Assessment of the Impact of Heat-Compression Therapy Time on Muscle Biomechanical Properties and Forearm Tissue Perfusion in MMA Fighters—A Pilot Study

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Recovery in sport.

Abstract

Background: The study aimed to evaluate the immediate effect of Heat-Compression-Therapy with Game Ready equipment (GR-HCT) on biomechanical properties at different treatment times. Reducing the duration of stimulation through HCT can help optimize recovery processes in sports. Methods: Twenty male MMA fighters (26.5 ± 4.5 years, BMI 24.75 ± 3.0, training experience: 10.3 ± 5.0 years) performed two experimental sessions: (a) 5 min heat-compression therapy (HCT) stimulation (GR-HCT-5); and (b) 10 min heat-compression therapy (GR-HCT-10). All registered parameters, tissue perfusion (PU), tissue temperature (°C), muscle tone (T-Hz) stiffness (S—N/m), elasticity (E—arb), pressure pain threshold (PPT—N/cm) and isometric muscle strength (Fmax—kgf), were measured at rest (r—2 min before GR-HCT stimulation) and 1 min after GR-HCT (p-post) for the flexor carpi radialis muscle (MFCR) and the brachioradialis muscle (MBR). Results: The Friedman’s test for MBR revealed significant differences between GR-HCT5min vs. GR-HCT10min for PU (p < 0.0001), S (p = 0.008), and for MBR and MFCR for °C (p < 0.0001). The pairwise comparisons for PU, S, and °C showed significant differences between rest vs. post values for 5 min as well as between rest vs. post values for 10 min. Further, there was a significant interaction effect (5 min-10 min × Rest-Post p < 0.0001) for PU for MFCR. The post hoc comparison for the interaction effect showed significantly higher PU for post- compared to premeasurement in 5 min PU and 10 min PU (p < 0.001). Further, there was a significant main effect of condition for Fmax both for MFCR and MBR (5 min vs. 10 min p < 0.001); The post hoc comparison for the main effect of condition showed a significantly higher Fmax for post- compared to premeasurement in 5 min and 10 min (p < 0.001). Further, there was a significant main effect of condition for pressure pain threshold [N/cm] both for MFCR and MBR and for T for MBR time of measurement rest vs. post (p < 0.001; p = 0.006; p < 0.001, respectively). The post hoc comparison for the interaction effect showed a significantly lower PPT for post vs. rest in 5 min (p < 0.001 only for MFCR) and in 10 min (p < 0.001). The delta values for PU (post-rest) and ΔTemp °C showed significant differences between the 5 min and 10 min conditions (MFCR; p < 0.0001); (MBR p < 0.001) and (MFCR p < 0.0001); (MBR p < 0.001). Conclusion: Our results show that GR-HCT significantly affects recovery of muscle biomechanical parameters, pain threshold, strength, tissue perfusion, and temperature. Our findings show that a 5 min effect can be sufficient when compared to a 10 min one which is a clue for designing effective recovery protocols.

1. Introduction

Training in combat sports, including mixed martial arts (MMA), emphasizes the development of muscular strength and improving the effectiveness of actions by raising their dynamics. This type of training greatly affects the athletes’ organisms, including muscle damage, and this increases the significance of recovery [1]. The forearms, uniquely utilized in MMA fights for striking, holding, and strangulation, have a pivotal role in selected techniques of this sport discipline [2]. From a physiological point of view, the forearm flexors play a significant role, especially in the context of isometric contraction. During the fight, the athlete also performs a combination of concentric–eccentric contractions, which can cause excessive muscle stiffness and tension and lead to high intramuscular pressure [3]. Physical exertion causes faster muscle fatigue and increases the risk of injury [4].

Physical conditioning training programs for MMA athletes should aim to improve various aspects of fitness, including improving anaerobic and aerobic metabolism, specific endurance, strength, power, and rate of force development. Excessive muscle mass can negatively affect sprinting performance in trained MMA athletes [5]. Therefore, it is essential to plan appropriate training methods that improve the biomechanical properties of muscles, such as muscle tone, stiffness, elasticity, and perfusion. These methods aim to improve muscle regeneration, increase muscle strength, and reduce muscle pain, which all affect the final performance of the athletes [2].

A popular method described in the scientific literature used for sports recovery is heat-compression therapy (HCT) [6,7,8]. An example of such therapy is Game Ready, which can be used as local cryotherapy and heat or contrast therapy [9,10]. This method is widely used because it is portable and an easy-to-use alternative to other traditional methods of HCT. Game Ready heat-compression (GR-HCT) combines a heat stimulus that is applied with the use of a compression cuff put on the lower or upper limbs. We can use the pressure from 5 to 25 mmHg and the heat stimuli of 3 to 45 degrees Celsius temperature. The duration of this procedure varies from 10 to 30 min [9,11]. The identified gap in knowledge shows that the scientific literature needs more information on the minimum time required to obtain positive results and the time after which the effects of this therapy do not increase. Time, in turn, determines the optimization of both clinical and regenerative interventions.

