The effect of Liquid ice after high-intensity exercise on muscle function compared to Block ice.

Cryotherapy is used to recover muscle damage after exercise and to treat acute sports injuries. Liquid ice (LI) can keep cold for a long time, and is assumed more effective than block ice (BI). From this, the aim of this study was to investigate the effects of LI on the change of passive stiffness (PS) as muscle function and to validate the effectiveness of LI compared to BI. We performed the experiment as part of a case series of verification of the effects of cryotherapy. 22 healthy men (target area: right leg) were randomized to two groups: LI group and BI group. PS was measured three times during experiment protocol, pre: before exercise; post; after treating each cryotherapy after exercise; 48h: 48 hours after pre. Statistical analysis compared the PS, the amount of change in PS, and the rate of change in PS between the two groups. The rate of change between pre and 48h in LI was significantly lower compared to that in BI (p = 0.03). There was no significant difference regarding other results between groups. It revealed that the difference of effect between LI and BI for PS of muscles after high-intensity exercises. These results could be helpful for the choice of intervention for reducing muscle stiffness after exercise and at sports field.

Introduction

High-intensity exercises, especially those with eccentric contractions cause muscle damage1, 2, 3 which leads to injuries, such as decreased muscle strength and decline in range of motion (ROM). Since these muscle injuries affect sports performance, it is important to repair muscle damage after exercise.

The main functions of skeletal muscles are to generate force and power, maintain posture, and produce movement. Among them, muscle flexibility is important for movements such as walking and sports performance. ROM is commonly used as an objective indicator of muscle flexibility as a part of muscle function. Passive stiffness (PS) can also be used to objectively and quantitatively assess muscle flexibility.

Several cryotherapy methods have been used to repair muscle damage8, 9, 10 and treat acute sports injuries., The popular method of cryotherapy involves an ice bag. As it is easy and multipurpose, it is commonly used in the outside, such as in sports. However, maintaining the temperature of ice bag is difficult as it depends on prevailing weather and temperature condition. Liquid ice (LI), a new cryotherapy material, is gel ice made from saline water (Fig. 1). Cryotherapy using LI may be more effective than using block ice (BI) as LI can be kept cold for a longer time and has higher contact with body surfaces. In addition to cryotherapy using BI, there have been previous studies on cryotherapy using materials such as BI and cold water or examining multiple outcomes for a single intervention,,13, 14, 15, 16, 17, 18 however, no study has been conducted on the effects of different types of ice using the same material.

Fig. 1

Liquid ice can be seen on the left side and block ice on the right.

This study aimed to investigate the effects of LI on changes in muscle flexibility and to validate its effectiveness., We focused on PS and used it as an index of flexibility as it can objectively and quantitatively assess muscle flexibility. We hypothesized that cryotherapy using LI would have a lower PS compared to that of BI after high-intensity exercise. Moreover, we performed a case series to verify of the effects of cryotherapy.

Methods

Participants

The participants of this study were 22 healthy men, and their right legs were used. Some participants had prior experience with cryotherapy. We confirmed that the participants did not undergo cryotherapy during the study period. They had not been diagnosed with muscle strain in their right hamstrings in the past six months and did not have diseases which prohibited cryotherapy such as Raynaud's disease. The participants were explained the experimental procedures and risks before they provided informed consent to participate in the study. The study was approved by the ethics committee and complied with the principles of the Helsinki Declaration.

Procedures

Experimental protocol

The subjects were randomized into two groups, depending on the intervention method, using the permuted block method: LI group (n = 11) and BI group (n = 11). The study was conducted for two days to measure the PS three times (Fig. 2). On the first day, we measured the PS of each participant (pre). Then, each participant exercised, received cryotherapy by LI or BI for 20 minutes, rested for 30 minutes, and PS was measured again (post). The PS was measured for the third time after 48 hours from the two initial tests (48h). We focus on the changes in muscle flexibility under general body temperature. Therefore, we chose these three time points.

Fig. 2

Experimental protocol and measurement time points.

Exercise protocol

The participants performed efferent exercises using a dynamometer (Biodex System 4.0, Biodex Medical System, Inc. Shirley, New York, USA). They were positioned on a chair, and the angle of the right knee was set from 90° to 40°. They performed eight sets of 10 right maximum knee efferent flexion and the speed of knee flexion was 60°/s. The first set was submaximal in order to familiarize them with the machine. The rest time was 60 seconds.

Cryotherapy intervention

The ice bags contained approximately 300 g of LI (Nippon Electric Heat Co.) or BI. After exercise, participants were provided with a cold application for 20 minutes with either LI or BI and placed at the center of the right posterior thigh. The time period of the cold application has been described in a previous study.

Measurements

The PS of all participants was measured as muscle function and it was assessed using the same dynamometer. The subjects lay on a bed, and their waist was held in position using belts. The right leg was tightly secured to the dynamometer leg plate, and the starting position of the right knee and hip joints was at 90° flexion. For passive testing, the subject's right knee joint was extended at a speed of 5°/s. Participants were instructed to push a stop button just before they felt pain in their hamstrings. When the button was pushed, the passive knee flexion torque (Nm) and knee extension angle (°) were measured. The PS was calculated as the slope of the passive knee flexion torque-angle curve. In the curve, the values of knee flexion torque were obtained at two points: the maximum knee extension angle and its half angle. Based on this, the value obtained by dividing the torque change in the torque angle curve by the angle change during that period was calculated as passive stiffness (Nm/deg)., Along with the values at each time point (pre, post, and 48h), the amount and the rate of change between pre and post, pre and 48h, as well as post and 48h were calculated. We measured the PS three times at each point and used the average values of the second and third measurements.

