Using multi-channel near-infrared spectroscopy to assess the effect of cupping therapy on the spatial hemodynamic response of the biceps muscle: A preliminary study

Abstract

BACKGROUND:

The local hemodynamic response after cupping therapy has been considered as a contributing factor for improving muscle tissue health; however, the effects of cupping pressure and duration on the spatial hemodynamic response have not been investigated.

OBJECTIVE:

The objective of this study was to investigate the hemodynamic response inside and outside the cupping cup under various pressures and durations of cupping therapy.

METHODS:

A 3-way factorial design with repeated measures was used to investigate the main and interaction effects of the location (areas inside and outside the cup), pressure ( $-$ 225 and $-$ 300 mmHg) and duration (5 and 10 min) on the hemodynamic response of the biceps muscle. A functional near-infrared spectroscopy was used to assess hemodynamic changes in 18 participants.

RESULTS:

A significant three-way interaction of the location, pressure, and duration factors was observed in oxyhemoglobin ( $p=$ 0.023), deoxy-hemoglobin ( $p=$ 0.013), and blood volume ( $p=$ 0.013). A significant increase was observed in oxyhemoglobin, blood volume, and oxygenation compared to pre-cupping ( $p<$ 0.05) in the area outside the cup.

CONCLUSION:

Our findings indicate that an appropriate combination of cupping pressure and duration can effectively affect the spatial hemodynamic response of the biceps.

Keywords

Blood volume dose response hemoglobins negative pressure oxygen oxyhemoglobins skeletal muscle

1. Introduction

Cupping therapy is performed by applying cups to the selected skin area for creating negative pressure by flame or suction [1], and is usually used for relieving musculoskeletal pain [2, 3]. Although cupping therapy is a popular intervention, evidence of cupping therapy for managing musculoskeletal impairment remains insufficient and conflicting results of clinical trials on cupping therapy have been reported in the literature [2, 4, 5, 6]. A recent survey of 158 healthcare professionals in the United States on their practice of cupping therapy indicates that clinical guidelines for selecting appropriate cupping pressure and duration are not sufficient to guide their clinical practice [7]. There is a need to establish the dose-response relationship of cupping therapy for improving the clinical effectiveness of cupping therapy on managing musculoskeletal impairment [3, 8].

In order to better understand the effects of cupping therapy on managing musculoskeletal impairment, local hemodynamic responses have been proposed as a potential mediating factor for promoting musculoskeletal healing after cupping therapy [9, 10]. Skin hyperemia after cupping therapy has been reported in the literature [10, 11, 12]. Kim et al. demonstrated that cupping therapy resulted in an increase in muscle blood volume after cupping therapy [13]. Such an increase in skin and muscle blood flow may accelerate the removal of metabolic wastes and the supply of oxygen to the treated area for improving muscle tissue health [9, 10]. This hyperemic response is considered a protective increase in blood flow following cupping therapy [14, 15].

Understanding the effects of cupping pressure and duration on the hemodynamic regulation may help establish the dose-response relationship of cupping therapy for improving the clinical effectiveness [9, 10]. However, there is only limited evidence on the effects of pressure and duration of cupping therapy on the hemodynamic response of the skin and muscle [10, 11, 12]. The regulation of muscle hemodynamic response from other interventions (e.g. foam rolling massage [16] and muscle stretching [17] could be referenced to predict the hemodynamic response after cupping therapy. Soares and colleagues provided the first evidence that foam rolling massage improves skeletal muscle oxygenation [16]. They demonstrated that forearm muscle oxygenation increased after 30 sec of foam rolling massage while there was no change in muscle oxygenation after 2 min of rolling massage. In another example, the mechanical force acting on the muscle induced by muscle stretching has been demonstrated to be a force-dependent response to congestion of venules or arterioles in rats [17, 18]. Although muscle stretching is different from cupping therapy, it is logical to assume that the muscle hemodynamic response after cupping therapy is also a force-dependent response because of the muscle blood flow regulation in response to mechanical stresses [19, 20, 21]. In order to better understand the effects of pressure and duration of cupping therapy on the hemodynamic response of the muscle, more research studies are needed.

Near-infrared spectroscopy (NIRS) has been used to quantify the changes of the concentration of oxyhemoglobin, deoxy-hemoglobin, blood volume, and oxygenation in the tissue (e.g. muscle and brain) [14, 22, 23, 24]. NIRS is also been used to investigate muscle oxygen metabolism [16, 25], a factor identified as a critical determinant of the hyperemic response after stretching and exercise [14, 22, 23]. NIRS may be a useful tool to assess how various intensities of cupping therapy affect the hemodynamic response of the muscle. The use of NIRS for measuring muscle hemodynamic responses after cupping therapy is usually operated at a single site [13, 26]. However, spatial heterogeneity of the microcirculatory system is found in the muscle [27, 28, 29]. Wolf et al. used NIRS to measure 22 locations of the calf muscle hemodynamics and demonstrated a significant spatial heterogeneity of muscle hemodynamics [27]. Mizuno and colleagues also demonstrated that muscle hemodynamic response is highly heterogeneous based on its location, for example, proximal and distal sites [28]. Thus, a single-channel NIRS may not be sufficient to catch the whole hemodynamic response of the muscle inside and outside the cup. To date, there is no study using multi-channel NIRS to investigate the spatial hemodynamic response (i.e. both areas inside and outside the cupping rim) of the muscle after cupping therapy.

