Physical activity is important for maintaining skeletal muscle mass and strength, and for improving the body’s ability to utilize energy. In contrast, physical inactivity such as sitting, which is common in modern society, can reduce muscle mass and strength and impair metabolic function. Prolonged periods of inactivity can also lead to anabolic resistance, which is characterized by a reduced ability to use dietary amino acids from food to create new muscle proteins.^[1] Anabolic resistance due to repeated bouts of prolonged inactivity may accelerate the loss of muscle mass, especially in older adults who may be more susceptible to this phenomenon.

Protein supplementation and exercise can prevent some of the adverse metabolic effects of inactivity.^[2] The exercise required to exert these beneficial effects doesn’t have to be substantial, either. “Exercise snacks”, or brief periods of physical activity throughout the day, improve glucose tolerance and may help to maintain protein synthesis during long bouts of sitting.

In this randomized controlled crossover study, 12 men and women (average age of 23) completed three separate conditions:

• 7.5 hours of uninterrupted sitting (SIT)
• 7.5 hours of sitting interspersed with 15 body-weight squats every 30 minutes (SQUAT)
• 7.5 hours of sitting interspersed with 2 minutes of walking every 30 minutes (WALK)

The participants were provided with high-protein breakfast (7 a.m.) and lunch (10 a.m.) during each trial.

The primary study outcomes were the incorporation of amino acids (from protein contained in the meals) into myofibrillar protein (ΔMyo) and myofibrillar protein synthesis (MyoPS). Secondary outcomes included the phosphorylation of anabolic signaling proteins, including mechanistic target of rapamycin (mTOR), eukaryotic initiation factor 2 (eIF2), ribosomal protein S6 (rpS6), extracellular regulated kinase 1/2 (ERK1/2), and 4E binding protein 1 (4EBP1). All of the study outcomes were assessed every 30 minutes during each trial with a blood draw and before and after each trial with a muscle biopsy.

ΔMyo and MyoPS were both higher in the SQUAT and WALK conditions compared to the SIT condition. The phosphorylation of rpS6 was higher in the SQUAT condition compared to the SIT condition, but there were no differences in the phosphorylation of any of the other anabolic signaling proteins.

The effects of sitting, exercise snacks, and intermittent walking on MPS

The participants in this study were classified as inactive, meaning that they were getting less than 150 minutes of activity each week. It would be helpful to see if these results would hold up in a study population with greater habitual activity levels.

There’s been a rising interest in the health effects of inactivity. This is a worthwhile research topic, given that the average adult spends about 6 hours sitting each day.^[3] Prolonged sitting has been shown to acutely impair cardiovascular function and elevate postprandial glucose, insulin, and triacylglycerols.^[4][5] In this way, people who are chronically sedentary put themselves at risk for cardiovascular and metabolic diseases. Furthermore, sedentary time is associated with worse health outcomes even when sedentary people meet physical activity guidelines. This phenomenon is known as the “active couch potato.”^[6]

On the other hand, brief periods of activity can reduce or completely prevent the consequences of prolonged inactivity. Light physical activity every 20–30 minutes seems to be ideal for obtaining the benefits for blood glucose regulation and cardiovascular function. As the current study shows, the activity can be something as simple as a brief walk, body-weight squats, or climbing a few flights of stairs. It can also be a bit more intense. One study found that performing a total of five 4-second cycle sprints every hour (for a total of 160 seconds per day) can prevent some of the adverse metabolic effects of inactivity.^[7] The important thing is to spend around 1–2 minutes doing some type of physical activity. However, the optimal intensity and duration of activity needed will likely depend on individual characteristics. Not only can these practical “exercise snacks” reduce the adverse cardiometabolic effects of prolonged sitting, but they may also improve aerobic fitness.^[8]

Consuming dietary protein and performing resistance exercise (and more potently, their combination) stimulates an increase in muscle protein synthesis (MPS). The repetitive stimulation of MPS is what drives the increase in muscle strength and hypertrophy during progressive resistance training. While adequate protein and exercise are important for muscle health throughout life, these factors become even more crucial for older adults because muscle mass begins to decline by approximately 3%–8% per decade after age 30.^[16] A decline in muscle mass and strength along with impairments in mobility and physical function is called sarcopenia.

A major contributor to the age-related decline in muscle is anabolic resistance of skeletal muscle to protein ingestion and exercise. Older adults demonstrate approximately 10% less protein digestion and absorption rates than younger adults.^[1] The reduced anabolic sensitivity of older adult muscle is likely compounded by higher levels of inactivity and lower levels of exercise, which independently reduce anabolic sensitivity and impair muscle mass and strength.^[2] This underscores not only the importance of increased protein requirements for older adults but also the need for strategies to enhance MPS throughout the day, especially during periods of prolonged inactivity, which may exacerbate skeletal muscle anabolic insensitivity.

Resistance exercise is a potent stimulator of MPS and is recommended to prevent the age-related decline in muscle mass. But, as the current study demonstrates, even brief periods of skeletal muscle stimulation with “exercise snacks” can keep MPS elevated throughout the day. These findings are important, as there are several physical and environmental barriers to obtaining the recommended levels of exercise.

The acute (short-term) incorporation of amino acids into myofibrillar protein, as seen in this study, is related to elevated rates of MPS.^[17] However, it’s important to note that the acutely elevated levels of myofibrillar protein synthesis observed in the activity conditions may not directly translate into chronic improvements in muscle mass or strength. Further studies over a longer duration are required to determine the effects of breaking up activity on muscle mass and function.

In addition to engaging in prolonged sitting, participants in this study also decreased their daily step count. The average daily steps for participants in the lead-up to the study was around 8,000 per day, which was reduced to 2,100 in the SIT condition, 2,800 in the SQUAT condition, and 5,700 in the WALK condition. Reducing steps to 750–1,500 per day has previously been shown to decrease MPS by 13%–16%.^[2] Thus, activity snacks could be able to prevent the adverse metabolic effects of both step reduction (seen in other studies) and prolonged sitting (observed in this study). Unfortunately, this study didn’t include another condition in which participants maintained their average daily steps and consumed identical test meals. This could have provided more insight into how the SIT condition affected MPS compared to habitual daily activity.

If simple physical activity interventions like “exercise snacks” can beneficially affect MPS and glucose regulation in healthy young adults,^[18] the implications for older adults may be even more profound. Not only are older adults more likely to engage in prolonged bouts of inactivity, but they’re also more susceptible to the consequences of inactivity on skeletal muscle health, glucose control, and inflammation.

Alhough structured exercise is necessary for maintaining and improving fitness, the relevance of daily activity patterns to health is becoming increasingly apparent. It’s prudent to move more, and move more often, avoiding long periods of sedentary time whenever possible.