Statistical analyses

The LI and BI groups were compared in terms of PS using the amount and the rate of PS. The Shapiro-Wilk test was used to determine whether the data were normally distributed. Unpaired t-tests were used for the normal distribution. Mann-Whitney U tests were used for the non-normal distribution. The statistical significance level was less than 5%. The data was analyzed with JMP Pro 14 statistical software package (SAS Institute Inc., Cary, NC, USA).

Results

The participants’ data regarding age, height and weight are shown in Table 1, and they revealed no significant differences (mean ± SD: age, 23.4 ± 1.0 years; height, 173.2 ± 5.9 cm; weight, 75.3 ± 1.1 kg). The rate of pre and 48h LI was significantly lower than that of BI (p = 0.030), and other results did not differ significantly between LI and BI (Table 2).

Table 1

Characteristics of 22 participants. Each data showed mean ± SD.

	Block ice (BI)	Liquid ice (LI)
Height (cm)	172.89 ± 4.18	173.64 ± 7.43
Weight (kg)	74.32 ± 11.69	76.15 ± 10.55
BMI (kg/m2)	24.82 ± 3.44	25.26 ± 3.11
Age (years old)	23.45 ± 1.29	23.36 ± 0.92

Table 2

The value of passive stiffness (PS), the amount of change of PS, and the rate of change of PS.

		BI group	LI group	p value
PS (Nm/deg)	pre	0.28 ± 0.10	0.39 ± 0.20	0.13
	post	0.33 ± 0.01	0.38 ± 0.18	0.36
	48h	0.36 ± 0.13	0.40 ± 0.22	0.64
The amount of change of PS (Nm/deg)	pre and post	0.04 ± 0.10	−0.00 ± 0.12	0.36
	pre and 48h	0.08 ± 0.05	0.01 ± 0.15	0.17
	post and 48h	0.04 ± 0.13	0.01 ± 0.09	0.65
The rate of change of PS (%)	pre and post	0.21 ± 0.41	0.04 ± 0.24	0.23
	pre and 48h	0.32 ± 0.30	0.05 ± 0.39	0.03∗
	post and 48h	0.20 ± 0.44	0.02 ± 0.28	0.27

Each data showed mean ± SD. The results of t-test or Mann-Whitney U tests between LI group and BI group. ∗p < 0.05.

Discussion

This study focused on the effectiveness of LI compared to BI for the muscle function. The results showed that cryotherapy using LI tended to reduce muscle stiffness 48 hours after high-intensity exercises; it was more effective than that the BI.

Muscle damage caused by high-intensity exercises causes inflammation and pain, leading to increased muscle stiffness. Damaged muscle during and immediately after exercise becomes stiff over time with further exercise., Early and rapid cryotherapy after exercises is helpful in controlling acute inflammation; as a result, the inflamed area and deterioration of muscle stiffness are suppressed., Cryotherapy tends to slow the oxygen demand of mitochondria, thereby decreasing the metabolic demand and inflammation of the muscle. Therefore, it is useful for controlling aggravating secondary hypoxia and stiffening muscles. Furthermore, a study on cryotherapy reported that decreasing the tissue temperature by 10°–15°maximizes its effect. It is speculation that it can effectively decrease the temperature of muscle surfaces and muscles and help muscles recover faster compared to BI via the wide range of contact and coolness due to the gel constitution of the LI and a temperature lower than that of BI. The current study showed that LI tended to reduce muscle stiffness 48 hours after high-intensity exercises more quickly than that the BI. This suggests that acute cooling controls the further occurrence of muscle damage, and, as a result, repairs the damage faster, compared to BI, by decreasing metabolic and metabolic demands and controlling stiffening muscles.

The rate of change between pre and 48h was considered as an index of whether the muscle returned to the pre state. LI showed a significantly lower value for this rate of change. In other words, LI contributes to faster muscle recovery than BI. There was no significant difference in the rate of change between pre and post, and post and 48h. This means that there is no difference in the immediate and long-term effects between LI and BI. Some reports have shown that muscle damage is repaired 48h after exercise and cryotherapy., However, there was no change in muscle function due to differences in the ice itself. These results show that there is a difference in the recovery speed of the muscles but no difference in the immediate or long-term effects. This revealed a difference in the effect of LI and BI on the PS of muscles after high-intensity exercises. These results could be helpful for reducing muscle stiffness in sportsperson.

This study had several limitations. First, we did not set a non-treatment group; therefore, we cannot ascertain whether cryotherapy using ice bags is better than no treatment. Second, the relationship between muscle stiffness and symptoms is unclear. Third, it is necessary to consider the reliability of PS before starting our study. However, since PS measurement was used in the previous research,, it was used in our research without validation. Fourth, we cannot refer to the physiological and performance aspects of the intervention, since we did not measure the differences in muscle temperature, blood indicators or exercise performance indicators by differences in LI and BI. Future studies should verify the physiological effects of LI and the effects of cryotherapy as a case series.

Conclusion

The present study showed that cryotherapy using LI after high-intensity exercise has a positive effect on muscle flexibility and can be useful in sports.

Funding/support statement

No financial or material support of any kind was received for the work described in this article.

Declaration of competing interest

The authors have no conflicts of interest relevant to this article.