The use of warm stimuli in medicine is common and often studied [12]. Heat transfer takes place when there is a temperature difference. Thermal energy can be transferred not only within a given matter but also from one body to another by thermal conduction, convection, and radiation [13]. The relationship between tissue physiology and tissue response to heating is shown [14]. The most crucial physiological parameter emphasized by the authors is blood flow, which is the main method of heat removal from tissues. This aspect causes the blood supply to the tissues to impact the ability to heat the tissues significantly. Various physiological changes, most of which are secondary, are induced as soon as any tissue is heated.

The beneficial effect of locally applying thermal stimuli is widely described in research, but the mechanisms causing this effect remain unclear [15]. The same applies to the methodological diversity of heat application and various proposals for the application time [16]. Particular attention is paid in the literature to the effect of heat on the muscular system [15,16,17]. Some observations suggest that cycling muscle heating may be helpful in increasing muscle mass [16,17,18]. Kobayashi et al. suggest that a single session of whole-body heat stress in rodents increases muscle calcineurin expression regardless of fiber type [19]. Among the mechanisms by which heat stress induces an increase in muscle mass, apart from calcineurin, the scientific literature describes an increase in the activity of serine-threonine phosphatase [20]. In turn, Stadnyk et al. observed no clear benefits of muscle heating in terms of hypertrophy during resistance training. At the same time, the authors also did not observe statistically significant differences in the dynamic force measured by 3-RM knee extension. Thermal stimuli also did not negatively affect the measured variables [21].

Heat stress can act as an effective post-exercise treatment and appears to be clinically beneficial for people who have difficulty participating in sufficient physical training, such as elderly or injured athletes [22,23]. Interactions between relevant heat shock proteins after heat therapy and factors related to muscle hypertrophy are proven, but the mechanisms driving them remain unclear [24]. Among the factors regulating muscle mass, attention is paid to the effect of the thermal stimulus on the inhibition of protein degradation and mitochondrial clearance [22]. Heat has been shown to have a beneficial effect on muscle atrophy [24,25].

Numerous beneficial angiogenic effects of heat and improvement of microcirculatory function are observed [14,26,27,28]. Heat application compared to continuous moderate-intensity training in sedentary young men induced a similar increase in skeletal muscle capillarization [29]. The literature reports that heat stimulation increases shear stress, which may act as a secondary signal to stimulate angiogenesis further and contribute to its increase by releasing factors into the circulation that activate endothelial cell proliferation and migration [30]. The anti-inflammatory effect of heat is also well described in research [31].

In post-exercise muscle regeneration methods, it is assumed that warm stimuli have a relaxing effect on muscle tone and improve muscle elasticity and stiffness; still, more research is needed in this area [32,33,34,35]. The evidence describing the mechanisms and the minimum duration of the therapy needed to achieve changes in the biomechanical properties of muscles is insufficient [36].

The analgesic effect of a warm stimulus is quite common; however, research results vary [32,37]. This is primarily the result of a different methodology (time of application), different forms of its exposure (conduction, convection, radiation), and variable types of pain measures (neuropathic, receptor) [38]. Most authors evaluate the analgesic effect of heat as positive, an effect also seen in DOMS (delayed onset muscle soreness) syndromes [39,40,41,42]. Researchers still did not observe significant differences in pressure pain threshold (PPT) compared to baseline measurements [43].In contrast, studies found that high-intensity thermal stimulation of human muscles causes muscle pain, possibly mediated by polymodal muscle nociceptors [44].The authors suggest that muscle thermo-sensitivity is weaker than nociception if a small volume of muscle tissue is stimulated.

Our study aimed to investigate the effect of GR-HCT duration (5 vs. 10 min) on important muscle biomechanical parameters such as muscle tension, stiffness, elasticity, pressure pain threshold, isometric strength, and tissue perfusion in MMA athletes. Our central hypothesis was that a longer GR-HCT stimulation time would show higher statistically significant changes in the measured parameters. Determining the effective duration of this therapy without reducing its efficacy may help to optimize recovery processes in sports in the future. The results of this pilot study focused on generating practical suggestions regarding the timing of GR-HCT.

2. Materials and Methods

2.1. Participants

Twenty healthy male MMA fighters (age: 26.5 ± 4.5 years; BMI: 24.75 ± 3.0; professional training experience: 10.3 ± 5.0 years) were randomly selected. The inclusion criteria were age 18–40, minimum training experience in MMA—3 years, minimal training frequency—4 times a week. All included fighters were registered on the platform tapology.com and had participated in at least 3 semi-pro or pro fights. In all cases, the dominant hand was the right hand—all participants declared right hand as their dominant hand (writing) and claimed using a right-hand-dominant stance during fights. Exclusion from the study concerned elevated blood pressure before the study (blood pressure > 140/90 mm Hg), people treated after injuries, any injuries in the area of measurements of the study including skin and myofascial and muscle injuries. Participants could not have any tattoos at the measurement site because of tissue perfusion measurements recommended by the methodology. Exclusion of participants covered also the case of excessive fatigue of the subject, fever, or infection [9,11]. Participation in training 24 h before and 24 h during the study was excluded. During the research, the participants did not undergo any weight loss and were outside the preparation period. Additionally, due to tissue perfusion measurements, the consumption of ergogenic drinks was prohibited (the list of forbidden drinks was delivered to the participants and contained information that every drink containing caffeine or taurine is prohibited and giving example names of popular drinks) for 12 h before the study. Every participant was informed that they could quit the study at any time on their own request. Informed consent was signed by every participant before taking part in the study and being informed about the aims and possible risks. The National Council of Physiotherapists ethics committee approved the study (written consent number 9/22 of 6 April 2022) and the study was conducted according to the Declaration of Helsinki.

