Latash and Zatsiorski [14] defined three possible approaches to the concept of muscle tone, from which the respective methods of its objectification proceed. The first one is based on the definition of tone as the resistance of a muscle to movement at a given joint. This approach was used by McPherson [8] and Brennan [23] in their works and proposed devices. They used the resistive force of the spring that kept the segment of the respective musculoskeletal system out of the physiological position in a given joint to determine the degree of spasticity. The very same principle is the basis of the widely used subjective assessment of spasticity based on the Ashworth scale or its modified version [3,24,25]. However, these methods assess the mechanical properties of the entire musculoskeletal chain, including the joint. Thus, they are not focused on a single muscle and small changes in muscle tone and the mechanical properties of a particular muscle (elasticity, viscosity) are difficult to determine from them, see [10,26].

The second approach is to use an EMG, where muscle tone is taken as the initial resting signal level without muscle activation. This is the approach taken by Jacobson [27], but as views on muscle tone have changed, this method has proven inadequate: Adrian and Bronk [28] found that completely relaxed normal muscle shows no spontaneous electrical activity, but under such conditions, quantifiable muscle tone (in terms of hypo-, eu-, or hypertonia) can still be detected. Latash and Zatsiorski [14] also point out that complete relaxation might not be possible in certain patients or for specific measurement scenarios.

The third group of methods are indentation stress tests. Their principle is to press a tip (indenter) of known geometry into a body surface and monitor the mechanical response, or the load characteristics, of the underlying tissue. Two types of such devices, often called myotonometers, can be distinguished. In the first case, the oscillatory response of the tissue to a single, short, rectangular pulse of the indenter is evaluated. Probably the most well-known representative of these devices is the Myoton, or the latest model MyotonPRO [29,30]. The reliability of the method varies between 0.74–0.93 (95% CI_Avg), in different measurements [31,32,33,34,35,36,37,38,39,40,41,42,43,44,45]. Its application is so far limited to superficial muscles and other soft tissues. It cannot examine deep, hard-to-palpate muscles and cannot measure thin or small muscles. A major shortcoming of the device is that it uses tapping, an impulse different from normal palpation, which can elicit an unwanted reflex response in the tissue that can distort the result.

The second subgroup consists of myotonometers whose indenter pushes against the tissue to a predefined depth or to a predefined resistance force at a defined but relatively low speed, and then it returns. Such devices essentially simulate the palpation technique described above. The output is the dependence of the resistive force of the tissue on the indentation depth of the indenter and takes the form of a hysteresis curve. The first devices of this type were originally developed to determine the pressure required to induce pain in soft tissue [46]; only later was a measurement of pliability added [47], but this was not very reliable [48]. Gradually, different authors [49,50,51,52,53,54,55,56] worked on developing more accurate and user-friendly solutions. For some of these first devices, it was necessary to perform the indentation manually, which did not allow a constant force or speed of tip indentation to be ensured [49]. Consequently, some authors developed manual instruments that perform the indentation and calculation of rheological properties automatically, but the necessity of manual stabilization causes inaccuracies in the obtained results. For example, Leonard et al.’s Myotonometer [52], which shows variable reliability between 0.42 and 1.00 when used on selected muscles in different experimental settings [57,58,59], is worth mentioning. Other instruments [55,56,60] have a fixed design that minimizes the unreliability associated with examiner participation at the cost of less user friendliness. For example, the CMT (computerized muscle tonometer) has been found to have a reliability between 0.82 and 0.97 (95% CI_Avg) [55,61]. Another option is to attach the device directly to the segment to be measured. In combination with constant tissue deformation by the indenter, this option is used by the MC Sensor [62].

A general criticism of Latash and Zatsiorski [14] against indentation methods is that they treat tone as a passive property only. Thus, they do not consider its participation in active movement and posturing since the examinee is always instructed to relax. However, it can be argued that, in manual palpation, complete relaxation is a requirement for correct examination as well according to some authors [2], and even Latash and Zatsiorski [14] admit that muscle tone can be defined based on the state when the muscle is “relaxed to the maximum of the examinee’s abilities”.

From a physical point of view, it is important to note that the measurement of the load characteristic alone only allows a limited assessment of the mechanical properties of the tissues under examination. It is usually based on a descriptive characterization of the tissue response curve as a function of its load. Only by using a suitable mathematical model can the viscoelastic properties of the indented tissue layers be identified. However, the validation of these models is quite complicated and often tied to computational simulations.

