As shown in Figure 1, there was a reduction in strength on the day after exercise in the control group. This significant reduction (p < 0.01) was 23.8% less than the resting (before exercise) strength. Both heat immediate and cold immediate groups also had a significant reduction in muscle strength of 4.5% by the first postexercise day and there was no significant difference between the groups, but both hot and cold application groups had significantly more strength than the control group on this day. The heat immediate group recovered by the second day after exercise to the pre-exercise strength, whereas the cold immediate group still showed a significant loss in strength at 3 days after exercise (p ≤ 0.05) compared to the pre-exercise strength.
When cold and heat were applied 24 hours after exercise, as shown in Figure 1, there was no statistical difference in strength between the 3 groups at rest or 1 day after exercise. The group that had cold applied after 24 hours appeared to recover their strength faster than the group with heat applied at 24 hours, with the slowest recovery in the control group. However, there was no statistical difference between the recovery in these 2 groups at day 2 or 3 after exercise (p = 0.4). Both groups had faster recovery than did the control group (p ≤ 0.05).
The results of analog visual pain scale are shown in Figure 2. As can be seen in Figure 2, all subjects showed an increase in pain the day after the exercise. The pain peaked by 2 days after exercise. The least pain was felt 1 day after exercise and was in the heat immediate and cold immediate groups; there was no statistical difference between the hot and cold groups 24 hours after exercise. By the second and third day after exercise in the cold and heat immediate groups, the fastest recovery was observed in the cold immediate group.
For the cold and heat at 24 hour groups, there was no statistical difference between the control and heat at 24 hours groups. However, for the cold at 24-hour group, there was less pain 2 days after exercise than in observed the control group, as shown in Figure 2.
The force needed to flex the knee was measured from the knee at 90–125°. Figure 3 shows the force that was measured at 110° of knee flexion. This measuring point was used because the measurement was well after the start of movement (90°) and when the inertia of the leg was brought to constant motion and was steady state. At this point, there were some differences in the forces 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 the force to flex the knee before the exercise in each subject. There was no difference in the force to flex the leg one day after the exercise bout. In the group that had heat or cold immediately after exercise, the force stayed statistically constant over the next 2 days. For the group that had no heat treatment, force to move the leg increased significantly on the second and third day (p < 0.01). For the groups that had heat or cold applied 24 hours after exercise, the force was the same on the first, second, and third days after exercise (p > 0.05).
Figure 4 shows the hysteresis curve for the same measurement. The force to flex the knee at the 110° point and to allow it to extend to the 110° point is different. This difference is called the hysteresis. As shown in Figure 4, for the 4 groups that received heat and cold treatment, the hysteresis stayed constant over the 4-day period. But for the control group, there was an increase in the difference between the force of flexion and extension that peaked on the second day after exercise and was still significantly higher than the rest at the last day of measurements (p < 0.01).
The same was true for the heat and cold at 24-hour groups. Unlike the controls where hysteresis increased in the second and third day after exercise, for the heat and 24-hour–cold and 24-hour–heat groups it stayed constant over the 4 day period.
Figure 5 shows the change in myoglobin of the serum after exercise. The average myoglobin before exercise was 33.0 ± 4.6 micrograms per liter of blood. The largest increases in myoglobin were after dry, moist, and cold at 24 hours. These changes in myoglobin (% above rest in Figure 5) were significantly higher than the baseline but not different form each other. (p > 0.05) Heat and cold immediately did allow for an increase in myoglobin, but it was not significantly different than the resting data (p > 0.05).
