4. Are any precautions needed for DG when he exercises, particularly with regard to his occasional knee issues?
(Continued)
AEROBIC EXERCISE PRESCRIPTION FOR T2D
Mode
Any form of aerobic exercise (including brisk walking) that utilizes large muscle groups and causes sustained increases in heart rate is likely to be beneficial for blood glucose levels and cardiovascular risk management in individuals with T2D (Hu 2001), and undertaking a variety of modes of physical activity is recommended to optimize training effects and lower injury risk (Physical Activity Guidelines Advisory Committee 2008). Examples of acceptable aerobic activities include both weight-bearing and non–weight-bearing ones, including walking, jogging, running, cycling, swimming, water aerobics, aquatic activities, conditioning machines, dancing, chair exercises, rowing, and cross-country skiing, among others. When choosing exercise modes for older and deconditioned individuals in particular, be mindful that walking and moderate-intensity physical activities are associated with a very low risk of musculoskeletal complications, whereas jogging, running, and competitive sports are associated with increased risk of injury (Garber 2011).
Intensity
For optimal fitness gains, aerobic exercise should be at least at a moderate intensity, or 40–59% of heart rate reserve (HRR, as discussed in chapter 4), although vigorous-intensity exercise, defined as 60–89% HRR, is likely to confer greater gains in fitness (Garber 2011, Physical Activity Guidelines Advisory Committee 2008). A severely deconditioned person can start as low as 30% HRR and progress from there to higher moderate levels over time. However, doing even 60 min of low-intensity work (30–39%) can improve blood glucose levels over 24 h in individuals with diabetes, maybe even more so than engaging in only 30 min of high-intensity exercise (Manders 2010).
Using subjective ratings, moderate- to vigorous-intensity exercise corresponds to an overall body rating of “somewhat hard” to “hard” (Garber 2011). Less fit people generally require a higher level of effort to complete the same activity, and their initial subjective intensity may need to be lower than “somewhat hard.” Subjective intensity can be estimated using a scale of 0 to 10, where resting is 0, maximal effort 10, moderate-intensity activity 5–6, and vigorous-intensity activity 7–8 on that scale (Physical Activity Guidelines Advisory Committee 2008).
For most people with T2D, brisk walking is a moderate-intensity exercise due to their lower cardiorespiratory fitness capacity (Boulé 2003). Including some faster intervals, such as pick-up-the-pace training that involves exercising at even 10% above a person’s normal walking or other exercise intensity, can lead to additional fitness gains in individuals with T2D (Johnson 2006, 2008). Greater cardiovascular protection and other benefits may be gained from engaging in vigorous exercise, however, especially if individuals already have a fairly high fitness level or greater physical activity participation. A recent meta-analysis showed that exercise intensity predicts improvements in overall blood glucose control to a greater extent than exercise volume, suggesting that those already fit enough to exercise at a moderate intensity should consider undertaking some vigorous physical activity, at least on occasion, to obtain additional glycemic and cardiovascular benefits (Boulé 2003).
Frequency
Aerobic exercise should be performed at least 3 days/week with no more than 2 consecutive days between bouts of activity due to the short-lived nature of improvements in insulin action (King 1995, Boulé 2005). Most exercise interventions in T2D have used a frequency of three times per week (Boulé 2001, Snowling 2006, Thomas 2006, Sigal 2007), but current guidelines for adults generally recommend five sessions of moderate activity (Haskell 2007, Nelson 2007, Physical Activity Guidelines Advisory Committee 2008). Recently, a study involving adults with diabetes required them to engage in either 30 min of daily, moderate-intensity aerobic activity or 60 min of similar-intensity exercise every other day (van Dijk 2012c). Although both trials substantially reduced the prevalence of hyperglycemia throughout the subsequent day, with total work matched, daily exercise did not lead to further improvements in glycemic control compared with exercise done every other day. The recommended frequency, therefore, is a minimum of 3–5 days/week, with equal caloric expenditure regardless of how many days physical activity is undertaken.
Duration
Individuals with T2D should engage in a minimum of 150 min/week of exercise undertaken at moderate intensity or greater. Aerobic activity should be performed in bouts of at least 10 min and should be spread throughout the week. Around 150 min/week of moderate-intensity exercise is associated with reduced morbidity and mortality in observational studies in all populations (Physical Activity Guidelines Advisory Committee 2008); however, not all health benefits of physical activity occur at 150 min/week, and engaging in less than this amount still benefits deconditioned individuals. As an individual person moves from
150 min/week toward 300 min (5 h) a week, he or she usually gains additional health benefits. The risk of musculoskeletal injury does rise with increasing exercise durations beyond the recommended amounts, but the type and intensity of the exercise are likely more important factors in the incidence of injury, with the volume of exercise (i.e., total work done or calorie expenditure through activities of varying duration, frequency, or intensity) apparently having less importance (Garber 2011).
Most studies in T2D populations have employed durations of 135–150 min/week of activity (Boulé 2001, Snowling 2006, Thomas 2006), even those that included higher-intensity aerobic exercise (Mourier 1997). Unfortunately, most people with T2D lack a sufficient aerobic capacity to work out vigorously enough to meet federal and other physical activity guidelines that allow for a shorter duration of vigorous work (Haskell 2007, Nelson 2007, Physical Activity Guidelines Advisory Committee 2008, Garber 2011). In a meta-analysis, the mean maximal aerobic capacity in diabetic individuals was only 22.4 ml/kg/min (6.4 metabolic equivalents [METs]), making their highest sustainable intensity only a moderate one in absolute terms (i.e., 4.8 METs, or 75% of maximal) (Boulé 2003). Accordingly, most diabetic individuals will require at least 150 min of moderate to vigorous aerobic exercise per week to achieve optimal cardiovascular disease risk reduction, although some glycemic and other health benefits are likely attained from lower exercise doses. Individuals with higher aerobic capacities may be able to exercise at a higher intensity for less time to gain the same benefits.
Progression
Deconditioned individuals need to start out on the low end of the intensity scale and work toward gradual progression of both exercise intensity and volume to minimize the risk of injury or noncompliance, particularly if health complications are present (Physical Activity Guidelines Advisory Committee 2008). Initially, their focus should be on increasing frequency and duration of the exercise rather than intensity. Progression over 4–6 months usually has an ultimate goal of inclusion of vigorous aerobic exercise, but for many with T2D, doing moderate-intensity workouts may be an appropriate endpoint (although frequency and duration may progress over time). See Table 5.1 for a summary of these exercise prescription recommendations for individuals with T2D.
Table 5.1 Recommended Aerobic Exercise Rx for Type 2 Diabetes
| Mode | Walking, jogging, cycling, swimming, rowing, aquatic activities, seated exercises, dancing, conditioning machines, and more |
| Intensity | 40–89% HRR (initial intensity may need to be lower, such as 30–39% or light intensity, for sedentary, deconditioned, or overweight individuals) Perceived exertion of “somewhat hard” to “hard” (but possibly easier initial intensity only for sedentary, deconditioned, or overweight individuals) |
| Frequency |
3–7 days/week (including structured and lifestyle activity)
No more than 2 consecutive
|