Individuals can do either moderate-intensity or vigorous-intensity aerobic activities, or a combination of both. It takes less time to get the same benefit from vigorous-intensity activities as from moderate-intensity activities. In general, 2 min of moderate-intensity activity is the equivalent of 1 min of vigorous-intensity activity (Physical Activity Guidelines Advisory Committee 2008). It is also possible to benefit from interspersing faster intervals into any low- or moderate-intensity workouts to make greater gains in fitness levels (Johnson 2006, 2008).
Engaging in moderate- to vigorous-intensity physical activity is generally recommended to achieve aerobic and metabolic improvements in people with diabetes (Sigal 2004, 2006; Colberg 2010). Lower-intensity activities will expend calories and help with weight maintenance, but they may or may not have much of an acute impact on blood glucose levels or enhance cardiovascular fitness as much (Duncan 2005, Hansen 2009). Appropriate measures of intensity include the “talk test,” perceived exertion, and target HR.
“Talk test.” When starting out with an exercise program, the easiest measure of intensity for anyone to use is the “talk test.” An individual should be able to carry on a conversation during aerobic activity without struggling to breathe. If he or she is breathing too heavily to talk, the intensity has exceeded the ventilatory threshold and is harder than a moderate workload. The only drawback to the use of this test is that it cannot discern when exercise intensity is too low, but upon starting an exercise program, it is safer for the individual to err on the low side than on the high side.
Perceived exertion. Use of perceived exertion is another easy way for individuals to estimate their exercise intensity. This is a subjective rating based on general fatigue and can be used along with target HR estimations or as a substitute to guide the intensity of activity. Several scales, including the Borg scales, can be used to assign numbers to intensity levels, or individuals can simply state how they feel. When subjective perceived exertion is used, a person should focus on full-body feelings of exertion and general fatigue. Generally, a moderate- to vigorous-intensity exercise corresponds to a subjective rating of “somewhat hard” (for moderate) to “hard” (for vigorous) (Garber 2011). Less fit people generally require a higher level of effort than fitter people require to do the same activity. Relative exertional intensity can be estimated using a scale of 0 to 10, where sitting is 0 and the highest level of effort possible is 10. Moderate-intensity activity is a 5 or 6 out of 10. Vigorous-intensity activity is a 7 or 8 (Physical Activity Guidelines Advisory Committee 2008).
Target HR. Exercise intensity can be prescribed and monitored using HR. The ACSM recommends the following HR measurement: exercise intensity of 40–89% of HR reserve (HRR). (This range roughly corresponds to 64–93% of maximum HR, or HRmax.) The recommended intensity range of 40% to 89% is broad because deconditioned individuals can gain improvements in cardiorespiratory fitness at lower intensities, whereas individuals with greater fitness typically require a higher minimal threshold to achieve similar gains (Garber 2011). Therefore, the prescribed intensity range should be based on an individual’s fitness level, duration of diabetes, presence and degree of complications, and personal goals. Using this method, moderate-intensity exercise is in the range of 40% to 59% HRR, whereas vigorous-intensity exercise is defined as 60–89% HRR (Garber 2011, Physical Activity Guidelines Advisory Committee 2008). A severely deconditioned person can start even lower than 40% HRR (e.g., light intensity is considered 30−39% and very light is <30%) and progress from there to more moderate levels over time.
Exercise intensity can be calculated most accurately if an individual’s maximal HR (MHR) has been determined with an exercise stress test. A true resting HR (RHR) is best measured upon awakening; measuring at other times of day, even at rest, may not yield the same results. The difference between the MHR and RHR is defined as the HRR.
The Karvonen formula, which relies on the calculation of HRR, commonly is used to calculate a single target HR or HR range in bpm (Garber 2011):
HRR = MHR – RHR
Percent HRR (% HRR) = HRR × Desired Intensity
(% as decimal)
Karvonen Formula: Target HR = [(MHR – RHR) × Desired Intensity] + RHR
If someone’s actual MHR has not been measured, target HR ranges can be approximated using the following equation to estimate maximal values:
MHR = 220 – Age
This equation should be used with caution because of the large standard deviation, which can cause the HR estimation to be off by 12–15 bpm (Garber 2011). This procedure may overestimate the maximal HR of some individuals with diabetes, particularly those with autonomic neuropathy (Colberg 2010, 2011). If cardiac autonomic neuropathy is present, exercise intensity is better prescribed using the HRR method with MHR directly measured, rather than estimated, for better accuracy (Colberg 2003).
An alternate equation that may be more accurate for estimating maximal HR in an older population utilizes 70% of age rather than full age (Garber 2011):
MHR = 208 – (0.7 × Age)
Table 4.2 Sample Calculation Using Karvonen Formula for Target Heart Rate Range of 40–89% of Heart Rate Reserve
In general, exercise performed at lower levels (<40% HRR) has a lesser glucose-lowering effect than exercise done at higher intensities—unless it is done long enough. Glucose disposal during high-intensity exercise is roughly proportional to the total work performed (time × intensity). On the other hand, vigorous exercise (≥60% HRR) may result in transient hyperglycemia and cause an excessive rise in systolic blood pressure (Colberg 2010).
Frequency
Physical activity sessions can be performed in a variety of combinations of frequency and duration. Most research concludes that physical activity should be performed 3–5 days/week to achieve significant health benefits. The latest guidelines for T2D suggest that individuals should undertake at least 150 min a week of moderate to vigorous aerobic exercise spread over at least 3 days, with no more than 2 consecutive days without aerobic activity, if their goals are to improve glycemic control, enhance fitness, and achieve target caloric expenditure (Colberg 2010, 2011; American Diabetes Association 2013). Exercise that is limited to 2 days/week results in less improvement in cardiovascular fitness.
Moreover, since the duration of glycemic improvement after an exercise session is usually >2 h but <72 h, regular physical activity is needed to lower blood glucose (Boulé 2001, O’Donovan 2005). For individuals taking insulin, being active on a daily basis may help to balance caloric needs with insulin dosages, as well as maintain higher levels of insulin sensitivity to allow for reduced insulin dosing. Obese individuals may need to be active more frequently (5–7 days/week) at lower intensities to optimize weight loss and maintenance (Ross 2000, 2004). In addition, to achieve sustained major weight loss, the optimal level of activity needed is typically greater than that needed to improve glycemic control.
Energy