Under different cupping pressure and duration, the muscle hemodynamic responses may be further influenced by the spatial heterogeneity issues. Specifically, it is largely unknown whether the muscle outside the cupping cup could show a significant increase in blood flow after cupping therapy. Given the importance of oxidative metabolism to the functional capacity of muscle, a large spatial difference in oxygenation and blood volume between the area inside and outside the cup rim may confound the efficacy of cupping therapy Despite there is no standard guideline on the location for applying cupping therapy specific acupoints and trigger points are usually used in cupping therapy practice [8, 30]. However, it is unclear whether an offset of the cup to the predetermined acupoint would still benefit the treated muscle or not. Thus, it is needed to study the muscle hemodynamic response at the area inside the outside the cup, especially under various pressures and durations of cupping therapy.

Therefore, this study aimed to investigate if there is a spatial difference of the hemodynamic response of the biceps muscle between the area inside and immediately outside the cup under various pressures and durations of cupping therapy. In order to investigate the spatial effect of muscle hemodynamic response to various treatment characteristics of cupping therapy, a three-way factorial design was conducted to assess the effect of the location, pressure and duration of cupping therapy on muscle hemodynamic responses using multi-channel NIRS. We hypothesized that: 1) there is an interaction between the location, pressure and duration of cupping therapy on muscle hemodynamic responses including oxyhemoglobin, deoxy-hemoglobin, blood volume and oxygenation (hypothesis #1); and 2) cupping therapy could significantly improve muscle blood volume in the area outside the cup (hypothesis #2). The long-term goal of our research is to establish the dose-response relationship of cupping pressure, duration and location (areas inside and outside cup) on the muscle hemodynamic response for improving clinical effectiveness of cupping therapy.

2. Methods

A 2 $\times$ 2 $\times$ 2 factorial design with repeated measures was used to investigate the main and interaction effects of location, pressure and duration factors of cupping therapy on the hemodynamic responses of the biceps. The selection of the biceps muscle is because the biceps curls is one of established exercise protocols to assess muscle fatigue and the efficacy of an intervention on reducing muscle fatigue. The location factor included two levels at the area inside and outside the cup. In this study, we intended to compare the hemodynamic responses between the area inside and outside the cup; thus, both areas were investigated. The pressure factor included two levels at $-$ 225 and $-$ 300 mmHg. Our previous research showed that both a higher value ( $-$ 300 mmHg) and a lower value ( $-$ 225 mmHg) of cupping therapy are safe and effective for increasing skin blood flow [10]. Therefore, the cupping pressure intensity tested in this study were $-$ 225 and $-$ 300 mmHg. The duration factor included two levels at 5 and 10 min. The selection of 5 and 10 minutes was based on popular durations of cupping therapy [10, 11, 12, 20]. Each participant completed four protocols including (A) $-$ 225 mmHg for 5 min, (B) $-$ 225 mmHg for 10 min, (C) $-$ 300 mmHg for 5 min, and (D) $-$ 300 mmHg for 10 min. The counterbalanced design was implemented to minimize the order effect of 4 cupping protocols conducted in 4 different days. The separation of at least two days allowed the soft tissue to recovery from the applied cupping therapy. Our lab has been conducting a series of studies on cupping therapy using various biomedical devices, including laser Doppler flowmetry, elastographic ultrasound electromyography, and near infrared spectroscopy [10, 11, 12, 20]. This is part of a bigger project investigating various combinations of pressure and duration of cupping therapy on muscle hemodynamic responses. The data reported in this study are not presented elsewhere. This study was approved by the University Institutional Review Board of the University of Illinois at Urbana-Champaign (IRB #22900). All participants gave written consent before participating in this study.

2.1 Participants

The inclusion criteria were healthy individuals aged between 18 and 40 years. The exclusion criteria were as follows: non-blanchable response of the red skin areas over the biceps and triceps of the dominant side, open wounds, scar or tattoo over the tested area, diagnosed ischemic cardiovascular hypertension (SBP $>$ 140 mmHg or DBP $>$ 90 mmHg), diabetes and smoking history. The power analysis was performed based on the large effect size of 0.4 the power of 0.8 and 4 measurements of protocols A, B, C and D for a total of 10 subjects. The rationale to choose a large effect size is based on our previous studies indicating that various pressures and durations would cause very different tissue responses [10, 11, 12, 20]. Considering the variations of hemodynamic responses in different days, we increased the subject number to 18 for this study. The participant was recruited from people who responded to our flyer posted in the bulletin board and university e-newsletters. Subjects were briefed on the experimental procedures before signing the informed consent form.