2.2. Study Design

Seven days before the pilot study, every subject underwent familiarization with the intervention by receiving 3 min heat-compression GR-HCT stimulation and the Fmax examination. Therapy duration times of 5 and 10 min were investigated. In every session, the temperature was set at 45 °C, pressure was changing from 5 to 25 mmHg with 1-min intermittent cyclical compression, and the inflatable cuff was applied on the forearm of the dominant hand (all right side) (https://gameready.com/, accessed on 15 May 2023) (Figure 1). The measurements and data collection were performed in the same way at rest and after the intervention as previously described in our study about cryo-compression with the same Game Ready equipment published in the Journal of Clinical Medicine (2024) [45] (Figure 2).

The same group underwent 2 experimental sessions of different GR-HCT time (5 min—GR-HCT-5, n = 20; and 10 min—GR-HCT-10, n = 20) with a one-week break (Figure 2). We used ultrasound control to determine in the dominant upper limb the widest part of the flexor carpi radialis (MFCR) and brachioradialis (MBR) muscles and marked this place with a marker [2]. The performed measurements were perfusion unit—microcirculation response described in non-reference units (PU), tissue temperature [°C], muscle tone—T [Hz], stiffness—S [N/m], elasticity—E [arb], pressure pain threshold—PPT [N/cm], and maximum isometric force—Fmax [kgf]. The research was conducted in the same time conditions (from 10 a.m. to 12 p.m.); subjects were sitting on a medical chair in a standard resting position with elbows bent at 60 degrees [2]. The project about recovery strategies in MMA fighters was held in the Medical Center Provita Żory with methodology in accordance with our previous publication about cryo-compression [45]. The measurements were taken 2 min before GR-HCT stimulation (r-rest), and 1 min (start of first measurement—PU and temperature)—5 min (end of last measurement—Fmax) after GR-HCT (p-post). Professional physiotherapists who were trained in using the equipment conducted all of the measurements in the following order: (1) PU, (2) °C, (3) T, (4) S, (5) E, (6) PPT, and (7) Fmax.

2.3. Measurements—Tissue Perfusion (PU) and Tissue Temperature (°C)

For registering the PU, the method of Laser Doppler Flowmetry (LDF) was utilized with the help of Perimed LDF apparatus (Perimed, Järfälla, Sweden). Simultaneously, tissue temperature was measured. The standardized LDF test proposed by Liana et al. was used to assess skin microcirculation augmented in response to heat therapy [46]. A special contact head-emitting laser light was placed on the skin to measure the temperature and speed of movement of erythrocytes on the skin surface in the firstly determined place [47]. The measurement depth was 2.5 mm, the volume was 1 mm³, and the procedure lasted 2 min. The LDF method, due to its repeatability, high sensitivity, and non-invasiveness, provides a precise assessment of the microcirculation at rest and in response to a physical stimulus.

2.4. Measurements—Myotonometry

The second measurement during rest and post stimulation conditions was myotonometry, performed with help of the MyotonPro myotonometer (Myoton Ltd., Tallinn, Estonia 2021) (Figure 3). The MyotonPro is a digital palpation device consisting of a device body and an indentation probe (Ø 3 mm). Through the probe, a pre-pressure (0.18 N) is applied to the surface that causes the material underneath to be compressed. A mechanical impulse (0.4 N, 15 ms) is used to deform the skin and underlying muscles for a short interval to detect differences in physical properties compared to stretched muscle fibers. Myotonometry is widely considered as a reliable method [48,49,50,51]. The device receives damped natural vibrations of soft biological tissue in the form of an acceleration signal and then simultaneously calculates the parameters of the stress state and biomechanical properties. The described method is used to evaluate the state of resting muscle tone (T) and dynamic stiffness (S). The stiffness assessed by myotonometry is based on the theory of free oscillations and results from the natural oscillation of tissues in response to short-term mechanical exposure on the skin [48,49,50,51]. Tissues can also regain their original shape after deformation. The feature measured in this study is referred to as elasticity (E). The greater the elasticity, the faster the tissue returns to its original shape [52].

2.5. Measurements—Pressure Pain Threshold (PPT)

As a third measurement both during the rest state and post stimulation, the PPT test was performed with the help of a FDIX algesimeter (Wagner Instruments, Greenwich, CT, USA 2013). The PPT method is widely used to objectively determine the pain threshold and its changes as an effect of different interventions. The reliability and repeatability of this method have been confirmed in the scientific literature [2,53,54]. We performed a three-time compression test with a testing probe of 4 mm diameter. Pressure is increased until the subject signals it as unpleasant. The equipment displays the force value expressed in [N/cm] and calculates the average of 3 measurements. In the case of too large a deviation of the measured value, the device signaled the need to repeat the test.

