Light intensity physical activities, including slower paces of walking, do not raise the heart rate sufficiently to be considered aerobic forms of exercise for cardiovascular health.^1–5 A systematic review of 72 studies noted that light intensity physical activities are associated with a beneficial acute and long-term lower risk of mortality. However, the authors stressed that there is an absence of an established biological mechanism underlying benefits, which inhibits clinical recommendations.¹

Current modalities of exercise are divided into two primary categories, aerobic and anaerobic exercise. Unless sped up to achieve an increased heart rate, exercises such as forms of tai chi, qigong, and slower-paced walking do not generate aerobic benefits. Possibly due to a need to fit tai chi into an existing category of exercise, tai chi is often categorized as a moderate intensity exercise.^6,7 However, existing evidence does not support tai chi as an effective way for increasing aerobic capacity.⁶ This begs the question, what is happening in the body during slower-paced exercises, beyond benefits for stress reduction?^3,8–11

Previous research and experience with tai chi indicated that slower-paced light intensity exercises might have a beneficial effect on oxygen use in the body, which might benefit conditions affected by hypoxia.^12,13 Preliminary investigations documented a slight increase in blood oxygen saturation (SpO2) during tai chi (mean percent change + 2.12%±1.25%; P < .001). But what was particularly notable was the large momentary drop in SpO2 following tai chi (mean = 90.78%±2%; P < .001). No significant change was noted during or following running.¹³

This prompted the question, if any slow-paced or light intensity exercise might result in a similar momentary significant post-activity drop in SpO2? And what if any are the implications? There is also the question of sleep. Sleep also involves a state of relaxation and slower respiration, similar to tai chi, qigong, and slower paces of walking. Sleep also results in periodic brief drops in SpO2 below 92%, although these drops are currently considered insignificant.¹⁴

A Nonin WristOx 3150 was used to record SpO2 levels. Unlike many oximeters, which can miss brief changes in SpO2 level, the WristOx 3150 can record changes in SpO2 at 1 second intervals with an accuracy of +/- 2%. This instrument is regularly used in military testing due to its durability and reduced error during challenging conditions. The motion tolerant software minimizes motion artifacts from being misinterpreted.^15,16

Initial measurements were taken of 11 participants at a fitness center. Ten of these participants reported being in good shape. One participant reported having been diagnosed with prostate cancer 3 months previously (male; age 77) and was excluded from the fitness center group, for independent comparison (see Fig. 1). The participants in the fitness center group ranged in age from 23 to 68 (mean 52.7 years). Participants in this group walked on a treadmill at 1.5 mph for 20 minutes, followed by 5 minutes of rest.

Additional measurements were taken of 10 members of a tai chi group for comparison, during and for 5 minutes following walking at 1.5 mph, tai chi (Yang style), and qigong (baduanjin). All members of this group noted a chronic or medical condition, including type 2 diabetes, recovering from hernia surgery, high blood pressure, chronic kidney disease, high cholesterol, chronic pain and neuralgia, chronic cytomegalovirus disease, abdominal cysts, lupus, and peripheral neuropathy. One participant who had high blood pressure also had asthma, and another who had type 2 diabetes also reported sleep apnea. But both participants reported the condition as well controlled and not impacting respiration during the measurements. No other respiratory conditions were reported. Measurements were also taken for comparison of the lead researcher, who has celiac disease and associated celiac neuropathy, including during sleep and moderate intensity aerobic exercise (using an exercise bike). Measurements following moderate aerobic exercise were taken until heart rate returned to resting levels. Sample measurements of respiration rate were taken during activities with a Go Direct Respiration Belt.

The research protocol was reviewed and approved by Mercer University's Institutional Review Board and the Office of Research Compliance, with all participants completing an informed consent form. Data was analyzed by the authors.

The fitness center group did not show a clearly distinct drop in SpO2 (Table 1), except for a potential post exercise drop in one participant (Fig. 1). The older participant with prostate cancer (excluded from the fitness center group) experienced a large momentary drop to 86% before returning to normal levels. See Fig. 1 for comparison.

However, the tai chi group experienced a momentary significant post activity drop in SpO2 before returning to normal levels (P = 0.001), dropping to 89.30 ± 1.70% following tai chi; 88.40 ± 1.43% following qigong; and 89.90 ± 2.02% following walking. SpO2 dropped for approximately 25 seconds to 2.5 minutes, within 5 minutes of stopping the activity. No significant change occurred following moderate aerobic activity. Measurements during sleep showed similar periodic drops. See Table 1 for statistical results, and Figs. 2 and 3 for sample graphs.