Figure 1.

(A) The drawing shows the location of 10 photodetectors and 4 near-infrared light sources for a total of 16 channels of the sensor band. The grey circular area indicates the area inside the cup. The channels 9, 10, 11, and 12 are used to calculate muscle hemodynamic responses inside cup, and the channels 7, 8, 13 and 14 are used to calculate muscle hemodynamic responses outside cup. (B) Examples of time-series data of oxyhemoglobin and deoxy-hemoglobin from different channels after cupping therapy. Channel 7 signals (CH7) are from the area outside the cup and Channel 9 signals (CH9) are from the area inside the cup.

2.2 Instrumentation

A functional NIRS sensor band (fNIR Imager 1000, fNIR Devices LLC, Potomac, MD, USA) was used to measure changes in hemodynamic responses of the biceps muscle, including oxyhemoglobin, deoxy-hemoglobin, blood volume and oxygenation (Fig. 1A). In this study, oxyhemoglobin represents the concentration change of oxyhemoglobin ( $\Delta$ [HbO2] in $\mu$ M), and deoxy-hemoglobin represents the concentration change of deoxy-hemoglobin ( $\Delta$ [Hb] in $\mu$ M) [33, 34]. The blood volume is calculated as the sum of oxyhemoglobin and deoxy-hemoglobin ( $\Delta$ [HbO2] $+$ $\Delta$ [Hb] in $\mu$ M) and is used as an index to estimate the oxygen supply of muscle, and oxygenation is calculated as the difference between oxyhemoglobin and deoxy-hemoglobin ( $\Delta$ [HbO2] $-$ $\Delta$ [Hb] in $\mu$ M) and reflects a balance between oxygen utilization and oxygen delivery [16, 25, 35, 36]. The fNIRS band consisted of 10 photodetectors and 4 LED light sources, resulting in sixteen channels. Alternating wavelengths of 730 and 850 nm are emitted via four LEDs of this device. The oxyhemoglobin shows an absorption peak at about 850–900 nm, whereas deoxy-hemoglobin has its absorption peak at about 730–750 nm [37]. Changes in the absorption and scatting of light in the 700–850-nm range have been successfully used to evaluate tissue oxygenation [38]. The source-detector separation distance is 2.5 cm, which allows about 1.25 cm measurement depth [15]. The penetration depth is enough for recording the hemodynamic activity of the biceps muscle. The sampling rate was set at 2 Hz, which is a common setting to study muscle blood volume [39] In this study, fNIRS channels 9, 10, 11, and 12 were used to measure hemodynamic responses inside the cup and channels 7, 8, 13, and 14 were used to measure muscle hemodynamic responses outside the cup (Fig. 1A). This would allow the same area inside the cup (ie. spatial hemodynamic responses measured under channels 9, 10, 11 and 12) and outside the cup (ie. spatial hemodynamic response measured under channels 7, 8, 13, and 14). The examples of time-series of oxyhemoglobin and deoxy-hemoglobin (Fig. 1B) from different channels at different muscle areas are presented. The spectroscopy device settings were modified depending on the participant to maintain infrared light-emitting diode intensities within a functional range that avoided dark noise levels and saturation. The raw fNIRS signals were low-pass filtered with a finite impulse response filter of cut-off frequency at 0.14 Hz to eliminate possible respiration and heart rate signals and unwanted high frequency noise [40, 41]. The fNIRS signal of 5-min pre-cupping period was used to calculate the relative change of fNIRS signals with the concentration of oxyhemoglobin and deoxy-hemoglobin.

This study used an automatic suction device (P1000-PCS, California Medical Device Manufacturing Facility, CA, USA) to create a negative pressure inside the cup by using an electrical suction pump. The device consists of a vacuum gauge, a vacuum regulator, a power switch, and a vacuum cup. This device can produce negative pressure ranging from 0 to $-$ 760 mmHg by automatic suction. The cup with the inner diameter of 45 mm and the outer diameter of 53 mm (round shape with a range of 53–55 mm) was used in this study. The selection of this cup size was based on our preliminary research results showing that the 45-mm cup was more effective in reducing the stiffness of the triceps muscle compared with the 35-mm cup [21].