2.6. Measurements—Maximum Strength of the Forearm Muscles (Fmax)

A warm-up consisting of one set of ten repetitions of the maximum voluntary pressure of a small rubber ball lasting 1 s each, followed by a 10 s stretching of the forearm muscles was performed before the test. To measure the maximal force, an electronic hand dynamometer (Kern MAP 130K1, Balingen, Germany) was utilized. The test involved three tries of flexing fingers of the dominant hand with 60 s pause in between; the participants’ elbow was at 60-degree flexion while sitting in a standard position. The average value of the generated force was calculated [kgf] [2].

2.7. Statistical Methods

Statistica 13.1 was used for the statistical analysis. All results are presented as mean ± standard deviation. In order to verify the normality, homogeneity, and sphericity of the sample data variances, the Shapiro–Wilk, Levene, and Mauchly’s tests were used, respectively. A repeated two-way ANOVA (2 conditions × 2 times) was used to examine the differences between the GR-HCT5min and GR-HCT10min conditions. Furthermore, t-test comparisons for delta values (pre-post) were made between GR_HCT5min and GR_HCT10min conditions. Effect sizes for main effects and interactions were determined with partial eta squared (ηp2). Partial eta squared values were classified as small (0.01 to 0.059), moderate (0.06 to 0.137), and large (>0.137). Post hoc comparisons using Tukey’s test were conducted to show the differences between mean values when a main effect or an interaction was found. For pairwise comparisons, the effect size was determined by parametric effect sizes (Cohen’s d), which were defined as large (d > 0.8); moderate (d between 0.8 and 0.5); small (d between 0.49 and 0.20); and trivial (d < 0.2). Friedman’s ANOVA was applied and pairwise comparisons for variables lacking normal distributions. The effect size for the Friedman ANOVA was calculated as Kendal’s W. Kendal’s W values were classified as W < 0.1 very small; W 0.1–0.3 small; 0.3–0.5 medium; W > 0.5 large. For pairwise comparisons for Friedman’s ANOVA, the effect size was determined by nonparametric effect sizes (r_g) which were defined as large (r_g > 0.5); moderate (r_g between 0.5 and 0.3); small (r_g between 0.3 and 0.1); and trivial (r_g < 0.1). Percent changes with 95% confidence intervals (95CI) were calculated. Statistical significance was set at p < 0.05. The repeated measure ANOVA between interactions with an effect size of at least 0.25, α = 0.05, and 1-β = 0.95 gave a statistical power of 97.37% and the sample size of a minimum of 15 subjects (calculation performed with G*Power software (version 3.1.9.7) according to Faul et al. [55].

3. Results

The Shapiro–Wilk tests for the MFCR muscle indicated that the normality of the data had been violated for T, S, E, and °C. Therefore, Friedman’s test was used to indicate the statistical differences of these parameters. The Friedman’s test did not show significant differences between 5 min and 10 min for T (Chi-square ANOVA=5.16; p = 0.16; Kendall’s W = 0.085); Stiffness [N/m] (Chi-square ANOVA = 6.18; p = 0.10; Kendall’s W = 0.10); or E [arb] (Chi-square ANOVA = 4.63; p = 0.20; Kendall’s W = 0.077). The Friedman’s test showed significant differences between 5 min and 10 min for °C (Chi-square ANOVA = 57.28; p < 0.0001; Kendall’s W = 0.95). The pairwise comparisons for °C showed significant differences between rest and post values for 5 min (p < 0.0001) as well as between rest and post values for 10 min (p < 0.001); r_g = 0.88 for both.

The Shapiro–Wilk tests for MBR muscle indicated that the normality of the data has been violated for PU, S, E, and °C. Therefore, Friedman’s test was used to indicate the statistical differences of these parameters. The Friedman’s test showed significant differences between 5 min and 10 min for PU (Chi-square ANOVA = 54.1; p < 0.0001; Kendall’s W = 0.90); S (Chi-square ANOVA = 11.83; p = 0.008; Kendall’s W = 0.20); and °C (Chi-square ANOVA = 57.30; p < 0.0001; Kendall’s W = 0.95). The pairwise comparisons for °C showed significant differences between rest vs. post values for 5 min (p < 0.0001) as well as between rest vs. post values for 10 min (p < 0.001); r_g = 0.88 for both. The pairwise comparisons for the PU showed significant differences between rest and post values for 5 min (p < 0.0001) as well as between rest and post values for 10 min (p < 0.001); r_g = 0.88 for both. The pairwise comparisons for S showed significant differences between rest vs. post values for 10 min (p = 0.049), r_g = 0.43, but did not show significant differences between rest and post values for 5 min (p = 0.57). The Friedman’s test did not show significant differences between 5 min and 10 min for E (Chi-square ANOVA = 2.06; p = 0.55; Kendall’s W = 0.034).

The two-way repeated measures ANOVA for muscle MFCR indicated a significant main effect of condition for PU (5 min PU vs. 10 min PU; p < 0.0001; ηp2 = 0.80) as well as for time of measurement (rest vs. post; p < 0.001; η2 = 0.91). The two-way repeated measures ANOVA indicated a significant interaction effect (5 min-10 min × Rest-Post for PU p < 0.0001; ηp2 = 0.80). The post hoc Tukey comparison for the interaction effect (5 min-10 min × Rest-Post) showed a significantly higher PU for post compared to premeasurement in 5 min PU (d = 1.77) and 10 min PU (d = 6.37) (p < 0.001 for both).

