Introduction. Moderate-intensity aerobic exercise has been noted to improve wound healing rates in mice and people, but different intensities of exercise may have different impacts on healing rates. It is important to determine the most beneficial exercise intensity for improving wound healing in people with type 2 diabetes to help prevent wounds from becoming chronic, greatly reduce pain and immobility, and lower the high cost of health care associated with treatment.Objective. The purpose of this study is to determine the impact of low-intensity exercise compared with high-intensity exercise in terms of the rate of wound healing in diabetic mice. Materials and Methods. Twenty-one 10-week-old female diabetic mice were randomly assigned to a sedentary control group (CON), low-intensity treadmill exercise (LEX) group, or high-intensity treadmill exercise (HEX) group. Mice were exercised for 30 minutes, 5 days per week, for 3 weeks. Mice were wounded on their upper back with a 3.5-mm punch biopsy instrument, and wounds were photographed at the same time every day. Results. In terms of the length of time it took wounds to fully heal, CON mice healed in an average of 14.4 ± 2.4 days (number of days to decrease to less than 10% of their original size ± standard deviation) and HEX mice in 14.0 ± 3.0 days (P = .396). However, LEX mice healed faster than CON in an average of 10.1 ± 2.3 days (P = .004). Conclusions. In this preliminary investigation, low-intensity exercise accelerated wound healing rates in diabetic mice but high-intensity exercise did not. Future studies should investigate the mechanisms behind this effect and evaluate different intensities of exercise on wound healing in humans with type 2 diabetes. 

Many countries are experiencing tremendous growth in the number of people being diagnosed with type 2 diabetes (T2D). Current estimates in the United States put the total number of people with diabetes at 22.3 million,¹ and the worldwide estimate is 387 million, which is projected to increase to 592 million by the year 2035.² Some with diabetes are unaware of the serious potential health problems associated with poorly controlled diabetes, including increased risk of heart disease, retinopathy and blindness, nephropathy, neuropathy, peripheral vascular disease, and delayed wound healing. In some cases, wound healing is so compromised that limbs must be amputated. Therefore, any method that can help wounds heal faster in people with diabetes is not only of great personal value and reassurance to the individual but also of great importance to the health of the US economy and beyond.  

Several studies^3-5 have reported that moderate-intensity aerobic exercise improved cutaneous wound healing rates in mice and humans. However, the latter authors⁵ suggest that different intensities of exercise have different impacts on healing rates. Indeed, young rats that performed low-intensity exercise on a treadmill had accelerated healing rates,⁶ which raises the important question of what is the most beneficial exercise intensity for improving wound healing in people with type 2 diabetes. This information would help physicians and other health care professionals prescribe the most effective exercise for their patients, with the end result that chronic wounds would heal more quickly, greatly reduce pain and immobility, and lower the high cost of health care associated with treatment. 

Physical activity is important for not only decreasing potential negative health consequences of hyperglycemia but also helping to prevent T2D.⁷ In a recent meta-analysis,⁸ several levels of physical activity, including light-, moderate-, and high-intensity exercise, reduced the risk of developing T2D. In addition, high-intensity training may be effective for decreasing postprandial hyperglycemia,⁹ and low-intensity exercise has been demonstrated¹⁰ to provide a sufficient stimulus to decrease blood glucose levels. Based on these studies and others, it is possible that all levels of exercise intensity can have a positive impact on diabetic wound healing. The purpose of this study is to determine the impact of low-intensity exercise compared with high-intensity exercise in terms of the rate of wound healing in diabetic mice; the investigators hypothesize that both exercise groups would heal faster than sedentary mice. 

Ethical approval. All procedures were approved by the Bowling Green State University Institutional Animal Care and Use Committee (Bowling Green, OH) prior to any data collection. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted.

Animals. Twenty-one 10-week-old female diabetic mice (dbdb or B6.BKS(D)-Leprdb/J) were purchased from Jackson Labs (Bar Harbor, ME). Mice were randomly assigned to a sedentary control (CON), low-intensity treadmill exercise (LEX), or high-intensity treadmill exercise (HEX) group. Sedentary controls were kept in their cages during exercise sessions, without access to food or water, in close proximity to the treadmill, with cage racks attached by clamps so all mice experienced the same vibration and noise. 

Exercise protocol. Exercise was performed on a modified Jog-A-Dog (Ottawa Lake, MI) adapted with lanes for individual mice with protective pads placed on the back of each lane to prevent injury. No electric shock or any other kind of stimulus was used in this study. Low-intensity exercise was performed at 12 meters per minute on a 5% incline while high-intensity was at 18 meters per minute on a 5% incline. This represents about 60% and 85% of maximal capacity for these mice at 10 weeks of age (data unpublished). Mice were exercised for 30 minutes, 5 days per week, for 3 weeks. 

Wound healing protocol. Three days after exercise began, mice were wounded on their upper back with a 3.5-mm punch biopsy instrument (Robbins Instruments, Inc, Chatham, NJ), as previously described,⁴ and wounds were photographed at the same time every day until fully healed (< 10% original size). Wounds were quantified using ImageJ (Version 1.48b; National Institutes of Health, Bethesda, MD), and differences in healing time between groups were compared by independent t test. The pattern of wound healing over time was analyzed by repeated measures analysis of variance (ANOVA) and differences between groups by Tukey post hoc test (SPSS Version 22; IBM, Armonk, NY). Significance level was set at α = 0.05. 

The CON mice healed in an average of 14.4 ± 2.4 days and HEX mice in 14.0 ± 3.0 days (number of days to decrease to less than 10% of their original size ± standard deviation). However, LEX mice healed in an average of 10.1 ± 2.3 days (Figure 1). 