In addition to indentation stress tests, there are efforts to determine the mechanical properties of musculoskeletal soft tissues using free vibration techniques (e.g., [63]). Again, information on the rheological properties of the affected muscle tissue can be extracted from these using a suitable mathematical model. However, there are once more limitations associated with the validation of mathematical models, see above.

An alternative to strain stress testing, standing outside of Latash and Zatsiorski’s [14] categories and currently coming to the fore, is non-invasive imaging. Ultrasonic elastography [64,65,66], where the propagation of acoustic waves is sensed by ultrasound, has a relatively long history and widespread use. There are already many approaches for the use of ultrasound in the assessment of tissue rheological properties [67], the most widely used being the supersonic shear wave imaging (SSI) [68]. The method has so far been attributed with a variable reliability ranging between 0.67 and 0.92 (95% CI_Avg) by various studies [33,40,41,69,70,71,72,73,74,75,76,77,78,79,80,81].

Magnetic resonance elastography (MRE) is a more costly and time consuming but also more accurate option [82,83,84,85]. Magnetic resonance imaging captures the propagation of mechanical waves in the tissue, and stiffness can then be inferred from both the wavelength and the speed of propagation. Low [86] aptly called this technique “virtual palpation.” MRE can even be used for measurement under dynamic conditions in real time [87]. The most advanced technique uses the 3T magnetic field, with a reliability of 0.65–0.98 (95% CI_Avg) on selected muscles [88,89].

Given the high variability of the reliability indices, a variance decomposition was used for selected methods (MyotonPRO, CMT, SSI). Results show that the intra-class variability of the reliability indices is significantly higher than the inter-class variability in all the analyzed methods. This virtually makes it impossible to compare the results from different measurements, even if the objective method used was the same.

In order to supplement this, infrared thermography can be mentioned as an indirect method of muscle tone assessment offered by Maršáková and Nováková [90]. In this technique, the body surface temperature is compared with the palpation findings, and the method can only be described as indicative.

Despite the shortcomings described above, objective methods offer an important advantage over subjective methods. While palpation can only assess muscle tone in terms of several levels (hypotonic/eutonic/hypertonic muscle), objective methods provide a numeric value. Therefore, provided that an appropriate methodology is followed, they allow quantification of intra- and possibly inter-individual differences in the measured components of muscle tone. This allows implementation of quantitative methods into a wide range of research areas.

In physiotherapy and rehabilitation medicine, or even neurology and other related fields, objective methods can play an important role in assessing muscle tone. First and foremost, objective muscle tone data can help the health care professional determine the lesion or other deviation from the physiological norm, its extent and degree, and possibly its nature or cause. For example, using both USE (ultrasound elastography) [91] and MRE, it has been possible to detect changes in various myopathies [92], conditions preceding pressure ulcers [93], and other muscle pathologies. Furthermore, these methods or Myoton can be used to detect stiff fascicles characteristic of myofascial trigger points [66,94,95], to assess rigidity [96,97,98,99,100] or spasticity [101]. Furthermore, Myoton, MRE, or USE have been used to investigate how changes in the rheological properties of the muscle correlate with the occurrence of various pain [102,103], changes in mobility and position of body segments [104,105,106] or previous injuries [107,108], how masticatory muscle stiffness affects masticatory abilities [109], etc.

Secondly, these methods can supplement the missing link in assessing the effectiveness of various physiotherapy interventions, such as techniques targeted at influencing muscle tone (ultrasound, soft tissue techniques, Kinesio taping, post-isometric relaxation, etc.), but also techniques targeted at another problem (symptom) or holistic techniques in which the change in muscle tone is a secondary manifestation (mobilization techniques, techniques based on neurophysiology, etc.). Similarly, of course, they can be used outside physiotherapy itself, e.g., to assess the effectiveness of medication, surgical interventions, etc. This use can contribute not only to the general validation of methods according to the rules of evidence-based medicine, but also in clinical practice for the assessment of individually implemented interventions on specific patients. For example, MRE has already been used to assess the effect of eccentric exercise [110] or positive thermotherapy [111], USE in instrumental massage [112] and botulinum toxin application [113], Myoton to assess the effectiveness of petrissage (deep muscle massage) and lymphatic drainage [114] and ischemic compression of myofascial trigger points [115], neurodynamics and instrumental soft tissue mobilization [116], strengthening and stretching exercises [117], aquatic exercise and electrical neuromodulation [118], dry needling [119], botulinum toxin and shock wave application [120], mobilization [121], and many other therapeutic modalities.

