Muscle soreness is a common phenomenon especially in people who exercise infrequently or athletes who exceed their normal workout intensity.1–5 Delayed-onset muscle soreness (DOMS) symptoms can range from mild irritation to severe pain that forms a psychological barrier inhibiting performance in subsequent days.3,6 As a general rule, the intensity and duration of the symptoms vary with age, nutrition, and physical condition.6 The DOMS is greater in intensity and duration in older individuals and individuals with diabetes.6–8 However, other factors can also reduce the duration and intensity of DOMS.
Ingestion of branched chain amino acids has been shown to reduce DOMS in older people and people with diabetes.9,10 Many other strategies have also been tried.11 The most common strategies involve thermal modalities or massage.11,12 Vibration has also been used to reduce DOMS.13 But the most common modality used to reduce DOMS is heat and cold.11,12,14 Recent studies have shown a positive effect of heat in reducing DOMS.12 Although 20-minute heat modalities have poor results, sustained heat does seem to help.12 Cold is the most commonly recommended modality after heavy exercise.10,14 Cold water immersion, in some studies, may slow down the repair process in muscle after exercise that induces DOMS.15 However, in other studies, it did not.14 The issues in many of these previous studies were that, comparing one study to another, the muscles exercised were different, the actual exercise protocol was different, the exercise intensities and durations were different, and the temperature and type of thermal modality were different as were the measures of DOMS. Other modalities include nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, muscle relaxants, and antidepressants.16 Nonpharmacological intervention includes treatments such as hot and cold and was evaluated in a 2006 Cochrane Review.17,18 Only heat seemed to show a consistent reduction in pain. A problem with heat is the duration. Hydrocollator heat wraps are usually left for 15 minutes. Although this warms the skin, deep heat penetration is poor.19 Furthermore, the use of warm temperature modalities such as whirlpool and other forms of clinical thermal therapy have caused infections of the skin20 and burns in susceptible populations.20 For wounds, heat penetration is not an issue because the wound is superficial. But for deep tissues such as the shoulder, knee, or back, due to the influence of subcutaneous fat, heat penetration is very poor to none in 15 minutes of heat exposure.19 A key to increased healing is temperature penetration both to increase tissue blood flow and metabolism. Generally, for every 3 degrees increase in tissue temperature, metabolism doubles, promoting faster repair of tissue.21 Without good tissue penetration, there is pain relief but little effect on healing. Continuous low-level heat wraps have been shown to penetrate and warm deep tissues.19,22
Therefore, in the present investigation, we compared a common over-the-counter heat modality, ThermaCare heat wraps applied for 8 hours used either immediately after exercise or 24 hours after exercise. It has been well established that heat provided by clinical modalities such as hydrocollator heat packs reduces pain.23 This is mediated by inhibiting purine receptors in the peripheral pain nociceptor pathways through voltage-gated calcium channels sensitive to temperature.24,25 These channels, the TRPV1 and TPRV4 receptors on sensory neurons, are sensitive to hot and warm temperatures, respectively,24,26–28 an often overlooked value of heat in increased healing of damaged tissue.
When heat is applied to wounds, even wounds in people with diabetes that showed no healing for 2 years, there was rapid healing.29–32 Part of this was attributed to an increase in blood flow due to heat,33 but blood flow remained elevated for over 24 hours after a 30-minute heat session showing a carryover that kept the tissue warm. The same was seen for pain relief if a continuous low-level heat wrap was applied. Here 8 hours of continuous heat resulted in 16 hours of pain relief.34,35
The hypothesis was that sustained heat would be very effective immediately after exercise by increasing circulation and healing rates. Rather than simple analog visual pain scales, we used both objective and subjective measures of DOMS. Analog visual pain scales were used, and algometer muscle soreness scales were used as subjective measures, and muscle strength, blood myoglobin, and muscle stiffness were used as objective measures. Measurements were taken 1 day before and 3 days after exercise. A control group had no heat.