There was no statistical difference between the CON and HEX groups (P = .396), but there was a difference between CON and LEX mice (P = .004) and between LEX and HEX groups (P = .015). Figure 2 shows an example of the difference in wound size in a LEX mouse and a CON mouse. 

The pattern of healing (ie, size of the wounds over time) also was different between groups (Figure 3). Repeated ANOVA measures indicated there was a significant group main effect (F[1,18] = 4.7; P = .023). Tukey post hoc analysis indicated the LEX mice had significantly smaller wounds over time compared with the CON and HEX mice (P = .034). There was no statistically significant difference between the CON and HEX groups in terms of healing pattern (P = .980). 

In these diabetic mice, LEX mice healed faster than CON mice; however, the HEX mice did not. Several studies^3-5,11 have reported improved healing rates with moderate-intensity exercise. In addition, a recent study⁶ reported accelerated wound healing with low-intensity exercise in rats instead of mice. In this study, it was hypothesized that both LEX and HEX would accelerate healing, but that hypothesis was only supported in the LEX group. To the best of the authors’ knowledge, the present study is the first to show improvements related to low-intensity exercise but not for high-intensity exercise. 

Low-intensity exercise in people with diabetes can decrease elevated blood glucose levels,¹⁰ among many other potential positive health benefits such as burning calories, decreasing total fat, adding muscle, and other beneficial effects. Since hyperglycemia and insulin resistance can delay healing,^12,13 low-intensity exercise in these mice may have improved healing through decreased blood glucose and increased insulin sensitivity. In addition, because moderate exercise decreased some inflammatory cytokines in wounds of aged mice⁴ and may decrease systemic markers of inflammation in other populations,^14,15 an improved cytokine profile in the wounds and/or blood of these diabetic mice may have allowed faster healing. Future studies should evaluate the impact of reduced hyperglycemia and reduced inflammation on improved healing resulting from exercise. 

In contrast to the accelerated healing rate in the LEX mice, there was no significant change in the HEX mice. There are likely multiple reasons for this result. One possibility worthy of further exploration is that high-intensity exercise can be stressful to the mice and elicit a corticosterone response. Corticosterone (ie, cortisol in humans) can have a negative effect on wound healing rates,^3,16 in addition to other negative effects on physiology, and may have slowed healing in HEX mice (however, this was not measured in this study). Indeed, there are many factors that can delay wound healing^17,18 that may have mitigated the positive effects found in low-intensity and moderate-intensity exercise. The mechanisms behind exercise-induced changes to the complex processes involved in wound healing are not yet clear and make an exciting area of research.

High-intensity physical activity, including high-intensity interval training, has been associated with increased insulin sensitivity and better glucose tolerance.^19-21 However, in the present study, insulin sensitivity was not measured and the high-intensity exercise was continuous, so it is unknown whether or not there was a beneficial effect to improving insulin resistance.

Similar results have been reported⁵ in young mice that were not diabetic. In that study,⁵ moderate exercise (70% of maximal capacity) improved wound healing, but 80% and 90% of maximal capacity were not as effective. Moderate-intensity exercise resulted in better wound closure and more mature granulation tissue, but high-intensity and strenuous-intensity exercise had delayed contraction.

When comparing the healing patterns between groups (Figure 3), the HEX pattern diverges early on from CON, with HEX wound sizes increasing for a couple days. This pattern also is seen in aged Balb-c mice.⁴ As mentioned earlier, a stress response mediated by corticosterone could lead to this delay. In addition, there is a difference in the LEX healing pattern compared with the CON on days 5 to 10.  There are a number of possible explanations for this more rapid decrease in wound size in the LEX mice, including a change in the balance of pro-inflammatory versus anti-inflammatory cytokines in the wound environment.²² Another potential reason the investigators hope to explore further in future studies is the effect of exercise on macrophage function and polarization^23,24 (ie, M1 to M2 shift). The role of exercise in altering markers of oxidative stress and angiogenesis also could provide valuable insight into these effects. 

In the present study, exercise intensity was determined by maximal running speed rather than measuring maximum volume of oxygen (VO₂max). However, there is a close correlation between running speed and VO₂max, and the former can be used as a reasonable substitute when expensive and time-consuming VO₂max studies cannot be performed.²⁵ It is important to note that the exercise intensity in the present study is an estimate based on running speed rather than a direct measure of oxygen consumption. 

Some criticism²⁶ has been expressed of using mice as a model of wound healing because there may be some differences in healing patterns between mice and humans. However, the evaluation of this claim by Chen et al²⁷ suggests the mouse excisional wound model is a valid and useful one that heals by both reepithelialization and contraction.

In the diabetic mice used in this preliminary investigation, wounds healed faster in mice that exercised at a low intensity than in those that remained sedentary. In addition, mice that exercised at a high intensity did not heal faster than sedentary mice. Based on these results, low-intensity exercise should be recommended to improve the healing of wounds in T2D, but high-intensity exercise may not improve healing rates. The effect of exercise on the diabetic mice in the present study should be evaluated in humans with T2D.  Also, to establish the mechanisms of these findings, future research should evaluate inflammation, macrophage function, angiogenesis, and other factors (eg, oxidative stress) potentially affected by exercise of differing intensities, durations, and modalities. 

Affiliation: Bowling Green State University, Bowling Green, OH

Correspondence: Todd Keylock, PhD, Associate Professor, Exercise Science, Bowling Green State University, 215 Eppler South, Bowling Green, OH 43403; tkeyloc@bgsu.edu

Disclosure: This study was supported by a Medical Mutual of Ohio and Bowling Green State University Center of Excellence for Health and Wellness Across the Lifespan grant.