# Effects of Mechanical Vibration in Equine Osteoarthritis: A Pilot Study
## 1 Introduction  
A mechanical vibration is a periodic back-and-forth motion of the particles of an elastic body or medium; the phenomenon occurs when a physical system is displaced from its equilibrium condition and responds to the stimulation with an internal motion that tends to restore equilibrium [1]. In biological systems, it has been demonstrated that vibrations, in particular ultrasound (US), are able to activate afferent nerve fibers of the muscle spindle and, hence, the alpha motoneurons; this elicits the so-called \"tonic vibration reflex\". It consists of the sustained contraction of the vibrated muscle and simultaneous relaxation of its prime antagonist [2]. Therapeutic ultrasounds (TUs) are widely used both in human and veterinary medicine to treat a broad series of pathological conditions [3]. However, scientific evidence of TUs' favorable clinical effects is limited for the majority of therapeutic purposes [4]. The first studies on TUs conducted on animals were recently published [4, 5, 6, 7], and all of them provide a clinical and little speculative imprint on the effects of TUs. Despite this, in daily clinical practice, TUs are largely used by practitioners, who describe excellent empirical results for a wide variety of orthopedic conditions, especially in the field of equine medicine. Thus, if on one hand there is a lack of supporting literature, particularly in terms of frequencies and times of application [4], on the other hand, we must obey [PERSON] famous Latin axiom \"_quod factumtissue temperature and blood flow, and a non-thermal effect [8; 9]. Non-thermal effects include acoustic cavitation [9]. Observations also suggest that the absorption of mechanical energy may modify gene expression, growth factors, and collagens. Together, thermal and non-thermal effects on the target tissue have been proposed to result in increased local metabolism, circulation, extensibility of connective tissue, and tissue regeneration [10]. TUs have a frequency range of 0.75-3 MHz, with most devices set at a frequency of 1 or 3 MHz. The treatment parameters taken into account are:  
Frequency: The number of compression-rarefaction cycles per unit of time, expressed in cycles per second, or Hertz (Hz). Increasing the frequency of TUs causes a decrease in their depth of penetration and concentration of ultrasonic energy in the superficial tissues. There is an inverse relationship between penetration and absorption [1]. Intensity: The power per unit area of the sound head, expressed in watts per square centimeter (W/cm\({}^{2}\)). The World Health Organization (WHO) limits the average intensity output by therapeutic US units to 3 W/cm\({}^{2}\)[1]. Power: the amount of acoustic energy expressed in watts (W), per unit time. In order to characterize the exposure, the total power should be specified, as well as the following intensities: spatial average temporal average (SATA) intensity; spatial peak temporal peak (SPTP) intensity; spatial peak temporal average (SPTA) intensity; and, if applicable, spatial peak pulse average (SPPA) intensity and spatial average pulse average (SAPA) intensity [1]. Pulse: TUs can be delivered as continuous or pulsed waves. The continuous mode consists of the continuous delivery of ultrasound at a single frequency throughout the treatment period; the pulsed mode consists of the intermittent delivery of TUs during the treatment period, with a non-constant pressure amplitude (equaling zero for part of the time) [1]. In the equine industry, lameness due to osteoarthritis (OA) is the most prevalent cause of diminished athletic function and wastage in racing horses [11]. The scientific and therapeutic results of the use of TUs in human medicine seem very encouraging. In particular, low-intensity ultrasound stimulation (LIPUS) has been proposed as a tool to promote the chondrogenic differentiation of mesenchymal stem cells (MSCs) [12].