Duration
The exercise duration is directly related to caloric expenditure requirements and inversely related to the intensity of exercise required to achieve the same results. In other words, to gain maximum caloric expenditure and glycemic benefits, lower-intensity exercise needs to be performed for longer periods of time compared with higher-intensity exercise.
The recommended weekly exercise duration for adults differs somewhat by guideline. Recent joint guidelines from the ACSM and American Heart Association (2007) recommend 150 min of moderate activity (30 min, 5 days/week) or 60 min of vigorous physical activity (20 min on 3 days) for all adults (Haskell 2007, Nelson 2007). U.S. federal guidelines (2008) recommend 150 min of moderate or 75 min of vigorous activity, or an equivalent combination, spread throughout each week (Physical Activity Guidelines Advisory Committee 2008).
The latter recommends an exercise volume of 500–1,000 metabolic equivalents (METs) min/week, where an MET is the ratio of the rate of energy expended during an activity to the rate of energy expended at rest. For example, 1 MET is the rate of energy expenditure at rest, whereas a 4-MET activity expends four times the energy used by the body at rest. MET minutes are the metabolic equivalent of physical activity times the number of minutes the activity is done. Thus, levels of 500–1,000 METs min/week are achievable with 150 min of weekly walking at 4 mph (6.4 km/h; intensity of 5 METs, or five times resting levels) or 75 min of jogging at 6 mph (9.6 km/h; 10 METs).
Although both of these guidelines apply to most adults with T1D, the majority of people with T2D lack sufficient aerobic capacity to jog for that duration or they have orthopedic or other limitations. The mean maximal aerobic capacity in such individuals is estimated to be only 22.4 ml/kg/min, or the equivalent of 6.4 METs (Boulé 2003), making 4.8 METs (~75% of maximal) the highest, sustainable intensity. Thus, most individuals with T2D have to undertake ≥150 min of moderate to vigorous aerobic exercise per week to achieve optimal cardiovascular risk reduction (Colberg 2010). The ADA’s Standards of Medical Care make a similar recommendation that individuals with diabetes perform at least 150 min of weekly moderate-intensity aerobic physical activity (American Diabetes Association 2013).
Although a minimum dose has not been determined, some cardiovascular and glycemic benefits may be gained from lower exercise volumes (Hansen 2009). Additional benefits likely result from engaging in durations beyond recommended amounts, however (Jakicic 2003, Houmard 2004). If someone has a higher aerobic capacity (>10 METs), such as many younger individuals with T1D, T2D, or GDM, he or she may choose to exercise at a higher absolute intensity for less time to achieve the same cardiovascular and glycemic benefits. Bouts of physical activity lasting 60 min or more may be required to achieve weight loss or to prevent weight regain (Physical Activity Guidelines Advisory Committee 2008).
Whether the actual length of each exercise session matters is an interesting question. Although 20 min has been the minimal recommendation for cardiovascular benefits, multiple shorter bouts of 10 min have been shown to result in measurable improvements (Eriksen 2007). Similar gains occur when physical activity is accumulated throughout the day as three bouts of 10 min compared with a single prolonged activity session of similar duration and intensity (i.e., a 30 min bout of continuous activity) (Haskell 2007). Thus, if desired, physical activity may be broken down into sessions of a minimum of 10 min spread throughout the day (Garber 2011). Severely deconditioned individuals may need to exercise in multiple sessions of short duration (5–10 min), begin at low levels with brief rest intervals, and progress slowly to higher-intensity exercise. Initially, sessions can be done for 10–15 min, increasing progressively over time to >30 min (Garber 2011). Closer to 60 min of daily exercise may be necessary to achieve significant weight loss in adults (Ross 2000, 2004).
Progression
Appropriate progression of aerobic training is required to help individuals effectively and safely achieve aerobic exercise goals. Initially, the focus should be on increasing frequency and duration of the exercise rather than intensity. Individuals should increase physical activity gradually over time whenever more activity is necessary to meet guidelines or health goals, and inactive individuals should “start low and go slow” by gradually increasing how often and how long activities are done (Physical Activity Guidelines Advisory Committee 2008). Progressing in this manner increases the likelihood of creating and sustaining an activity habit and lowers the risk of injury or demotivation (Garber 2011).
A full exercise prescription, to be truly effective, should include both how to start a fitness program and what to reasonably expect in terms of progress, as shown in Table 4.3. The individual must understand the type, quantity, and rate of progress to expect. Over a period of time averaging at least 4–6 months, each individual moves through three distinct stages of a fitness plan: initial, improvement, and maintenance.
Table 4.3 Progressive Stages of Exercise Training
Initial stage. In this stage, individuals begin to form physical activity habits that can be integrated into their lifestyles. Unfortunately, most people never make it out of this critical stage, either because they attempt activities that are too hard for them given their fitness level or because they develop unrealistic expectations for what they will be able to accomplish and when, which leads to discouragement and a cessation of training. To help ensure success, individuals should be helped to understand that significant changes (like loss of body weight) may not occur during this period. Also, they need to understand that building fitness habits takes 4 weeks or longer, especially when people start out with very poor fitness levels. Setting specific fitness goals can help with motivation during this stage. For individuals who are starting with a higher fitness level, this stage often can be skipped.
Improvement stage. During this second stage, the focus shifts from developing habits to improving fitness levels. The individual now has improved stamina and endurance, is able to spend longer amounts of time doing physical activity, and can begin adding to duration or intensity of workouts. Over the 3 months (or longer for deconditioned and older individuals) of this stage, health benefits begin to become measurable. If individuals see these changes as permanent without further training needed, however, their exercise habit may start to weaken, and they may start missing workouts. Individuals who already have higher levels of fitness will begin their program in this stage when undertaking new types of training or setting new fitness goals. The progression shown is for individuals with an ultimate goal of participating in vigorous aerobic exercise. For many individuals with T2D, however, doing moderate-intensity workouts may be an appropriate endpoint, although frequency and duration may progress.
Maintenance stage. Once someone has successfully met his or her initial fitness or blood glucose management goals, it is important to develop new goals and plans to enhance motivation and continued participation. A new goal may be something like “prevent weight regain” or “train for a competitive event.” To ensure continued safety and effectiveness of training in all individuals with diabetes, medications may need to be adjusted or other changes made to training if any existing health comorbidities progress or new ones arise.
SAFETY CONSIDERATIONS
Everyone who engages in physical activity should do so with safety in mind. To do physical activity safely and reduce risk of injuries and other adverse events, people should understand the risks yet should exercise confidently given that physical activity is safe for almost everyone. They should start with and do types of physical activity that are appropriate for their current fitness level and health goals, increase their physical activity gradually over time, start