The effect of inactivity in ageing muscle

Our body is built to move. Our muscles, joints and bones have evolved to allow us to move from one place to the next and to interact with our environment as efficiently as possible. Being efficient also means adapting to how much the body moves, increasing the capacity to move as needed over time. The ability to adapt to our environment is one of the most advanced and intriguing capabilities of the human body, but it can also have significant negative consequences – the body adapts to physical inactivity possibly more efficiently than it does to increased physical activity.

Physical inactivity and muscle

Although our understanding of the effects of being more physically active are reasonably advanced, our understanding of the science of physical inactivity are quite new (Lee et al, 2012). That being said, what is clear is that physical inactivity is not just the opposite of physical activity, it’s potentially much more powerful and affects muscles in different ways.

Physical inactivity holds the potential to be potent because practically, we spend more of our day being inactive than active. We are spending more time sitting than ever before, with downstream dampening effects in muscle on genetic signalling for mitochondria (Rimbert et. al, 2004), protein synthesis and vasculature.

We can easily accumulate 5-6-7 hours of sitting in a day. It’s not surprising that only 40 minutes of structured exercise is not able to completely counteract the effects of the physical inactivity accumulated throughout the day. Indeed, research suggests that it may even be more beneficial to distribute any activity away from one single bout to being across the day, breaking periods of physical activity into smaller bouts (Healey et al, 2008). This science has some quite important behavioural messages too. For many people, it is easier to sit less than it is to take up exercise.

Physical inactivity and food absorption

Physical activity increases blood flow to the muscle, conversely, physical inactivity reduces blood flow. The flow of blood has a direct effect on the absorption of food. As blood flow to the muscles is reduced so too are the levels of fats, carbohydrates and amino acids that reach the muscle where they are supposed to be stored. If they are not stored in muscles they remain in the blood, where fats can promote damage to blood vessels and reduce insulin sensitivity, and sugars are absorbed into the blood vessels where they reduce their flexibility. This is supported by recent studies which show a 30% increase in circulatory glucose and a 20% increase in circulatory fats during prolonged sitting (Hensen et al, 2016).

There is an inevitable loss of muscle mass as we age. Although we do not completely understand why we lose muscle with age, we do know that physical activity and inactivity play a role. Part of that may relate to the ability to take up amino acids, the building blocks of proteins.

Physical inactivity and ageing; cause or consequence?

So which comes first, do we move less because we have less muscle or have less muscle because we move less? Well, both play a role. Even the most avid exercisers lose physical capacity as they age (Wright and Perricelli, 2008). If we look at running times and weights that can be lifted in people who compete, times and weights decrease with age, even in people who train regularly (look at Arnold Swartzenegger over the past 30 years, even his body shape has changed). Although moving more cannot prevent age-related decline in muscle mass, physical inactivity can leave you with less to start with.

There is one really important area of physical inactivity that affects older people, and that is hospitalisation. Bed rest is a form of extreme physical inactivity, as it takes away all muscle contraction and its downstream benefits (Kortebein et al, 2007). If an older person is hospitalised, they are more likely to need additional support or have an event after it (Gill et al, 2004).

The loss of muscle through bed rest and physical inactivity plays an important, and potentially preventable, part in that decline in function.

References

_{Lee, I-Min et al. (2012) Effect of physical inactivity on major non-communicable diseases worldwide: an analysis of burden of disease and life expectancy. The Lancet, Vol. 380 , Issue 9838 , pp219–229.}

_{Rimbert, Virginie, Yves Boirie, Mario Bedu, Jean-François Hocquette, Patrick Ritz, and Béatrice Morio (2004) Muscle fat oxidative capacity is not impaired by age but by physical inactivity: association with insulin sensitivity, The FASEB Journal 18, no. 6: 737-739.}

_{Healy, G.N., Dunstan, D.W., Salmon, J., Cerin, E., Shaw, J.E., Zimmet, P.Z. and Owen, N., (2008) Breaks in sedentary time beneficial associations with metabolic risk. Diabetes Dare, 31(4), pp.661-666.}

_{Henson J, Davies MJ, Bodicoat DH, Edwardson CL, Gill JM, Stensel DJ, Tolfrey K, Dunstan DW, Khunti K, Yates T. (2016) Breaking Up Prolonged Sitting With Standing or Walking Attenuates the Postprandial Metabolic Response in Postmenopausal Women: A Randomized Acute Study. Diabetes Care, 39(1): 130-8. doi: 10.2337/dc15-1240.}

_{Wright, V.J. and Perricelli, B.C. (2008) Age-related rates of decline in performance among elite senior athletes. The American Journal of Sports Medicine, 36(3), pp.443-450.}

_{Kortebein, P., Ferrando, A., Lombeida, J., Wolfe, R. and Evans, W.J. (2007) Effect of 10 days of bed rest on skeletal muscle in healthy older adults. Journal of the American Medical Association, 297(16), pp.1769-1774.}

_{Gill, T.M., Allore, H.G., Holford, T.R. and Guo, Z., 2004. Hospitalization, restricted activity, and the development of disability among older persons. Journal of the American Medical Association, 292(17), pp.2115-2124.}