Thirdly, these methods make it possible to assess and recognize some negative influences on muscle tone objectively. For example, Myoton has been used to determine the effect of army boots on the stiffness of the lower limb muscles [122] and the effect of dental protectors on the stiffness of masticatory muscles [123]. This use in ergonomics is a separate chapter (see below).

Since occupational therapy is closely related to physiotherapy, the use of these methods is, to some extent, identical in these areas. Objective methods of muscle tone assessment can be used to determine the extent of impairment [66,94,95,96,97,98,99,100,101,124], which can then help in designing appropriate therapy and possible compensatory aids.

They can also be used to assess the effect of a specific workload or work environment on the musculoskeletal system, either by comparing the results of tone measurements on specific muscles before and after working hours within a single day [125] or periodically over a longer time cycle [126], or by comparing measurements under normal circumstances and under specific working conditions [127]. At the same time, it is possible to investigate how the rheological properties found in response to work/stress correlate with factors such as age, duration of employment [126], as well as perceived pain [125].

Similarly, these methods can be used to assess the effectiveness of various ergonomic devices and measures to make work easier for workers or to minimize the adverse effects of their work on their health, especially the musculoskeletal system [128,129,130].

In the field of sports, objective methods of muscle tone assessment make it possible to investigate the influence of individual types of loads or even specific sports on the muscular apparatus or the rheological properties of the muscle. Myoton has been used for this purpose in many cases, either to measure the immediate effect (i.e., before and after exercise) [131,132,133,134] or to determine the long-term effect. The latter has been determined either by measuring it in a single individual before, during, and after a training program [135], or by simply comparing values in a specific group of athletes with the general population [136,137]. These measurements can give people information on how to enhance performance in some cases and in which cases muscle overload occurs. Thus, this information can be used to prevent injuries and chronic overuse, modify training and recovery methods, and so on. Similarly, the influence of various sports aids, such as the aforementioned dental protectors, can also be investigated [123]. Secondarily, how tone is influenced by other factors such as posture and positioning in different segments can be investigated, and how these may relate to pain [104].

Furthermore, these methods can be used to investigate how the rheological properties of skeletal muscles are related to specific sports performance. For example, measurements of pre-exercise muscle tone using Myoton have shown that, for some muscles, higher agonist muscle tone (or lower antagonist muscle tone [138]) predicts better performance both between different individuals [139,140,141] and within the same individual in the course of a day [142].

Finally, as in physiotherapy, these methods can be applied in sports to test the effectiveness of recovery procedures intended to affect muscle tone or to speed up the treatment of sports injuries. In addition to the cases in the Physiotherapy section and many others, for example, the use of negative ion patches in the field of alternative medicine [143].

Apart from one exception [137], researchers could not find any cases in which a device other than Myoton was used in sports. It can be assumed that this is mainly due to practicality, time and money savings, and availability of the method. Imaging elastography methods, especially MRE, are more likely to be available at healthcare workplaces, whereas sports research is most often performed in sports institutions and laboratories or directly at sports venues.

Methods of muscle tone objectification can also contribute to research on the nature of muscle tone and its behavior in specific physiological and pathological circumstances. Some authors have already used USE [144], MRE [145,146,147], or Myoton [106,109,142,148,149] to establish normative values and to investigate changes in muscle rheological properties during the day or as a function of age, gender, menstrual cycle phase, BMI, race, individual physical activity, or stride length. As mentioned above, Myoton has also served to investigate the effect of specific physical activities on the rheological properties of muscle [131,135,136,137] or how these characteristics further relate to endurance and contractile ability and muscle strength [138,140].

Others have used these methods to investigate the rheological properties of muscles and the nature of their changes in pathologies such as hyperthyroidism [150], myopathy [91,151], other neuromuscular disorders [152,153], changes due to irradiation of tumors [154], etc., but also under specific extreme conditions [127].