2.3 Procedures

All testing procedures were performed in the Rehabilitation Engineering Laboratory at the University of Illinois at Urbana-Champaign in a thermos-neutral environment (24–26^∘C). Subjects visited the laboratory for four days separated by at least 24 h and not more than 72 h. The participant rested at least 30 min in a supine position to acclimate to room temperature. Before the testing session, a mark was drawn on the biceps muscle (from the center of the cubital fossa to one-third of the acromion) to identify the location of applying the cup. Three marks on the four corners of the band of NIRS sensor were drawn to mark the location of the NIRS band. The NIRS sensor was used to measure the hemodynamic activity of the biceps muscle for 5 min. During the whole procedure, participants were asked to keep the same position and do not move. The participant fully extended the dominant-side elbow with the palm facing upward, and the angle between the arm and the body was 45 degrees. Elastic bandage was used to wrap the NIRS sensor to the arm to prevent any extraneous light from interfering with the NIRS signal. The pre-cupping hemodynamic activity was measured for 5 minutes. Then, the cupping cup with the inner diameter of 45 mm was applied on the biceps muscle area (in the center of cubital fossa to one-third of acromion) for either 5 min or 10 min at either $-$ 225 mmHg or $-$ 300 mmHg based on the predetermined test order of cupping protocols [10]. Then, the cup was removed and the NIRS sensor was placed back to the biceps muscle for 10 min (i.e. post-cupping hemodynamic response).

Several procedures were used to decrease biases in this study. Research participants were blinded for 4 cupping therapy protocols. They were instructed that different intensities of cupping therapy would be tested. One researcher performed data collection procedures and saved the NIRS files as protocols A, B, C, and D. Another researcher performed standard signal processing (low pass filter) to these files named as A, B, C and D who was not aware of the research hypotheses.

2.4 Statistical analysis

The three-way analysis of variance (ANOVA) with repeated measures was used to examine the main effect of location (areas inside and outside cup), pressure ( $-$ 225 and $-$ 300 mmHg) and duration (5 and 10 min) factors and the interaction effect among the location, pressure and duration factors (hypothesis 1). Each dependent variable was an average from 4 NIRS channels to overcome spatial heterogeneity (inside the cup from channels 9, 10, 11, and 12 and outside the cup from 7, 8, 13, and 14) (Fig. 1). The analysis was used to examine whether pressure and duration factors interact with the location factor (two-way ANOVA between pressure and location and two-way ANOVA between duration and location) (hypotheses #1 and #2). The test of sphericity was used to examine whether an assumption was violated, including the normal distribution. Bonferroni correction was used for repeated measures comparisons. The significance level was set at $p<$ 0.05. Statistical tests were implemented using SPSS 29 (IBM, Armonk, NY, USA).

3. Results

Eighteen healthy individuals (12 women, 6 men) volunteered to participate in the present study and their characteristics were: age 25.0 $\pm$ 4.6 years, systolic blood pressure 106.2 $\pm$ 9.7 mmHg, diastolic blood pressure 62.7 $\pm$ 8.0 mmHg, arm length 32.9 $\pm$ 2.2 cm, and arm circumference 27.4 $\pm$ 3.6 cm.

Table 1
Statistical results of the three-way ANOVA with repeated measures on the main effects of location (inside and outside the cup), pressure ( $-$ 225 and $-$ 300 mmHg), duration (5 and 10 min) and the interaction effect among location, pressure, and duration on muscle hemodynamic responses

		$F$ values	$P$ values	Effect size
Location $\times$ Pressure $\times$ Duration	Oxyhemoglobin	6.292	0.023^*	0.270
	Deoxy-hemoglobin	7.712	0.013^*	0.312
	Blood Volume	7.617	0.013^*	0.309
	Oxygenation	0.000	0.989	0.000
Location $\times$ Pressure	Oxyhemoglobin	2.967	0.103	0.149
	Deoxy-hemoglobin	0.041	0.842	0.002
	Blood Volume	1.461	0.243	0.079
	Oxygenation	4.356	0.052	0.204
Location $\times$ Duration	Oxyhemoglobin	6.852	0.018^*	0.287
	Deoxy-hemoglobin	0.689	0.418	0.039
	Blood Volume	0.093	0.765	0.005
	Oxygenation	4.313	0.053	0.202
Pressure $\times$ Duration	Oxyhemoglobin	2.319	0.146	0.120
	Deoxy-hemoglobin	2.625	0.124	0.134
	Blood Volume	2.978	0.103	0.149
	Oxygenation	0.018	0.895	0.001
Location	Oxyhemoglobin	1.605	0.222	0.086
	Deoxy-hemoglobin	0.001	0.974	0.000
	Blood Volume	0.402	0.535	0.023
	Oxygenation	3.021	0.100	0.151
Pressure	Oxyhemoglobin	13.295	0.002^*	0.439
	Deoxy-hemoglobin	0.709	0.412	0.040
	Blood Volume	6.671	0.019^*	0.282
	Oxygenation	9.260	0.007^*	0.353
Duration	Oxyhemoglobin	8.270	0.010^*	0.327
	Deoxy-hemoglobin	0.882	0.361	0.049
	Blood Volume	7.101	0.016^*	0.295
	Oxygenation	4.562	0.048^*	0.212

^*Indicates $P<$ 0.05.