The two-way repeated measures ANOVA for muscles MFCR and MBR did not indicate a significant interaction effect for Fmax (p = 0.41; η2 = 0.018). However, there was a significant main effect of the time of the measurement (rest vs. post; p < 0.001; η2 = 0.82). The post hoc Tukey comparison for the interaction effect showed a significantly higher Fmax for post- compared to premeasurement in 5 min (d = 0.84) and 10 min (d = 1.27) (p < 0.001 for both). The two-way repeated measures ANOVA did not indicate a significant effect for this condition (5 min-F max vs. 10 min-F max; p = 0.14; η2 = 0.055).

The two-way repeated measures ANOVA for muscle MFCR did not indicate a significant interaction effect for PPT [N/cm] (p = 0.35; ηp2 = 0.022). However, there was a significant main effect of time of measurement (rest vs. post; p < 0.001; η2 = 0.59). The post- hoc Tukey comparison for the interaction effect showed significantly lower PPT for post compared to premeasurement in 5 min (d = 0.88) and 10 min (d = 1.02) (p < 0.001 for both). The two-way repeated measures ANOVA did not indicate a significant effect for this condition (5 min PPT vs.10 min PPT; p = 0.14; η2 = 0.055).

The two-way repeated measures ANOVA for muscle MBR did not indicate a significant interaction effect for T [Hz] (p = 0.15; η2 = 0.053). However, there was a significant main effect of the time of the measurement (rest vs. post; p < 0.001; η2 = 0.30). The post hoc Tukey comparison for the interaction effect showed significantly a lower T for post- compared to premeasurement in 10 min (d = 0.69) (p = 0.002). The post hoc Tukey comparison for the interaction effect did not indicate significant differences in T for post compared to premeasurement in 5 min (p = 0.28). The two-way repeated measures ANOVA did not indicate a significant effect for this condition (5 min T [Hz] vs. 10 min T [Hz]; p = 0.14; η2 = 0.055).

The two-way repeated measures ANOVA for muscle MBR indicated a significant main effect PPT [N/cm] for the time of measurement (rest vs. post; p < 0.001; η2 = 0.51); but did not indicate a significant effect for this condition (5 min PPT [N/cm] vs. 10 min PPT [N/cm]; p = 0.15; ηp2 = 0.052). The two-way repeated measures ANOVA indicated a significant interaction effect (5 min-10 min × Rest-Post for PPT p = 0.006; η2 = 0.18). The post hoc Tukey comparison for the interaction effect (5 min-10 min × Rest-Post) showed a significantly lower PPT for post- compared to premeasurement only in 10 min PPT (d = 1.23) (p < 0.001); 5 min PPT did not show significant differences (p = 0.09) (

Table 1. Comparisons between the experimental conditions for all measured variables.

Conditions	Muscles
	MFCR		MBR
	Rest	Post	Rest	Post
Perfusion Unit
GR-HCT5min	7.04 ± 2.33 (5.94 to 8.13)	10.81 ± 1.92 (9.91 to 11.72)	7.04 ± 2.23 (5.99 to 8.08)	11.22 ± 2.49 (10.05 to 12.38)
GR-HCT10min	7.88 ± 1.34 (7.24 to 8.50)	23.55 ± 3.21 (22.04 to 25.05)	6.75 ± 2.00 (5.81 to 7.67)	24.94 ± 4.28 (22.93 to 26.93)
Muscle Tone [Hz]
GR-HCT5min	17.89 ± 2.33 (16.91 to 18.86)	17.53 ± 1.23 (16.96 to 18.10)	15.79 ± 1.28 (15.18 to 16.38)	15.37 ± 1.10 (14.85 to 15.88)
GR-HCT10min	17.81 ± 2.35 (16.71 to 18.90)	16.95 ± 1.46 (16.26 to 17.62)	15.72 ± 1.31 (15.10 to 16.33)	14.82 ± 1.31 (14.20 to 15.43)
Stiffness [N/m]
GR-HCT5min	304.0 ± 60.75 (275.6 to 332.4)	290.6 ± 47.17 (268.5 to 312.7)	247.5 ± 42.93 (227.4 to 267.6)	244.6 ± 39.89 (225.9 to 263.3)
GR-HCT10min	299.05 ± 63.34 (269.4 to 328.7)	267.35 ± 48.83 (244.5 to 290.2)	245.35 ± 38.22 (227.5 to 263.2)	230.35 ± 34.19 (214.35 to 246.4)
Elasticity [arb]
GR-HCT5min	0.94 ± 0.13 (0.88 to 0.99)	1.01 ± 0.13 (0.95 to 1.07)	0.88 ± 0.09 (0.84 to 0.92)	0.88 ± 0.12 (0.82 to 0.94)
GR-HCT10min	1.00 ± 0.25 (0.88 to 1.12)	0.96 ± 0.14 (0.89 to 1.02)	0.94 ± 0.25 (0.82 to 1.05)	0.86 ± 0.08 (0.83 to 0.90)
Pressure pain threshold [N/cm]
GR-HCT5min	83.15 ± 11.89 (77.58 to 88.72)	72.43 ± 12.55 (66.56 to 78.30)	85.17 ± 9.46 (80.74 to 89.60)	78.21 ± 11.65 (72.75to 83.66)
GR-HCT10min	82.11 ± 12.89 (76.07 to 88.14)	68.26 ± 14.17 (61.62 to 74.89)	85.82 ± 16.08 (78.29 to 93.35)	67.07 ± 14.39 (60.33 to 73.81)
Tissue temperature (°C)
GR-HCT5min	36.51 ± 0.085 (36.47 to 36.55)	37.37 ± 0.35 (37.20 to 37.53)	36.51 ± 0.085 (36.47 to 36.55)	37.37 ± 0.35 (37.20 to 37.53)
GR-HCT10min	36.49 ± 0.081 (36.45 to 36.52)	38.09 ± 0.12 (38.03 to 38.14)	36.49 ± 0.081 (36.45 to 36.52)	38.09 ± 0.12 (38.02 to 38.14)
Maximum forearm muscle strength [kgf]
	Rest		Post
GR-HCT5min	50.43 ± 8.63 (46.39 to 54.46)		58.05 ± 9.56 (53.57 to 62.52)
GR-HCT10min	47.45 ± 5.39 (44.92 to 49.97)		54.17 ± 5.22 (51.72 to 56.61)

