The Science of Ultrasound Dosing

Dosing Therapeutic Ultrasound to Induce Vigorous Heating Prior to Stretching and Manual Therapy

By Joseph A. Gallo, DSc, AT, PT and Kevin J. Silva, MS, ATC, Salem State University, Sport and Movement Science Department

It has become increasingly clear in the electrophysical agent literature that a combined approach yields better outcomes when compared to the passive stand-alone use of modalities. The purpose of this article is to discuss how to effectively heat tissue to therapeutic temperature ranges in preparation for stretching and/or manual therapies.

Clinicians often choose either a superficial or deep heating agent prior to a “heat and stretch” intervention. Superficial heating agents, such as hot packs, have a limited depth of penetration of up to 1-2 cm. However, at depths greater than 1 cm, superficial heating agents are often not able to effectively elevate tissue temperatures to the appropriate therapeutic range.[1] Conversely, therapeutic ultrasound and shortwave diathermy are classified as deep heating agents which have the capacity to heat effectively up to 5 cm deep.[2]

Figure 1. Clinician performing thermal ultrasound to patellar tendinopathy with the tendon in a slightly stretched position.

Therapeutic ultrasound has the ability to effectively heat tissue to a therapeutic level that promotes an increase in tissue viscoelasticity, which is often referred to as vigorous heating. Vigorous heating is achieved by elevating the baseline tissue temperature by 4°C or reaching an absolute tissue temperature of 40°C (Table 1). It is important to note that baseline intramuscular tissue temperature is approximately 36°C. This 4°C increase is believed to maximize viscoelasticity of soft tissue during and immediately after treatment, and is the basis for the wide spread use of pre-heating tissue immediately before stretching and manual therapy techniques.[1,3] Additional research is needed to determine the comparative effectiveness of combining deep heat with manual techniques.

Early research using animal models indicated that an absolute tissue temperature between 40-45°C was required to increase viscoelasticity in tissue.[4] For many years, this was the prevailing thought expressed in the electrophysical agent literature and textbooks. However, it has been noted in more recent ultrasound research that human subjects commonly do not tolerate absolute tissue temperatures above 41°C. [5]

Draper et al. [2] established the dose response relationship for heating muscle with 1 and 3 MHz ultrasound. This study identified the heating rate of ultrasound in °C/min, allowing the clinician to select intensities (W/cm2) and treatment times that produce predictable heating in human muscle (Table 2). It is important to note that heating rates vary between manufacturers and devices; therefore, net tissue temperature increase will vary between manufacturers and devices. Additional research to determine the heating rates of contemporary devices is needed.

The frequency of ultrasound dictates the depth of penetration and impacts the efficiency of heating. To reach deeper tissues (up to 5 cm), a frequency of 1 MHz should be selected. When the target tissue is within 2.5 cm from the surface of the skin, a frequency of 3 MHz should be selected.[7] It is important to note that 3 MHz heats approximately 3x faster than 1 MHz, creating an efficiency in heating when compared to 1 MHz ultrasound.[2] Furthermore, 1 MHz ultrasound has the capacity to be a deep heating agent, however, it is an inefficient heater of deep muscle and thus requires greater sonation time (Table 3). Conversely, ultrasound is a reasonably efficient heater of superficial muscle and is the most efficient heater of superficial tendons because of the increased collagen content (Table 4).

Heating efficiency will also be affected by the application technique. It is important to remember that ultrasound is a very focused treatment, and the size of the treatment area should be no greater than 2x the size of the sound head. In order to maximize the heating effect, the sound head should be moved in an overlapping circular or longitudinal pattern at a rate of approximately 4 cm/sec.

A common treatment goal is to increase local blood flow and tissue extensibility, which can be achieved by combining vigorous heating with stretching and/or manual therapy. Clinically, it is important to note that the stretching window post-ultrasound treatment is limited to 3.3 minutes for muscle and 5 minutes for tendon and ligament.[8,9] It is during these post-ultrasound time periods that tissue has the greatest temperature and viscoelasticity. Near the end of an ultrasound treatment, place the target tissue on stretch to maximize tissue elongation, and immediately follow the treatment with stretching, joint mobilizations, or instrument assisted soft tissue mobilization. The literature is clear that ultrasound can elevate tissue temperature to a vigorous level prior to stretching and manual therapies when dosed and applied correctly.

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References:

1. Draper, D. Therapeutic ultrasound. In: Knight KL, Draper DO. Therapeutic Modalities: The Art and Science. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013.

2. Draper DO, Castel JC, Castel D. Rate of temperature increase in human muscle during 1 MHz and 3 MHz continuous ultrasound. J Orthop Sports Phys Ther. 1995;22(4):304-307.

3. Draper DO. Ultrasound and Joint Mobilization for achieving normal wrist range of motion after injury or surgery: A case series. J Ath Train. 2010;45(5):486-491.

4. Lehman JF, De Lateur BJ. Therapeutic Heat. In: Lehman J, and Therapeutic Heat and Cold. 4th ed. Baltimore, MD: Williams & Wilkins; 1990.

5. Merrick MA, Bernard KD, Devor ST, Williams JM. Identical 3-MHz ultrasound treatments with different devices produce different intramuscular temperatures. J Ortho Sports Phys Ther. 2003;33(7):379-385.

6. Chan AK, Myrer JW, Meason GJ, and Draper DO. Temperature changes in human patellar tendon in response to therapeutic ultrasound. J Ath Train. 1998; 33(2): 130-135.

7. Hayes BT, Merrick MA, Sandrey MA, Cordova ML. Three-MHz ultrasound heats deeper into the tissue than originally theorized. J Ath Train. 2004; 39(3):230-234.

8. Rose S, Draper DO, Schulthies SS, Durrant E. The stretching window part two: rate of thermal decay in deep muscle following 1 MHz ultrasound. J Ath Train. 1996; 31(2): 139-143.

9. Draper DO, Ricard MD. Rate of thermal decay in human muscle following 3 MH ultrasound: The stretching window revealed. J Ath Train. 1995; 30(4):304-307.