3.1 The oxyhemoglobin response

A significant three-way interaction of the location, pressure, and duration factors was observed in oxyhemoglobin ( $p=$ 0.023) (Table 1). To decompose this interaction, Bonferroni pairwise comparisons indicated that there was a significant interaction between location and duration on hemoglobin ( $p=$ 0.018) for oxyhemoglobin (Table 1). There was a significant main effect of the pressure factor on oxyhemoglobin ( $p=$ 0.002).

There was a significant main effect on the duration factor on oxyhemoglobin ( $p=$ 0.01). Oxyhemoglobin from 2.60 $\pm$ 1.54 $\mu$ M to 3.80 $\pm$ 2.44 $\mu$ M ( $p<$ 0.05) and from 3.73 $\pm$ 1.90 $\mu$ M to 5.01 $\pm$ 2.80 $\mu$ M ( $p<$ 0.05) for pressure at $-$ 225 mmHg and $-$ 300 mmHg, respectively, as the duration increased from 5 minutes to 10 minutes for the area outside the cup (Fig. 2A). Oxyhemoglobin from 2.60 $\pm$ 1.54 $\mu$ M to 3.73 $\pm$ 1.90 $\mu$ M ( $p<$ 0.05) and from 3.80 $\pm$ 2.44 $\mu$ M to 5.01 $\pm$ 2.80 $\mu$ M ( $p<$ 0.05) for duration at 5 minutes and 10 minutes, respectively, as the pressure increased from $-$ 225 mmHg to $-$ 300 mmHg (Fig. 2A). The interaction effect of pressure and duration of cupping therapy on the difference of oxyhemoglobin between the areas inside and outside cup is presented in Fig. 2B (Table 1).

3.2 The deoxy-hemoglobin response

A significant three-way interaction of the location, pressure, and duration factors was observed in deoxy-hemoglobin ( $p=$ 0.013). No main effect was significant on deoxy-hemoglobin of pressure, location, and duration (Table 1). Deoxy-hemoglobin from 0.45 $\pm$ 1.58 $\mu$ M to 0.96 $\pm$ 2.00 $\mu$ M ( $p>$ 0.05) and from 0.70 $\pm$ 2.30 $\mu$ M to1.37 $\pm$ 1.61 $\mu$ M ( $p>$ 0.05) for pressure at $-$ 225 mmHg and $-$ 300 mmHg, respectively, as the duration increased from 5 minutes to 10 minutes for the area outside the cup (Fig. 3). Deoxy-hemoglobin from 0.45 $\pm$ 1.58 $\mu$ M to 0.70 $\pm$ 2.30 $\mu$ M ( $p>$ 0.05) and from 0.96 $\pm$ 2.00 $\mu$ M to 1.37 $\pm$ 1.61 $\mu$ M ( $p>$ 0.05) for duration at 5 minutes and 10 minutes, respectively, as the pressure increased from $-$ 225 mmHg to $-$ 300 mmHg (Fig. 3A). The interaction effect of pressure and duration of cupping therapy on the difference of deoxy-oxyhemoglobin between the areas inside and outside cup is presented in Fig. 3B (Table 1).

Figure 2.

(A) The averaged oxyhemoglobin responses of the biceps muscle measured at the area outside the cup after four intensities of cupping therapy. The values are presented as the change compared to the pre-cupping oxyhemoglobin value ( $\Delta$ [HbO2]). ^#Indicates a significant increase in $\Delta$ [HbO2] compared to pre-cupping value ( $p<$ 0.01). ^*Indicates a significant difference of $\Delta$ [HbO2] between 2 cupping intensities ( $p<$ 0.05). Values are presented as mean $+$ SE. (B) The interaction effect of pressure and duration of cupping therapy on the difference of oxyhemoglobin between the areas inside and outside cup.

