Keywords

1 Introduction

Modern thermography is a relatively young science and it can be stated, that it starts in the early 30s of the 20th century, despite the infra-red radiation is a much earlier discovery. During the last 100 years, the thermal imaging technique gains in popularity and is often used for many purposes like security, control of electrical systems, buildings insulations, finding wet/humid areas. Nevertheless, its most prominent use should be identified with medicine and biomechanics. Detecting symptoms of diseases, vascular problems, or even scanning for cancer is one of the most common use. In biomechanics scanning muscle systems for its activity, overloading, faulty posture or musculoskeletal injury is just one of the most common applications [1, 2]. Definitely, this technique is getting more and more popular in the field of biomechanics; one can find plenty of research on this topic. According to Google Scholar, there is about 18,000 answers for the question “thermal imaging muscle system”.

The biggest advantage of infra-red is that it is a non-contact technique. Unfortunately, when speaking about a musculoskeletal system (human or animal), researchers can expect many variables that can influence the experiment results. When analysing State of the Art of thermal imaging regarding humans, one can find some propositions of standardization procedures. International Association of Certified Thermographers, American Academy of Thermology, European Association of Thermology, and papers [3, 4] are just examples of them. Nevertheless, it is worth to notice, that in recently published and analysed papers (see for example [3,4,5, 11, 12] authors are using different methods of volunteer preparation and providing conditions in laboratories, which sometimes even they are changing during an experiment. This generates a question if the published results are repeatable or even reliable. According to the author’s knowledge, the most advanced (and restricted) procedure is presented in [4], nevertheless, it can be improved and supplemented with new findings.

The aim of this paper is to present a method of infra-red experiments standardization in biomechanics. Method of volunteer and laboratory facility preparation along with guidelines for making thermal images are presented and described.

2 Standardisation Procedure of Thermal Imaging

The main points of the standardisation procedure are based on the International Association of Certified Thermographers, American Academy of Thermology, European Association of Thermology, and papers [4,5,6]. The most restricted advices are chosen and supplemented with recommendations created based on the author own experience.

2.1 Volunteer Preparation

The most important element of the standardization procedure is the correct preparation of the volunteer. The main recommendations are pointed out in Table 1. Listed recommendations should help to minimize the influence of external factors or the behaviour of the volunteer on the skin temperature prior to the experiment. The most important ones are connected with the actions which effects persist for a long time, especially like sunburns or muscle recovery after an extensive workout. Some of the recommendations, like not to use any lotions, creams, detergents, or similar minimum 3 days before the experiment should help to stabilize emissivity for body areas on the same level [7]. If the thermal image is to be done on a hairy area, hair should be removed three days before, preferably by shortening them at skin level, but not shaving, which can cause local inflammation. About one day before the experiment, the volunteer should stop consuming alcohol and should start to avoid greasy meals or other meals, spices, and substances that lead to greater thermogenesis (ex. spices with capsaicin, black pepper, ginger, mixed spices, and drinks like green or black tea and caffeine) [8]. Tight garment parts can restrict blood flow [13] and as a result change body segment temperature, what is the reason why they should be avoided before thermal imaging experiments.

Additionally, volunteer’s skin should be free from any inflammations, wounds, cuts, and other types of skin damage. Some tattoos can modify temperature readings, which makes it necessary for screening tattooed areas for unexpected, local differences in temperature [14]. Core temperature should not exceed 37 °C. It is recommended to have an interview regarding all volunteer injuries. Some of them, even considered as insignificant can have long-lasting (even life-lasting) effect, however, finding volunteer without any injury-history can be a real challenge, thus the researchers should be aware of a possible influence of the injury and faulty posture history on the thermal imaging.

Table 1. Main recommendation of the volunteer preparation.
Full size table

2.2 Laboratory Conditions

Not only the volunteer has to be prepared for the experiment. Very important is to correctly prepare the laboratory facility. The key actions are presented in Table 2. At first, the temperature and real humidity should be stabilised. It is advised to keep the room temperature in the range 21 °C–24 °C [9] and to allow the volunteer to choose his/her preferred temperature during the experiment in this range to ensure a proper thermal comfort level. The temperature should not change during the experiment. Temperature below 18 °C can cause shivering, above 24 °C – extensive sweating. Highly efficient heat sources, ACs, fans, humidifiers, or air dryers may be used, but they should be removed or turned off during the experiment. Especially IR radiators, like heaters should be removed or covered. On the day of the experiment, the advection and convection in the laboratory should be minimised to avoid cooling air movement. It is also advised to use window blinds, the best is with IR-mirrors. If it is possible, the best placement for the laboratory, where this type of experiment is performed is a northern site of the building or room without windows.

Table 2. Main recommendations for the laboratory preparation.
Full size table

2.3 Making the Thermal Image

Last but not least is to make a proper thermal image. Main recommendations are presented in Table 3. After the volunteer arrives at the laboratory an adaptation period should be performed. Volunteers stay in a garment in which thermal image will be done. No additional covers of the skin or body parts are allowed. The volunteer should not touch or leaning anything that can change his/her skin/body temperature. The position should be natural and symmetrical. No muscle part should be overload ex. by standing or sitting in an inconvenient or asymmetrical position. A slow gait is allowed.

The minimal time of adaptation is 15 min and it can be calculated according to the equation:

$$\tau_{min} \left[ {{\text{min}}} \right] = {15} + \left( {|T_{ext} - T_{int} |} \right)$$

where: τmin is minimal time of adaptation in minutes, Text and Tint – external (outside laboratory) and internal (in the laboratory) temperature in Celsius/Kelvin degree respectively. For the Fahrenheit scale, this rule should be adequately modified. The camera should be placed on stable support at a constant and repeatable distance from the volunteer. Its optical axis should be placed with respect to the normal of the object surface (ex. torso) due to the Lambertian nature of IR radiation [9]. The emissivity should be chosen for proper skin pigmentation. The average and accepted values in the literature are in range 0.97–0.98 [10].

It is also advised to choose a neutral background, for example, a wall with uniform temperature. Moreover, a gold standard should be to check and store all necessary data like temperature and humidity in the laboratory, date, and time of the experiment. A good practice should be to check the body fat of the volunteer (amount and distribution) and to make basic anthropometric measurements. Taking photos in S and F plane should facilitate the analysis. A questionnaire about performed sports/activities should also help to analyse and understand results.

Table 3. Main recommendations for making thermal image.
Full size table

2.4 Control List

It is advised to create a control list. It should help to understand all problems found during the analysis of thermal images and to distinguish between technical problems/errors in procedure and natural phenomena. An example of this type of control list, created on the basis of control list (translated and rearranged) used in the laboratory of Biomechanics Lodz University of Technology is presented in Supplementary Materials. It includes the most important questions about volunteer activity, faulty posture, photography in anatomical position (Table S1) and checklist (Table S2).

3 Conclusions

Standardisation of examination procedures is crucial for obtaining reliable and repeatable results. For some of the techniques, conditions are restricted. For example, when sEMG is made not according to the SENIAM regulations this makes results unpublishable. The variety of methods, equipment, conditions in the laboratory, and procedures in different research centres makes published results of thermal analysis often incomparable and even questionable. Such a standard procedure in this field is mandatory. Presented procedure should help to obtain a repeatable conditions of thermal imaging experiments and to obtain more reliable results. Main recommendations are presented in Tables 1–3.