The subjects for this study were 60 healthy individuals divided randomly into 3 groups of 20 (control, ThermaCare heat wraps applied immediately after exercise, and ThermaCare heat wraps applied 24 hours after exercise). All subjects had at least 6 weeks of physical inactivity, and their body mass index was less than 40. Subjects had no cardiovascular disease, hepatic disease, diabetes, lower limb neuropathies, or recent lower limb injuries. All had blood pressure measurements between 140/90 and 90/60 mm Hg at rest. Subjects were not on alpha or beta agonist/antagonists, any type of NSAID, Cox 2 inhibitors, calcium channel blockers, Pregabalins (Lyrica), or pain reducers. The demographics of the subjects are shown in Table. All methods and procedures were approved by the Institutional Review Board of Loma Linda University, and all subjects signed a statement of informed consent.
Muscle Strength Measurement
Muscle strength was measured with 4 strain gauges placed on opposite sides of a steel bar. The bar was fixed to a chair base with a leather ankle strap that was placed just above the malleolus and measured force developed during extension by the quadriceps muscle (knee bent at 90 degrees). When the bar was bent, the strain gauges, arranged as a Wheatstone bridge, were deformed and an electrical output was provided to a BioPac (BioPac Systems, Goleta, California) system DAC100 bioelectric amplifier module and MP150 system (gain = 5000, analogue to digital conversion = 1k 24 bits resolution). Data analysis and storage used the Acknowledge 4.1 software from BioPac Inc (BioPac Systems). Muscle strength was determined as the average of 2 contractions, each contraction lasting for 3 seconds in duration with at least 1 minute of rest separating the contractions.
To provoke DOMS, the subjects accomplished squats for 5 minutes. An exercise monitor set the pace, and they were instructed to flex the hip past 110 degrees. The pace was set at 3 seconds per squat for 5 minutes repeated after 3 minutes of rest 2 more times (total 3 rounds).
Subjective Pain Measurement
A 10-cm visual analog scale (VAS) horizontal line across a piece of paper was used. One end was marked “pain free” and the other “very, very sore.” The subject was asked to place a vertical slash across the line where appropriate. Only one visual analog pain scale was printed on a single sheet of paper. This technique has been recently reviewed and is reliable.36
Approximately 5 mL of venous peripheral blood was collected from an antecubital vein using a disposable needle and vacutainer for serum or plasma with a serum separator and whole-blood EDTA before (Pre) and at 48 hours after exercise. The blood was placed in a refrigerated centrifuge and spun at 3000 rpm for 10 minutes to separate the serum or plasma from the cells. The separated serum and plasma aliquots were stored at −80°C until analyses were conducted.
Plasma Biomarker Measurements
A complete blood count was performed using a Mindray BC-3200, including the hematocrit and an automated 3 part white blood cell count. For the measurement of plasma Myoglobin, commercially available enzyme-linked immunosorbent assay kits in a 96-well plate format were used according to the manufacturers' instructions.
- Plasma myoglobin (MG017C; CalBiotech, Spring Valley, California).
Force to Flex and Extend the Knee
The force to flex and extend the knee was measured from 90 to 125 degrees. The subject was in the seated position with the leg free to hang at an initial angle of 90 degrees with the foot off the floor. A linear actuator was connected through an ankle strap to passively move the knee through 35 degrees of flexion (Figure 1). The force needed to move the knee was measured as a measure of flexibility and elasticity of the quadriceps muscle and its tendons. The rate of movement was 45 degrees in 7.5 seconds. The knee was flexed and then extended, and the force was measured in each direction. Resistive strain gauges (350 Ohms) were arranged as a Wheatstone bride. The bridge output was amplified and conditioned with a DAC100 strain gauge amplifier with a gain of 500 (BioPac Systems). The amplified output was digitized at 2000 Hz with a resolution of 24 bits on an MP150 BioPac data acquisition system (BioPac Systems). A goniometer measured the angle of the knee to calculate the force needed per degree moved.