3.3 The blood volume response

A significant three-way interaction of the location, pressure, and duration factors was observed in blood volume ( $p=$ 0.013). There was a significant main effect of the pressure factor on blood volume ( $p=$ 0.019), and a significant main effect on the duration factor on blood volume ( $p=$ 0.016) (Table 1). Blood volume from 0.45 $\pm$ 1.58 $\mu$ M to 0.96 $\pm$ 2.00 $\mu$ M ( $p>$ 0.05) and from 0.70 $\pm$ 2.30 $\mu$ M to 1.37 $\pm$ 1.61 $\mu$ M ( $p>$ 0.05) for pressure at $-$ 225 mmHg and $-$ 300 mmHg, respectively, as the duration increased from 5 minutes to 10 minutes for the area outside the cup (Fig. 4). Blood volume from 0.45 $\pm$ 1.58 $\mu$ M to 0.70 $\pm$ 2.30 $\mu$ M ( $p>$ 0.05) and from 0.96 $\pm$ 2.00 $\mu$ M to 1.37 $\pm$ 1.61 $\mu$ M ( $p>$ 0.05) for duration at 5 minutes and 10 minutes, respectively, as the pressure increased from $-$ 225 mmHg to $-$ 300 mmHg (Fig. 4A). The interaction effect of pressure and duration of cupping therapy on the difference of blood volume between the areas inside and outside cup is presented in Fig. 4B (Table 1).

3.4 The oxygenation response

There was no significant interaction of the location, pressure and duration factors on oxygenation ( $p=$ 0.989). There was a significant main effect of the pressure factor on oxygenation ( $p=$ 0.007). There was a significant main effect on the duration factor on oxygenation ( $p=$ 0.048). Oxygenation from 2.15 $\pm$ 1.26 $\mu$ M to 3.04 $\pm$ 1.52 $\mu$ M ( $p<$ 0.05) and from 2.83 $\pm$ 2.31 $\mu$ M to 3.63 $\pm$ 2.10 $\mu$ M ( $p<$ 0.05) for pressure at $-$ 225 mmHg and $-$ 300 mmHg, respectively, as the duration increased from 5 minutes to 10 minutes for the area outside the cup (Fig. 5A). Oxygenation from 2.15 $\pm$ 1.26 $\mu$ M to 2.83 $\pm$ 2.31 $\mu$ M ( $p>$ 0.05) and from 3.04 $\pm$ 1.52 $\mu$ M to 3.63 $\pm$ 2.10 $\mu$ M ( $p>$ 0.05) for duration at 5 minutes and 10 minutes, respectively, as the pressure increased from $-$ 225 mmHg to $-$ 300 mmHg (Fig. 5A). The interaction effect of pressure and duration of cupping therapy on the difference of oxygenation between the areas inside and outside cup is presented in Fig. 5B (Table 1).

All individual data are presented in Fig. 6.

Figure 3.

(A) The averaged deoxy-hemoglobin responses of the biceps muscle measured at the area outside the cup after four intensities of cupping therapy. The values are presented as the change compared to the pre-cupping deoxy-hemoglobin value ( $\Delta$ [Hb]). ^#Indicates a significant increase in $\Delta$ [Hb] compared to pre-cupping value ( $p<$ 0.01). ^*Indicates a significant difference of $\Delta$ [Hb] between 2 cupping intensities ( $p<$ 0.05). Values are presented as mean $+$ SE. (B) The interaction effect of pressure and duration of cupping therapy on the difference of deoxy-oxyhemoglobin between the areas inside and outside the cup.

Figure 4.

(A) The averaged blood volume responses of the biceps muscle measured at the area outside the cup after four intensities of cupping therapy. The values are presented as the change compared to the pre-cupping blood volume value ( $\Delta$ [HbO2] $+$ $\Delta$ [Hb]). ^#Indicates a significant increase in blood volume compared to pre-cupping value ( $p<$ 0.01). ^*Indicates a significant difference of blood volume between 2 cupping intensities ( $p<$ 0.05). Values are presented as mean $+$ SE. (B) The interaction effect of pressure and duration of cupping therapy on the difference of blood volume between the areas inside and outside the cup.

4. Discussion

Our study demonstrated that the treated area outside the cup also showed a significant increase in oxyhemoglobin, blood volume and oxygenation of the biceps, and demonstrated a significant interaction effect between the location and duration factors and significant main effects of pressure and duration factors on oxyhemoglobin, blood volume and oxygenation This study found that the cupping intensity affects not only the area inside the cup but also the area outside the cup on oxyhemoglobin, blood volume and oxygenation of the biceps Our findings provide the first evidence of the dose-response relationship of cupping pressure, duration, and location on muscle oxygenation. Our findings indicate that an effective cupping therapy is determined by an appropriate combination of cupping pressure, duration, and location (the area inside and outside the cup).

Figure 5.

(A) The oxygenation responses of the biceps muscle measured at the area outside the cup after four intensities of cupping therapy. The values are presented as the change compared to the pre-cupping oxygenation value ( $\Delta$ [HbO2] $-$ $\Delta$ [Hb]). ^#Indicates a significant increase in oxygenation compared to pre-cupping value ( $p<$ 0.01). ^*Indicates a significant difference of oxygenation between 2 cupping intensities ( $p<$ 0.05). Values are presented as mean $+$ SE. (B) The interaction effect of pressure and duration of cupping therapy on the difference of oxygenation between the areas inside and outside the cup.

