combine 16 the obtained results with the standard linear solid model to extract information of 17 viscoelastic effect of ultrasound on muscle, and calculated muscle fibril content by 18 histological analysis. 19 Results: Ultrasound can accelerate muscle stress relaxation; the viscosity and 20 elasticity coefficient of the ultrasound group was higher than that of the control group, 21 and there was no significant difference between the three ultrasound intensities; H&E 22 staining showed that muscle fibrillar content decreased and the matrix substance 23 increased. 24 Conclusion: We considered that ultrasound can change the microstructure of muscle, 25 and the matrix substance plays a significant role in the relaxation process. In this 26 paper, the relationship between muscle viscoelasticity and passive muscle force is 27 obtained. The results provide an important theoretical basis and a feasible method for 28 monitoring muscle functional characteristics by measuring muscle viscoelasticity. 29

staining showed that muscle fibrillar content decreased and the matrix substance 23 increased.

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Conclusion: We considered that ultrasound can change the microstructure of muscle, 25 and the matrix substance plays a significant role in the relaxation process. In this 26 paper, the relationship between muscle viscoelasticity and passive muscle force is 27 obtained. The results provide an important theoretical basis and a feasible method for 28 monitoring muscle functional characteristics by measuring muscle viscoelasticity. As a muscle is passively stretched, it resists with passive force such as tension, 33 impeding joint movement, reducing muscle flexibility, and increasing the risk of 34 muscle injury [1,2]. Because this significantly affects the level at which an individual 35 can function and perform, it is particularly important for athletes to relieve muscle 36 tension and improve flexibility after exercise. 37 Proprioceptive neuromuscular facilitation (PNF) and static passive stretching are 38 two most commonly used stretching techniques in athletics and clinical practice [3,4]. 39 Another method-mechanical vibration that has been shown improve flexibility [5][6][7]. 40 The release of muscle tension can correct muscle imbalances, improve joint range of 41 motion, relieve muscle soreness and joint pressure, and help maintain physiological 42 muscle length. The most common and safe way to reduce muscle passive force is by 43 applying an external force, for example, through massage. Research shows that 44 3 massage can promote muscle relaxation [8,9], reduce muscle tension [10] and 45 soreness [11,12], accelerate muscle function recovery [13], and improve athletic 46 performance [14]. Other methods, such as self-myofascial release, exert pressure on 47 the muscles (in this case, using one's own body weight on structures, such as foam 48 rollers) to reduce passive force [15]. However, these methods mentioned above are   The standard linear solid model is shown in Fig. 1A. The model is composed of 104 two branches: one branch-a damper, with viscous coefficient η 1 and displacement 105 represented by x 1 , and spring S 1 , with elasticity coefficient μ 1 and displacement represented by x 1 ′, connected in series-is subjected to force 1 ; and the other 107 branch-spring S 2 , with elasticity coefficient μ 2 and displacement represented by 108 x 2 -is subjected to force 2 , when force is applied to the system. The model 109 satisfies the following relations [25]: After substituting eq.3 and eq.4 into eq.2, it can be written eventually as and factor = 2 is the relaxation modulus of elasticity. As → ∞, the dashpot 120 relaxes completely, and the change in passive force is primarily determined by the 121 spring, which is characterized by .

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If the muscle produces an elongation at = 0 that is maintained thereafter, the  and without ultrasound stimulation (ultrasound and control groups, respectively).

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The rats were deeply anesthetized by intraperitoneally injecting pentobarbital 163 sodium; the limbs of rats were fixed, the dorsal skin of the hind limbs was removed,

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tendons on both ends of soleus were ligated with silk thread, and the muscle was fully  The whole animal muscle test system for data acquisition consisted of a tissue 173 bath, in which the soleus specimen was fixed, and force sensor, which captured 174 passive muscle force data during the stress relaxation test. The experimental setup and 175 ultrasound temporal waveforms were illustrated in Fig. 2. As presented, the silk thread 176 at one end of the soleus was placed on the bracket inside the tissue bath, and the silk 177 thread at the other end was tied to the lever of the force sensor.

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To adjust the muscle to the optimal length, pre-strain was applied to both ends of The data were fitted to eq. 7. We set the minimum value of each set of data as .   Fig. 6), 308 and the results were showed in Fig. 7 we used a duration of 10 minutes (600 s) for the stress relaxation tests.

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The force-time curves that we obtained (Fig. 4A)  where in equation (8), is fibril content that defined as the ratio of the area A f 390 covered by fibril (dark circle) over the total fascicle cross sectional area A t (Fig. 6B).

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We observed microstructure of tissue by H&E staining. The fibrillar components are in a certain relative density to the embedding matrix