Measurement of Range of Motion
Range of motion of the knee was measured by a trained physical therapist with a digital goniometer. Measures were made of full active range of motion and the point during range of motion of the knee where pain was felt, if any, after the exercise.
Heat was applied by placing one ThermaCare heat wrap on each leg centered over the quadriceps and lying longitudinally over the muscle. ThermaCare heat wraps use the oxidation of iron through an oxygen-controlled mass reaction to provide constant heat over an 8-hour period through individual cells. The wrap is elastic and held over the sore area by Velcro bands.
On each day, subjects entered the room and relaxed in a thermally neutral environment for 20 minutes. Measurements such as leg strength, range of motion, analog visual pain scales, and force to move the leg were recorded. These data were collected on Monday, exercise was accomplished on Tuesday, and then measurements were taken again on Wednesday, Thursday, and Friday. Blood was sampled on Monday and Thursday. One group was the control group and did not use heat. Another group had heat applied by ThermaCare heat wraps immediately after exercise, and a final group had ThermaCare heat wraps 24 hours after exercise. Heat wraps were placed on the long axis of the quadriceps bilaterally for 8 hours.
Statistical analyses involved the calculation of means and SDs and analysis of variance (ANOVA). The level of significance was P < 0.05.
Blood analytes were measured and the data corrected for changes in serum volume after exercise as per Van Beaulmont et al.37 First, we corrected the hematocrit from venous blood to the true whole-body hematocrit, by multiplying the venous hematocrit value by 0.873. The change in plasma volume after the first day was then calculated, to correct for any shifts in plasma volume that would have affected the concentration of analytes in subsequent measures. The formula we used is shown below:
Ca = final analyte concentration,
Hct1 = hematocrit on the control day,
Hct2 = hematocrit on the test day,
Cb = analyte test concentration.
Changes in total plasma content of electrolytes and proteins with maximal exercise.37
There was a significant reduction in the quadriceps strength on the first day after the exercise in the control group (Figure 2). This reduction (P < 0.01) was 23.8% less than the resting (before exercise) strength. For the heat 24-hour group, there was no significant difference in any measure compared with the control group (ANOVA, P > 0.05). In contrast, the heat immediate group showed no significant reduction in strength from rest on any postexercise day (ANOVA, P > 0.05).
As can be seen in Figure 3, all subjects showed an increase in pain after the exercise. The pain peaked by 2 days postexercise. The least pain was felt 1 day postexercise and was in the heat immediate group. There was no difference between the other 2 groups after the first day (P > 0.05), whereas the heat immediate group was significantly lower on the pain scale than the other 2 groups on the first and second days postexercise (P < 0.01). By the third day postexercise, there was no significant difference between the groups (P > 0.05).
Knee Flexion Pain
The knee was passively flexed through full range of motion, and the point where, if any, pain was felt was recorded (Figure 4). Pain was felt on flexing the knee at less than full range of motion on the first, second, and third days postexercise. The only significant difference was seen on the first day postexercise where the heat immediate group could flex their knees much further without pain than was seen for the other 2 groups (P < 0.05). By the second day postexercise, the heat immediate and 24-hour groups showed no significant difference from the resting data (P > 0.05), whereas the control group was significantly less than at rest (P < 0.05).
Force to Passively Move the Leg
Figure 5 shows the force measured at 110 degrees of flexion. This measuring point was used because the measurement was well after the start of movement (90 degrees) and when the inertia of the leg was brought into motion and when motion was steady state. At this point, there were some differences in the force to move the leg depending on the leg length and girth of the leg from one individual to the next. Therefore, in this figure, all data were normalized in terms of force to flex the knee before the exercise in each subject. There was no difference in force to flex the leg 1 day after the exercise bout. In the group that had heat immediately after the exercise, force stayed constant over the next 2 days. For the group that had no heat, force to move the leg increased significantly in the second and third days (P < 0.01). For the group that had heat applied 24 hours after the exercise, force was significantly less in the second but not third day (P < 0.05). For the control group, force to move the leg stayed constant for the first day after exercise and then increased significantly (P < 0.01).