Figure 6.

Individual data of A) oxyhemoglobin ( $\Delta$ [HbO2]), B) deoxy-hemoglobin ( $\Delta$ [Hb]), C) blood volume ( $\Delta$ [HbO2] $+$ $\Delta$ [Hb]), and D) oxygenation ( $\Delta$ [HbO2] $-$ $\Delta$ [Hb]) after 4 cupping protocols ( $-$ 225 mmHg for 5 min, $-$ 225 mmHg for 10 min, $-$ 300 mmHg for 5 min, and $-$ 300 mmHg for 10 min).

An important finding of the present study is a significant interaction between the location and duration factors for oxyhemoglobin ( $p=$ 0.018) and oxygenation ( $p=$ 0.053) for both pressure levels. Previous studies indicate that an appropriate combination of cupping pressure and duration can effectively increase oxyhemoglobin but not deoxy-hemoglobin [13, 26, 31]. Our results further indicate that oxyhemoglobin was not uniformly distributed from the area inside the cup to the area outside the cup after cupping therapy at $-$ 225 mmHg and $-$ 300 mmHg. The difference between these two cupping pressures increased as the duration increased from 5 minutes to 10 minutes. This suggests that the duration of cupping therapy determines the spatial responses of oxyhemoglobin in the biceps muscle. When the cupping duration is sufficient, the response of oxyhemoglobin between the area inside and outside of the cup is significantly different. One research study demonstrated that increased hemodynamic response depends on the extent of ischemic insult [42]. The duration (standard duration of 5 min ischemic test, with a range of 4, 6, and 8 minutes) of tissue ischemia is longer, the magnitude of changed blood flow is larger [14]. Another research study indicates that a longer duration of stretching in studies of 5 durations (20 seconds and 1, 2, 5, and 10 minutes) elicits an increase in oxyhemoglobin after stretching [43]. In this research, our results demonstrated that a longer cupping duration induces an increase in the spatial difference of oxyhemoglobin of the biceps muscle between the areas inside and outside the cup. Cupping duration is an important factor affecting the spatial difference of oxyhemoglobin of the biceps muscle between the areas inside and outside the cup.

To date, it remains unclear whether the hemodynamic response of the muscle from the area inside and outside the cup would be correlated. Our results indicate that the muscle hemodynamic responses in the areas inside and outside the cup are similar, although the muscle area inside the cup is under tension according to the finding from a simulation study, the muscle directly under the rim of the cup is under compression, and muscle outside the cup is not under tension nor compression [44]. The tensile stresses appear to be larger in a bulb-shaped region under the center of the cup. The existence of high tensile stresses inside the cup rim and in the central region enclosed by the cup is believed to be the primary cause of ecchymosis, a discoloration of the skin caused by the escape of blood into the tissue from ruptured blood vessels. This study reveals that there are no heterogeneities of oxyhemoglobin, deoxy-hemoglobin, blood volume, and oxygenation in the biceps muscle between the areas inside and outside (within 2.5 cm) the cup because both areas show similar responses to cupping therapy. It seems that the existence of high tensile stresses inside the cup rim and in the central region inside the cup implies the same level of mechanical stimulation to the muscle blood volume and oxygenation of the biceps brachii. Based on our results, the cupping effect is not only limited to the area inside the cup, but also the area outside the cup. Cupping therapy is usually used for the reduction of musculoskeletal pain; however, it may be difficult to find the exact pain point. The application of cupping therapy on the surrounding area may be able to induce the same physiological response as applying the cup on the exact acupoint or pain point.

Mechanical deformation of the vasculature may initiate rapid vasodilation in the contracting skeletal muscle. It appears that smooth muscle cells can directly respond to changes in elevated extravascular pressure, generally referred to as myogenic autoregulation [19]. Thus, a highintensity cupping therapy can induce a larger mechanical deformation of the vasculature, which induces a greater reactive hyperemic response [13, 26, 31]. Capillaries in skeletal muscle are surrounding myocytes, lengthening of sarcomeres will create a kinetic sequence of events moving outward such that the capillary extension reserve will cause compression and reduced vessel diameter. This alters vascular resistance and subsequently alters blood flow [45, 46]. However, it seems that muscle stretching needs to be under a certain physiological range for decreasing the mean capillary diameter and increasing blood flow after the removal of the stimulation [46]. For the spatial responses of oxyhemoglobin, deoxy-hemoglobin, blood volume and oxygenation in the biceps muscle after cupping therapy at different intensities, the level of mechanical deformation and the reduced capillary diameter of the vasculature in the areas inside and outside the cup are the same when the duration is short. As the cupping duration is longer, the reduced capillary diameter of the vasculature is different between the areas inside and outside the cup. The effects of mechanical stimulation on oxidation between the areas inside and outside the cup need a longer duration to reach a significant change. This may be partly explained by the results of our research.