All data are presented as mean ± SD, 95%CI.

).

The Shapiro–Wilk tests show that the normality of the data has been violated for results Δeffect post-rest for PU, T, S, E, PPT, Fmax, and °C. Friedman’s test was used to indicate statistical differences for these parameters.

The Friedman’s test did not show significant differences between 5 min and 10 min for ΔFmax (Chi-square ANOVA= 0.20; p = 0.65; Kendall’s W = 0.01); for ΔT (Chi-square ANOVA = 1.68; p = 0.64; Kendall’s W = 0.03); for ΔS [N/m] (Chi-square ANOVA = 4.25; p = 0.26; Kendall’s W = 0.07); for ΔE [arb] (Chi-square ANOVA = 4.52; p = 0.21; Kendall’s W = 0.08); and for ΔPPT (Chi-square ANOVA = 6.38; p = 0.094 Kendall’s W = 0.11).

The Friedman’s test showed significant differences between 5 min and 10 min for Δ PU (Chi-square ANOVA = 49.2; p < 0.0001; Kendall’s W = 0.82). The pairwise comparisons for the ΔPU showed significant differences between 5 min and 10 min values for MFCR (p < 0.0001) as well as between 5 min and 10 min values for MBR (p < 0.001); r_g = 0.88 for both.

The Friedman’s test showed significant differences between 5 min and 10 min for Δ °C (Chi-square ANOVA = 60.0; p < 0.0001; Kendall’s W = 0.99). The pairwise comparisons for Δ °C showed significant differences between 5 min and 10 min values for MFCR (p < 0.0001) as well as between 5 min and. 10 min values for MBR (p < 0.001); r_g = 0.88 for both (

Table 2. Comparisons between the Δeffect_post-rest for GR-HCT 5min vs. 10min for MFCR and MBR.

Conditions	MFCR	MBR
ΔPerfusion Unit (post-rest)
GR-HCT5min	3.78 ± 1.45 (3.10 to 4.46)	4.18 ± 1.47 (3.49 to 4.87)
GR-HCT10min	15.67 ± 4.04 13.78 to 17.56)	18.19 ± 4.57 (16.05 to 20.32)
ΔMuscle Tone(post-rest) [Hz]
GR-HCT5min	−0.36 ± 1.88 (−1.24 to 0.53)	−0.42 ± 0.95 (−0.86 to 0.02)
GR-HCT10min	−0.86±2.00 (−1.79 to 0.07)	−0.90 ± 1.13 (−1.43 to −0.37)
ΔStiffness(post-rest) [N/m]
GR-HCT5min	−13.40 ± 66.71 (−44.62 to 17.82)	−2.90 ± 24.17 (−14.21 to 8.41)
GR-HCT10min	−31.70 ± 59.84 (−59.71 to −3.69)	−15.00 ± 32.81 (−30.35 to 0.35)
ΔElasticity(post-rest) [arb]
GR-HCT5min	0.07 ± 0.12 (0.01 to 0.12)	−0.003 ± 0.08 (−0.04 to 0.04)
GR-HCT10min	−0.04 ± 0.27 (−0.17 to 0.09)	−0.07 ± 0.26 (−0.19 to 0.05)
ΔPressure pain threshold(post-rest) [N/cm]
GR-HCT5min	−10.72 ± 7.47 (−14.21 to −7.23)	−6.97 ± 5.24 (−9.42 to −4.51)
GR-HCT10min	−13.85 ± 12.91 (−19.89 to −7.81)	−18.75 ± 17.61 (−26.99 to −10.51)
Δ Tissue temerature (post-rest) [°C]
GR-HCT5min	0.85 ± 0.36 (0.69 to 1.02)	0.85 ± 0.36 (0.69 to 1.02)
GR-HCT10min	1.60 ± 0.14 (1.53 to 1.67)	1.61 ± 0.15 (1.54 to 1.67)
ΔMaximum forearm muscle strength (post-rest) [kgf]
GR-HCT5min	7.62 ± 4.37 (5.58 to 9.66)
GR-HCT10min	6.72 ± 2.15 (5.71 to 7.72)

All data are presented as mean ± SD, 95%CI.