Figure 6 illustrates the hysteresis curve for the same measurement. The force to flex and extend to the 110 degree point was different (hysteresis). For the 2 groups that received heat, hysteresis stayed constant over the 4-day period. But for the control group, there was an increase that peaked on the second day postexercise and was still significantly higher than rest at the last day of measurements (P < 0.01).
The average hematocrit started at 41.8% ± 5.2%. After exercise, the hematocrit increased in the control group to 43.9% ± 6.1%, a significant increase (P < 0.01). This value was 106.0% of the resting data and was equivalent to a reduction in plasma volume of 11.5%, a significant hemoconcentration. When heat was applied just after exercise, there was a 3.9% reduction in hematocrit or, in other words, a shift in water from the cells into the blood by about 6.9%. The reduction in hematocrit with heat at 24 hours was only 2% but still showed significant hemodilution (P < 0.05).
The average corrected myoglobin before exercise was 33.0 ± 4.6 μg/L of blood. The largest increases in myoglobin were in the control and heat 24 groups. These changes in myoglobin (percent above rest in Figure 7) were significantly higher than baseline but not different from each other (P > 0.05). Heat immediately after exercise did allow for a significant difference from the resting data (P > 0.05).
Delayed-onset muscle soreness is a common problem in people who exercise infrequently or even in athletes who exceed their normal workout.38,39 For the last 3 decades, the DOMS phenomenon has gained a considerable amount of interest among researchers and specialists in exercise physiology, sports, and rehabilitation.40 There has been a variety of published studies investigating this painful occurrence regarding its underlying mechanisms, treatment interventions, and preventive strategies.8,11,41–49 However, it is evident from the literature that DOMS is not an easy pathology to quantify, as there is a wide amount of variability between the measurement tools and methods used to quantify this condition.40 It is obvious that no agreement has been made on one best evaluation measure for DOMS, which makes it difficult to verify whether a specific intervention really helps in decreasing the symptoms associated with this type of soreness. Even though needle biopsies of the muscle and blood levels of myofiber proteins might be considered a gold standard to some,40 large variations in some of these blood proteins have been documented,40,50 in addition to the high risks sometimes associated with invasive techniques. Thus, it can be seen that muscle soreness is somewhat ambiguous because many studies depend on measuring soreness using a VAS.44,51–54
Some studies call for cold immediately after exercise,55 whereas some call for heat,56 some diathermy,11 and others modalities such as ultrasound,57 massage,58 or changes in amino acid food intake59 to reduce DOMS. Cold is another possible treatment.60 The NSAIDs and steroids are also common for pain.17
In the present investigation, a standard exercise regimen was followed by all subjects and heat was standardized and applied for 8 hours either immediately after exercise or 24 hours after exercise. Generally, this differs from standard therapeutic modalities such as hydrocolator heat wraps, diathermy, or whirlpool heating, all of which are applied for less than 30 minutes.7 The advantage of ThermaCare heat wraps is that they slowly raise internal tissue temperatures and maintain the temperature for hours.35 This is especially important because if individuals have thick subcutaneous fat, a 20-minute heat modality may not even penetrate the skin and get into fascia and muscle.19,22 This is important because the legs and arms are shell tissues and as such have a temperature well below that of the core.61,62 Heat increases metabolism in tissues.7,63 This should cause healing to occur more quickly.64,65 Heat also increases tissue blood flow.31,66,67 This also should wash away metabolites and allow quicker healing postexercise. Another benefit of heat is that it reduces pain.12,68
Here, the only modality studied was heat. When it was used immediately after exercise, pain was less the next day and the reduction in muscle strength was less than was seen in the control group. Similar findings were seen in a previous study of heat wraps.69 However, this earlier study, although showing similar results, that is, better recovery after exercise with heat than with cold, was limited to self-reported pain and disability, and heat was not used until 18 hours postexercise. In the present investigation, numerous objective and subjective measures of DOMS were made.