Hypoxia underlies or complicates a wide range of chronic conditions.^12,17 Hypoxia consists of lower levels of oxygen content compared to normal states in organs, specific tissues, and cell types.¹⁷ Hypoxemia, low levels of oxygen in the blood, can be affected by blood flow to the lungs (perfusion), ventilation or airflow to the alveoli, and gas exchange through diffusion.^18,19 The latter is particularly relevant since enhanced perfusion and diffusion may affect SpO2 levels. Oximeters tend not to be impacted by cellar oxygen use, since venous blood is reoxygenated by the lungs, usually returning to normal levels above 94%.²⁰ However, some conditions, including chronic obstructive pulmonary disease (COPD),²¹ and extreme athletic performance,²² can result in low arterial blood oxygen saturation (SpO2) levels below 94%.

During the practice of tai chi and qigong, which are consciously focused on relaxation and the breath, it is common to feel a tingling in the extremities. Similar to when a limb falls asleep due to a lack of circulation, but pleasant instead of painful. It is suggested that this pleasant tingling is a result of enhanced oxygen perfusion. SpO2 levels below a certain threshold (typically 92%) can indicate that the body is not adequately oxygenated, signifying hypoxia/hypoxemia.²³ However, in the case of the slower-paced exercises measured in this study, and in previous research,¹³ the timing and short duration of these drops suggests a state other than hypoxia or hypoxemia.

Oxygen levels in the cells, in relation to hypoxia, can affect cellular metabolism and physiological function, through various mechanisms which underlie cellular oxygen sensing which allow cells to adapt to changes in oxygen availability.²⁴ The relaxed state coupled with slower respiration during slower-paced walking, tai chi, qigong, and during stages of sleep, may affect oxygen sensing mechanisms related to the increase in oxygen diffusion and perfusion, stimulating enhanced cell metabolism, and supporting healing and cellular function. Other similarities between sleep and relaxation exercises such as tai chi is a reduction in blood pressure.^10,14

That the individuals in the fitness center group with no reported health conditions did not experience a clear drop in SpO2 following walking raises interesting questions. Being within the normal range of SpO2, any potential drops may not as be as noticeable. The healthy participant in Fig. 1 showed a potentially clear drop in SpO2 to 94%, but not to the degree of the tai chi group, nor the participant with prostate cancer (Figs. 1 and 2). If the level of the momentary post-activity drop in SpO2 is tied to the health of the person, healthier individuals may experience less of a drop in SpO2, compared to individuals with chronic conditions. The participants who experienced the large post-activity momentary drop in SpO2 were all older adults, which may indicate a greater need for ongoing cellular regeneration due to aging. There is also the question if the level of relaxation plays a role. One of the fitness center participants noted how difficult it was to force themselves to keep to a slow pace. However, the participant with prostate cancer experienced a notable momentary post exercise drop to 86% but did not do tai chi.

Results support a potential category of exercise distinct from aerobic exercise, suggested as a “metaerobic” response, due to indications that relaxation combined with slower respiration engendered by various activities, may enhance vasodilation and oxygen perfusion, triggering a metabolic state which may enhance healing and cellular regeneration. These effects seem to be distinct from aerobic and anaerobic effects (Fig. 4). This may explain the momentary drop in SpO2 below normal levels, shortly after stopping the activity, possibly due to a shift from enhanced perfusion back to normal perfusion, oxygen use, and metabolism. A metaerobic effect may also occur during other activities as well, such as during sleep (see Fig. 3) and sitting meditation (discussed below).

The primary premise being considered as metaerobic theory, is that the momentary drops in SpO2 following slower-paced exercises, and periodically during sleep, may be a result of enhanced oxygen perfusion and a shift in the use and metabolism of oxygen, resulting from a more relaxed state and slower respiration. Potentially in response to hypoxic areas of the body, and a need for enhanced healing and cellular regeneration. In relation to sleep, this contrasts to current theory that lower levels of SpO2 during sleep result from more shallow respiration, due to the body needing less oxygen during sleep, resulting in hypoxemia. However, based on Fig. 3, periodic drops in SpO2 may be consistent with the transition between stages of sleep and may signify a metaerobic effect.

In a review by Elkehenany and others, it was noted the healing process is enhanced during sleep, but also that mechanisms which underly faster healing during sleep has not been studied to any satisfactory extent.¹⁴ The authors note that the literature states benefits of sleep may be due to mild hypoxic states, which they also note does not make sense. One current explanation for why hypoxia occurs during sleep is that mild to low levels of oxygen might minimize damage caused by oxidation.^25,26 On the other hand, this theory is not universally accepted, since hypoxia has also been reported as increasing rather than reducing oxidative stress.^26,27 So an alternative explanation or hypotheses may be metaerobic theory. Ongoing levels of mild hypoxia during sleep may be a result of a notable increase in oxygen demand related to cellular regeneration and healing, rather than as a result of reduced oxygen intake during sleep.