There is controversy as to the effectiveness of cold and heat after strenuous exercise (1,4,11,50). Some studies show that heat is better, whereas others favor cold, and others show little effect of either (5,16). The problem seems to be that when comparing heat to cold or examining either, there is no clear definition of what heat is or what cold temperature should be and the population of people being studied. Thus the literature is in total confusion. Another complicating factor is the duration of cold and heat. The issue is one of physics and heat flow. Warming the muscles to the core temperature is a slow process (27,33). First, subcutaneous fat buffers the gains in heat from the skin to the muscle (28,33). Second, basic heat flow equations show that the steeper the gradient the greater the heat flow (27). But if heat is too elevated above the core, the skin can burn (14,44). This limits rapidly applied heat modalities such as hydrotherapy to 20–30 minutes exposures (37). The same argument can be made for cold treatment except that cold can be tens of degrees less than the core temperature so that heat flow from the muscle can be more rapid. But there is still a vast difference in the tissue penetration of hydrotherapy vs. 50° cold packs vs. therapy ice packs separated from the skin by 6–8 layers of towels (37). Therefore, in this investigation, we used ThermaCare cold wraps (ice) directly on the skin for 20 minutes, and for heat treatment, we used ThermaCare heat wraps applied for 8 hours to standardize the heat and cold application. The 8-hour exposure allows the heat to gradually penetrate into deep tissue and keep it warm for hours. The exercise was standardized and all subjects were students with similar exercise training, age, and BMI so that variability in exercise and subjects could be eliminated to make a proper comparison of the effects of cold and heat.
Compared to the control subjects, who were very sore and lost a lot of strength after exercise, cold or heat both were both effective in reducing muscle damage and pain. Control subjects lost almost 24% of their strength in the days after exercise. The results show that to preserve muscle strength after strenuous exercise, heat immediately applied after exercise was best. In addition, using heat immediately after exercise seemed to result in the least damage to muscle, as assessed by myoglobin and the force of passive movement hysteresis curve. If heat and cold are not applied until 24 hours later, the reverse is true; cold is better than heat in preserving strength and reducing tissue damage.
For reducing pain, the control subjects were very sore in the days after exercise. Cold immediately after exercise or 24 hours later was superior to heat in reducing pain, alhough both did reduce pain. Pain measured by a visual analog scale showed that soreness was reduced with cold immediately or at 24 hours and was more than it was found with heat, although both were superior in reducing soreness compared to the control subjects who did nothing.
The effect of heat and cold on pain is well documented. There are both voltage-gaited calcium hot and cold sensitive channels that interact with P2X2 purine channels that modulate pain in peripheral tissue (6,8,9). Although heat or cold is applied, the P2X2 pain receptors are inhibited and hence pain is reduced. But here pain was measured 24 hours after each modality and not during the application of heat and cold. In all probability, the effect on pain is caused by a reduction in tissue damage with heat or cold, causing less pain 24 and 48 hours after exercise. Strength data supports this fact, but the preservation in strength with cold and heat was best with heat, and pain reduction was greatest with cold. Therefore, there may be a carryover effect on pain seen here. Heat has been shown to have a carrier over pain relief 24 hours after heat is removed, (25,49) and perhaps there is some receptor inhibition for both heat and cold.
The difference in this study of cold and heat that allowed us to see benefits is that here we used a uniform exercise and subject population to test heat and cold. Mayer et al. (22) also used ThermaCare heat wraps but did not apply them until 18 hours after exercise. Their measure of effectiveness was subjective, and no hard measures were taken, although there seemed to be good results. Here objective and subjective measures were used; most previous studies of heat and cold used objective measures. In other studies, athletes were examined with ice baths. Ice baths with lower-body immersion seem to help, but they also cause a release of free radicals in the body, thus increasing whole-body inflammation. Further, in many studies lower-body cold immersion was for less than 5 minutes (5). This is not enough time for cold to penetrate into deep tissue (37). The same is true of contrast baths, which are too brief for either heat or cold penetration (31,39).
The results of this study however show what can happen with matched exercise and age of the subjects and condition of the subjects. Athletes, the elderly people, and people with diabetes may respond differently.
For some reason in athletics, it is believed that cold after exercise is the best modality to prevent swelling and damage to muscle. At least for this age group, this is not true. There is a definite advantage of heat after exercise, if applied or used immediately. Cold and heat both prevent muscle damage, but on balance, heat actually has small advantages over cold in increasing healing after a heavy workout. The target population here was college undergraduate- and graduate-age students. Data would need to be collected on other population or even in this age range to examine these effects in athletes.
This work was supported by a contract from Pfizer pharmaceuticals under contract number WI173615.
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