The results in the present investigation imply much less muscle damage with heat applied immediately after exercise. It is not clear why heat was not applied immediately after exercise in previous studies. The first objective evidence of less tissue damage is in the continuous passive movement data on the leg. Here, the hysteresis remained constant for 3 days after the exercise if heat was applied immediately after exercise. Hysteresis in the flexion extension curve is a measure of stored elastic energy in the muscle and its tendons.70 Usually it takes more force to flex than extend the leg due to the fact that elastic energy is imparted in the series and parallel elastic components when the knee is flexed.71–74 One published effect of heat is to remove stiffness in muscle and joints.71,74 For the controls, the muscle stiffened in the second and third days after the exercise as shown by the increased force difference from flexion to extension. It is reasonable to assume that this is related to muscle or soft tissue damage in the quadriceps or its tendons. For the control group, it also took more force to move the leg on the second and third days, also implying more tissue damage. This was also seen in the myoglobin data. If heat was used immediately after exercise, myoglobin was unchanged from resting data. But with heat delayed to 24 hours, myoglobin increased to the same level as the controls showing muscle damage.
The subjective data were similar. Algometer data and range of motion data agreed with this premise that heat was helpful in preventing and healing potential damage. The best use of heat was for 8 hours after exercise because soreness was less as was the apparent muscle and tendon damage.
Although most studies recommend cold after heavy exercise,75 here we used heat. The premise is that cold will reduce swelling and therefore reduce tissue damage.75 And yet, despite routine use of cold after exercise, studies show little effect on reducing muscle damage, although most studies have different temperatures for cold and different durations of cold.14 The fact that heat seems to reduce damage if used immediately but not 24 hours later is probably related to the mechanism for DOMS. There are 6 existing theories for the cause of DOMS. These are lactic acid accumulation,76 muscle spasm,77 microtrauma,78 connective tissue damage,79 inflammation, and electrolyte efflux.80 It would be hard to believe that lactic acid accumulation could cause a change in the mechanical properties of muscle (eg, stiffness and strength). The same argument would hold for electrolyte efflux. The fact that the hysteresis curve changes so dramatically would be a strong indicator of traumatic damage to muscle or connective tissue. But if it was to connective tissue, then myoglobin would not increase in plasma because this comes from muscle. Thus, the evidence here supports a muscle damage theory with DOMS.
The fact that heat works well when applied immediately but not 24 hours later would also show that once there is cellular disruption in muscle, damage has occurred and is hard to stop. Although heat helps at 24 hours, this may be due to increased healing rates from higher temperatures of tissue. The fact that the results were less for the same commercial heat wrap in the Mayer study12 is probably due to the fact that they waited 18 hours postexercise to apply the heat wrap.
There were some limitations to the study. First, measures of damage were only made every 24 hours and only for 3 days. Subjects were also young and somewhat fit. Data may be different on fitter subjects such as elite athletes and older subjects or subjects with diabetes where blood flow is not increased as much with heat in tissue,65,75,81,82 and further investigation is needed. Here, there was no difference in the response of men and women studied here. But these were young women, and the effect of estrogen and the birth control pill on motor control and movement are pronounced,55,71–73 and further investigation is needed. From these data, continuous low-level heat packs just after exercise seems to be a good recommendation for the practitioner. Heat before exercise has been poorly studied, but it also seems to reduce muscle damage and injury. Unlike active warm-up, passive warm-up involves heat or heat-producing sources such as infrared or ultrasound. The idea is that the benefits of active warm-up can be achieved by passive heat. Heat increases circulation8,22,66,83 and tissue flexibility71 and increases metabolism. In one study, when examining the effect of warm-up on muscle strength, for strength, passive warm-up was just as effective as active warm-up to increase strength.84 For swimming, performance was increased with either an active or a passive warm-up by similar amounts.84 Jumping performance also increased with either an active or a passive warm-up. The evidence on passive warm-up using heat to increase performance and prevent injury is weak.
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