During tai chi, respiration slowed down from the 12 to 20 breaths per minute typical in healthy adults during rest,²⁸ to as low as 6 to 10 deep breaths per minute during tai chi, which is supported in the literature.^29,30 Breathing rate was also observed while walking at 1.5 mph in the current study to fall between 10 to 12 breaths per minute. Stages 2 and 3 of sleep result in a similar reduction of breathing rate.³¹

The generally lower levels of SpO2 during sleep (Table 1 and Fig. 3) may be the result of less demand for oxygen by the large muscle groups, which occurs during wakefulness. During sleep there is a reduction in skeletal muscle blood flow.^14,32 Under awake resting conditions, oxygen extraction in the muscle tissues ranges between 20% and 40%, while during heavy exercise this demand can increase to 70–80% of the oxygen carried by the blood.³³ The relaxed state of slow-paced exercise and sleep may reduce the muscular need for oxygen and enhance oxygen use by the rest of the cells of the body. Particularly in the organs, which are more prone to disease. Cancer development is very rare in skeletal muscle tissue.³⁴ One reason relates to the larger demand for oxygen and the increased blood flow and perfusion in skeletal muscle tissue. Sleep and relaxation focused exercises may lead to an increase in oxygen delivery to the organs, compared to the musculature during more active exercises, and during waking hours.

Relative to the role of the skeletal muscles in oxygen use, is a study which documented greater declines in SpO2 during walking than when cycling, in patients with COPD.³⁵ Desaturation of approximately 7% occurred during a treadmill test, compared to 3% during a cycling test. The authors had difficulty explaining the higher declines during walking than during cycling due to the greater muscle demand during cycling. Based on metaerobic theory, the higher level of desaturation occurring during walking in patients with COPD might be due to greater oxygen perfusion and metabolism during periods of healing and cellular regeneration. It may be worth noting that in a COPD sleep graph showing SpO2 levels over a 3-hour period, within the generally lower oxygen levels, periodic momentary drops similar to those in Fig. 3 occurred.²¹

Another area to consider, which may support a metaerobic premise, is sitting meditation, which also focuses on relaxed breathing. Sitting meditation has been associated with a range of physical health benefits, including benefits for oxidative stress, telomere length, and associated reduction of cellular aging. However, how meditation affects these factors is unclear.³⁶ Metaerobic theory may offer one explanation. In the lead author’s practice of tai chi and qigong, minor modifications to enhance the relaxation aspect, while also increasing the amount of practice, corresponded to a reversal of neuropathy symptoms.

However, various limitations and cautions are worth noting. The first being the small number of participants in the current study, and that the participants in the tai chi group were all older adults with chronic conditions. It is recommended to replicate these measurements with a larger number of participants in various age cohorts, including those who do not do tai chi, as well as for a wider range of practices, such as sitting meditation. Also to be considered is the potential of undocumented COPD or other respiratory conditions in the participants in the tai chi group. Another limitation is the lack of sleep graphs in the literature documenting SpO2 for healthy adults for comparison. In speaking with different sleep labs and researchers, periodic drops in SpO2 in healthy participants occurred similar to those experienced by the lead researcher, but they were not able to share the graphs due to privacy concerns.

The observations presented in this research presents potential relationships, but these relationships may be independent of actual cause and effect. As an exploratory study, possible associations have been presented which might explain the momentary drops in SpO2. Future research will confirm, deny, or provide alternate associations. It will be important to conduct a range of measurements beyond SpO2 which may detect unique effects of tai chi and other light-intensity exercises, potentially through cerebral oximetry, transcutaneous oxygen measurement (TCOM), near-infrared spectroscopy (NIRS) and other measures.

One last and potentially particularly important point. Since SpO2 dropped significantly in the participants with chronic conditions, measurements might be able to be used to detect potential chronic conditions or illness. It might be worth conducting a range of medical evaluations in those without a documented chronic condition, yet who exhibit a large post slow activity drop in SpO2, to see if these drops correspond to an unknown illness or condition. Carlson and others note that declines during sleep in regional cerebral oxygen saturation in older adults from resting baseline may help identify individuals who are at risk for cognitive decline.³⁷

It is also important to note that for overall health, aerobic and resistance exercises are still key for overall health^4,38 However for those who have an inability to engage in vigorous exercise, slower-paced and light intensity exercises may offer a valuable supplement for health. Understanding the physiological mechanisms underlying the benefits for health may enhance more effective research and promotion of tai chi, qigong, and other slow-paced and light intensity exercises, similar to what followed with the understanding of aerobic exercise following the Framingham Heart Study, and the publication of Aerobics.^39,40