).

Differences (Δ) between post-rest results which are a measure of effects of the therapy are greater for GR-HCT5min when comparing to GR-HCT10min for T, S, E, and PPT for both examined muscles. The only parameters showing a greater post-rest Δ for GR-HCT10min when comparing to GR-HCT5min were PU (p < 0.0001 for MFCR and p < 0.001 for MBR) and tissue temperature (p < 0.0001 for MFCR and p < 0.001 for MBR) (Figure 4).

4. Discussion

The assessment of the impact of therapy duration on the immediate effects of short-term GR-HCT application on specific biomechanical properties of muscles, such as tone, stiffness, and elasticity, as well as the effect on improving muscle perfusion and tissue temperature, reducing pain, and increase in effectiveness of recovery of isometric strength of the forearm were the main objectives of our research. The obtained results show that the duration of 5 min of GR-HCT versus 10 min is sufficient to evoke significant changes and that increasing the time of GR-HCT from 5 to 10 min does not bring better effects in the aspect of recovery effectiveness.

In our studies, we demonstrated the importance of the influence of the time of GR-HCT application on MFCR and MBR perfusion. Some authors suggested that skin perfusion measured by LDF may have a parallel in muscle hyperemia responses [56,57,58]. Research on the effects of heat on the vascular system published by Brunt and colleagues in The Journal of Physiology indicated the ability of heat therapy to exert improvement in many subclinical markers of cardiovascular disease [13]. One of the analyzed functional variables is tissue perfusion, which is also included in our research. Functional changes (flow-dependent dilation; FMD) were determined by improved endothelial function [59,60]. A variety of interventions, including heat stress, can prompt a rapid improvement in these responses. The hemodynamic changes that occur after these interventions are described as mechanistic adaptations of the vascular system [14,60,61].

Akasaki et al. showed, in an animal experiment, that 15 min of daily heat stimulation in a dry far infrared sauna at 41 °C improves limb perfusion [26]. The authors showed that repeated thermal therapy increased eNOS protein expression, blood flow, and capillary density in an ischemic hind limb in mice. The endothelium plays a vital role in the mechanism of local microcirculation autoregulation as a source of numerous mediators, of which NO and prostacyclin are the strongest vasodilators, and EDCF2 and endothelin (EDCF1) are the most potent vasoconstrictors [62]. Roberts et al. showed that the time of tissue heating is essential for the hyperemia reaction [63]. In our study, the GR-HCT application provided a rapid temperature rise. Differences in plateau protocols and skin heating rates trigger different contributions of vasodilatory pathways to the local heat response [63]. Rapid local heating (0.5 °C for 5 s) induces a transient axonal reflex (5–10 min) triggered by the activation of heat-sensitive sensory and adrenergic nerves. This is followed by a vasodilatory response (20–30 min) mediated by NO. Some authors suggest that maintaining the plateau phase at 39 °C leads to a higher proportion of NO in the plateau phase [64].

One of the important results in our study shows that despite the increase in temperature and PU during 10 min of GR-HCT, it did not show statistical significance in reducing the value of individual biomechanical properties of muscles (T, S, E). The immediate effect of hyperemia in 5 min GR-HCT was sufficient to obtain similar results for 10 min GR-HCT of MFCR and MBR. The explanation for this phenomenon underlies recent evidence suggesting that up to about 80% of the capillaries are perfused in a resting state for many animal preparations. Thus, any stimulation, including heat, is unable to recruit more than once, and capillary recruitment alone is not a major contributor to congestion and gas exchange in contracting skeletal muscles [65,66].

Muscle tone, elasticity, and muscle stiffness are mainly maintained by the complex interplay of spinal and supraspinal mechanisms, the disruption of which can cause many changes in the biomechanical properties of muscles [44,67]. It is widely recognized that a thermal stimulus causing an increase in muscle congestion reduces muscle tone and improves muscle stiffness and flexibility [15]. Although these mechanisms are unclear, a non-myogenic regulation of muscle tone related to perfusion is also accepted. The increased concentration of Ca2+ in the cytosol, which occurs in the preconditions of hypoxia resulting from impaired perfusion, triggers muscle contraction by activating myosin light chain phosphorylation, followed by actomyosin cross-bridging, which generates an increase in tension. Activation of the capillary system, eliminating subclinical symptoms of tissue hypoxia, may reduce muscle tone [15,68].

Overall, there are few studies evaluating the effect of HCT on muscle properties. The scientific literature focuses mainly on muscle stiffness and tension, while flexibility and its correlation with PU are mostly not considered. Our results regarding muscle tone and stiffness contribute to the existing evidence and are broadly in line with the current literature [14,16]. Improving sports performance and preventing injuries are the primary goals in the sports recovery area. There are many DOMS mitigation strategies in the literature, but no precise protocols are available [69,70]. Muscle fatigue is one of the main factors disrupting the biomechanical properties of muscles, which can reduce athletic performance [71]. The scientific literature confirms the effectiveness of the warm stimulus, but there is insufficient evidence regarding the impact of short-term stimulation.