One of potential benefits of cupping therapy is to increase metabolic demands of the treated muscle. Based on our results, cupping therapy may not be able to induce a higher metabolic rate of the muscle. Our results indicate that cupping pressure at $-$ 300 mmHg induced a significant increase in oxyhemoglobin and oxygenation than pressure at $-$ 225 mmHg, as the duration increased from 5 minutes to 10 minutes (i.e. no significant difference between $-$ 300 mmHg and $-$ 225 mmHg at 5-min duration but a significant difference at 10-min duration). This is expected because a larger cupping pressure would induce a greater tension on the biceps muscle, which may increase the intramuscular pressure leading to an alteration in oxygen supply [44]. These results support previous studies demonstrating that a higher value ( $-$ 300 mmHg) of negative pressure is more effective for increasing skin blood flow compared to a lower value ( $-$ 225 mmHg) [10, 47]. Although there was no significant change of deoxy-hemoglobin after different intensities of cupping therapy between the areas inside and outside the cup, our results suggest that a greater O₂ supply rather than an increased metabolic demand to the biceps muscle in both areas inside and outside the cup after cupping therapy [48]. It appears that cupping therapy could not significantly increase the metabolic demand of biceps muscle. The mechanical stimulation of cupping therapy is different from exercise and could not induce muscle metabolic activity.

There are limitations of this study. First, this study was conducted in a homogenous group of participants with normal body mass index. The findings of this study may not be generalized to people who have obesity because of the potential influence of adipose tissue thickness on the penetration depth of the light of NIRS device as well as the mechanical forces induced by cupping therapy. Moreover, there were more females than males that may influence the results. Keller and Kennedy demonstrated that men exhibited faster skeletal muscle tissue desaturation than women after fatiguing handgrip exercise [55]. Future studies should investigate the gender and race effects on the hemodynamic response after cupping therapy. Second, a multi-channel fNIRS device was used in this study with a continuous-wave spectroscopy mode that only measures the relative changes of muscle oxyhemoglobin and deoxy-hemoglobin. It is unclear whether the setting of fNIRS could be directly used to measure muscle hemodynamic responses. It is noted that muscle oxygenation measured from NIRS could be cofounded by cutaneous microcirculation. Based on the findings from our previous studies, cupping therapy can significantly increase skin blood flow using laser Doppler flowmetry [10]. Third, the intensities of cupping therapy tested in this study included 4 common combinations of pressure ( $-$ 225 and $-$ 300 mmHg) and duration (5 and 10 min). Future studies need to investigate different combinations of pressure and duration of cupping therapy to validate our findings.

5. Conclusions

The spatial response of oxyhemoglobin, deoxy-hemoglobin, blood volume, and oxygenation of the biceps muscle after various intensities of cupping therapy was investigated using a multi-channel NIRS system. Our results support that the clinicians should pay attention to cupping pressure and cupping duration for effectively improving muscle oxygenation. Our findings also indicate that the cupping duration factor is important when treating a wider area of soft tissue impairment because a sufficient duration is needed to induce a spatial response for improving muscle oxyhemoglobin of the area outside and inside the cup. Our findings indicate that an effective cupping therapy could benefit the area inside the cup and immediately outside the cup.

Author contributions

YJ conceptualized the study; YL, PM, JG, and ZS collected data; and all authors contributed to the analysis of data and writing of the manuscript.

Data availability statement

The dataset generated during this study is not publically available due to planned computational analyses for publication, but is available from the corresponding author upon reasonable request.

Ethical approval

The study protocol was approved by the Institutional Review Board at the University of Illinois at Urbana-Champaign (IRB #22900).

Funding

This study received no funding.

Informed consent

The participants gave written consent for participating in this study.

Footnotes

Acknowledgments

None to report.

Conflict of interest

The authors declare that they have no conflicts of interest.

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Using multi-channel near-infrared spectroscopy to assess the effect of cupping therapy on the spatial hemodynamic response of the biceps muscle: A preliminary study

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Methods

2.1 Participants

2.3 Procedures

2.4 Statistical analysis

3. Results

Table 1 Statistical results of the three-way ANOVA with repeated measures on the main effects of location (inside and outside the cup), pressure ( - 225 and - 300 mmHg), duration (5 and 10 min) and the interaction effect among location, pressure, and duration on muscle hemodynamic responses

3.2 The deoxy-hemoglobin response

3.4 The oxygenation response

Author contributions

Data availability statement

Ethical approval

Funding

Informed consent

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Statistical results of the three-way ANOVA with repeated measures on the main effects of location (inside and outside the cup), pressure ( $-$ 225 and $-$ 300 mmHg), duration (5 and 10 min) and the interaction effect among location, pressure, and duration on muscle hemodynamic responses