#### Movement

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Movement is a physical phenomenon which occurs in the 3-dimensional space and has a dynamic element to it defined by time.

Bodily movement can take many forms. We tend to use the anatomical reference axes which define a local coordinate system within the body (see Figure P.1.3). This is not the same as the global coordinate system which is absolute and defined relative to the Earth.

Figure P.1.3 Three axes of human movement for the center of mass. The vertical axis (a global coordinate system reference) is aligned with the longitudinal axis (a local coordinate system reference) when the person is upright.

Most human movement occurs at variable speeds; change in movement speed with respect to time is termed acceleration. For example, when we run, the anatomical segments of the body will accelerate and decelerate within each step cycle. We can measure acceleration in one, two, or three directions with accelerometers attached to specific body segments. Such measurements are in the local coordinate system but Gravitational acceleration will also be distributed across measurement axes, relative to vertical in the global coordinate system. Simpler devices include pedometers which also measure movement as a frequency count.

Another class of movement is rotation. The angular speed at which something is rotating can be measured with a gyroscope, which can capture one, two or three dimensions of rotation. Measurements are in the local coordinate system.

Movement can also be captured with a magnetometer which measures the orientation of the device relative to the Earth's magnetic field, i.e. North, South, East and West. Measurements are in the global coordinate system. A combination of triaxial accelerometer, gyroscope and magnetometer is called an inertial measurement unit; this combination allows acceleration measurements to be transformed from local coordinates into global coordinates via the use of Quaternions.

We also capture movement when repeatedly measuring location over time, e.g. using proximity sensors or global satellite technology.

Energy expended by the muscles to produce forces acting across our joints may result in bodily movement. If forces are equal on either side of a joint (antagonists), there will not be any physical movement around that joint. A measure of the movement of one or more of the body's anatomical segments can be used to infer the energy expenditure associated with that movement; the validity of such an inference would depend on the degree of antagonist muscular activity. If the inference is applied to the whole-body level, validity would also depend on the degree to which the movement of one body segment correlates with the movement of all other body segments.

1. Ekelund U, Yngve A, Brage S, Westerterp K, Sjöström M. Body movement and physical activity energy expenditure in children and adolescents: how to adjust for differences in body size and age. The American Journal of Clinical Nutrition. 2004;79:851-6
2. da Silva IC, van Hees VT, Ramires VV, Knuth AG, Bielemann RM, Ekelund U, Brage S, Hallal PC. Physical activity levels in three Brazilian birth cohorts as assessed with raw triaxial wrist accelerometry. International Journal of Epidemiology. 2014;43:1959-68
3. Doherty A, Jackson D, Hammerla N, Plötz T, Olivier P, Granat MH, White T, van Hees VT, Trenell MI, Owen CG, et al. Large Scale Population Assessment of Physical Activity Using Wrist Worn Accelerometers: The UK Biobank Study. PloS One. 2016;12:e0169649
4. Rahmani MH, Berkvens R, Weyn M. Chest-Worn Inertial Sensors: A Survey of Applications and Methods. Sensors (Basel, Switzerland). 2021;21:2875
5. Stevens ML, Gupta N, Inan Eroglu E, Crowley PJ, Eroglu B, Bauman A, Granat M, Straker L, Palm P, Stenholm S, et al. Thigh-worn accelerometry for measuring movement and posture across the 24-hour cycle: a scoping review and expert statement. BMJ Open Sport & Exercise Medicine. 2020;6:e000874
6. van Hees VT, Renström F, Wright A, Gradmark A, Catt M, Chen KY, Löf M, Bluck L, Pomeroy J, Wareham NJ, et al. Estimation of daily energy expenditure in pregnant and non-pregnant women using a wrist-worn tri-axial accelerometer. PloS One. 2011;6:e22922
7. van Hees VT, Gorzelniak L, Dean León EC, Eder M, Pias M, Taherian S, Ekelund U, Renström F, Franks PW, Horsch A, et al. Separating movement and gravity components in an acceleration signal and implications for the assessment of human daily physical activity. PloS One. 2012;8:e61691
8. White T, Westgate K, Hollidge S, Venables M, Olivier P, Wareham N, Brage S. Estimating energy expenditure from wrist and thigh accelerometry in free-living adults: a doubly labelled water study. International Journal of Obesity (2005). 2018;43:2333-2342