Although our PPT results did not show an effect on reducing muscle pain, they do not contradict the general hypothesis of the analgesic effect of a warm stimulus [40,41]. Our results are consistent with Graven-Nielsen et al. and Vargas e Silva et al. [43,44]. We observed a slight decrease in PPT, which did not change significantly between 5 and 10 min of GR-HCT. The differences should be seen in the different types of pain measured in the scientific literature. Nociceptive pain measured during PPT is different from neuropathic pain. Hyperalgesia to these stimuli can be divided into two different phenomena: trigger points and tender points. The pressure applied to the skin can activate the nociceptive afforestation in multiple tissue layers depending on the shape of the transducer [72]. If pressure is exerted on a large area of skin (e.g., 2 cm²) and the head can deform more intensively healthy muscle tissue that has been relaxed after HCT, this can lead to irritation of more neuromuscular spindles, i.e., mechanoreceptors (vater—Pacini corpuscle). This, in turn, triggers the PPT. Consistent with local anesthetic experiments, the contribution of other nociceptors, in contrast to neuropathic pain, is small in blunt pressure pain [73]. Regardless of our results, most authors reported a 95% agreement on the analgesic effects of this stimulus in the 2021 Delphi International Multidisciplinary Consensus on Heat Therapy. It declares that local compression used in superficial heat therapy increases its effectiveness and can activate proprioceptors and block the processing of nociceptive signals [41]. It has been proposed that 40 °C is optimal while suggesting that the higher the temperature of surface contact thermotherapy, the better the effects achieved [41]. A temperature of 45 °C was used in our tests. This was a completely safe temperature and well tolerated by the study participants. The adult human skeletal muscle capillary bed contains approximately 9+ billion capillaries in total, with a total length of more than 8 km and surface area [66]. For nearly a hundred years, when the Nobel laureate August Steenberg Krogh (1874–1949) made observations of muscle microcirculation, followed by Edward F. Adolph (1895–1986) pioneering the physiological regulation of microcirculation, the undeniable role of microcirculation function has been constantly growing, not only in physical exercise and sports recovery but in many diseases of the cardiovascular system [56,66]. Current results of numerous studies, such as near-infrared spectroscopy (NIRS) or time-resolved spectroscopy (TRS) for deep muscle sampling, enable extremely precise measurements [74]. It has been shown that a slight increase in tissue temperature (1–2 ℃) does not cause excessive heat stress, which is considered an inducer of oxidative stress in skeletal muscles. A slight increase in temperature triggers adaptive mechanisms that protect against protein degradation through increased proteolysis and autophagy. As a consequence, there may be a better metabolism and optocoupling of proteins in skeletal muscles. Additionally, it has been described that a slight increase in temperature may affect muscle plasticity. Contrast therapy slightly affected tissue temperature, which may suggest a positive, stimulating effect on skeletal muscles [75].

In our studies, the effect of GR-HCT improved the maximum isometric force score. Stimulation time was not statistically significant for 5 and 10 min. It indicates that 5 min is the optimal time for improving isometric forearm strength. Numerous studies focus on the improvement of muscle strength, power, and hypertrophy after the application of a thermal stimulus on the activity of heat shock proteins (HSPs) [76,77]. HSPs are considered molecular chaperones that play a universal role in maintaining cellular homeostasis. It has been confirmed that HSPs are expressed in skeletal muscles, the induction of which varies depending on the histological and even functional characteristics of the muscles [76]. Heat increases the expression of genes involved in the growth and differentiation of muscle cells [78].

5. Limitations of the Study

The lack of a control group was the main limitation of these experimental studies. Our aim was only a preliminary assessment of different types of GR-HCT time. This was a pilot experiment, which is the first step in a series of research projects. In subsequent stages, studies with a control group and studies after exercise loading are planned to be performed. The drawback is that the results report an immediate effect. Thus, the measured tissue response may reflect a thixotropic effect, although the magnitude of this effect remains unclear [79]. The Laser Doppler Flowmetry method is very sensitive and requires observance of strict procedures. In our study, an assessment of changes in response to the applied stimulus was used to avoid the consequences of high sensitivity of the LDF procedures. Twenty MMA fighters participated in the study—undoubtedly, future studies should use larger groups and longer observation. The participants were randomly selected from a bigger group of fighters using the randomizer.org website tool; however, future studies should use randomized controlled trial protocol and examine people of different age groups, gender, and level of physical preparation.

6. Conclusions

Our research provides evidence that GR-HCT affects muscle biomechanical parameters, pain threshold, strength, and tissue perfusion, which can be used to assess regeneration processes in sports in the future. A 5 min effect must be sufficient treatment time to provide immediate changes in stiffness, tension, muscle elasticity, and hyperemia. This may provide a basis for using shorter stimulation times with the Game Ready device and indicate directions for future researchers analyzing regeneration optimization processes in sports

7. Practical Application

Using 5 min GR-HCT is the minimal recommended therapy time for the forearm muscles in conditions where there is not enough time for more complex regeneration. When there is need for using GR therapy in other body areas, the time-saving protocols should be utilized; however, practitioners should be aware that larger muscle groups may require a longer time of therapy to be effective.

8. Future Research Directions

Future research directions should include both female and male participants of different age groups and different levels of sports mastery while assessing the impact of HCT to make more general conclusions. Future research should analyze the effects of HCT on muscle parameters after standardized fatigue protocol or after fatigue coming from real fight conditions.