Accelerometers

Accelerometers are electromechanical devices which measure acceleration from the movement of a small mass mounted to the case of the accelerometer itself. The measurements can be made using capacitive, mechanical,piezoelectric or piezoresistive methods. In capacitive accelerometers, the mass presses two capaci-tor plates together and acceleration is measured from the difference in capacitance. Mechanical accelerom-eters have the mass attached to a spring and the a niount that the spring deflects measures the amount of acceleration. In a piezoelectric accelerometer, the mass is attached to a piezoelectric crystal (typically quartz). When the body of the accelerometer experi-encesacceleration the piezoelectric crystals deform, resulting in the development of an electric charge which is proportional to the induced acceleration. Piezoresistive accelerometers use a piezoresistive sub-strate in place of the piezoelectric crystal and the force exerted by the mass changes the resistance across a whetstone bridge, which changes the voltage output. The voltage output is proportional to the acceleration.

Accelerometers are available commercially as either uniaxial, in which accelerations can be quantified in one direction, or triaxial, whereby accelerations can be measured in each of the orthogonal axes. Most accelerometers used in biomechanics are very light, weighing less than 10 g. Accelerometers typically serve two purposes in biomechanical analysis: either to measure movement of the body or body segments, or to measure impact transients. For movement of the body or body segments, the accelerometers only need to measure at a lower frequency, typically 60-100 Hz for walking, and with a smaller range, typically 6-9 g (nine times the acceleration due to gravity). For measuring impact transients, modern devices are capable of measuring accelerations at much higher frequencies, typically 1-5 kHz and up to 1000 g (one thousand times the acceleration due to gravity).

Accelerometers allow a direct and immediate signal output which can provide real-time visualization and biofeedback. They tend to be relatively low cost, although if data are required from multiple segments, the cost can become more significant. They can provide clinically important information on shock attenuation or the acceleration and deceleration of body segments, although they do not. give any direct information on segment angles and joint posision.They have been widely used in clinial studies that have to track movment pattern during walking and are also in use in clinical practice.

For measurement of acceleration and deceleration patterns of the body during walking, an accelerometer is usually fixed to the trunk with straps or a tunic, or the head with a cap or band. There are many different commercial products available and each product will provide outputs from the acceleration signal in differ-ent ways. Usually these include temporal variables (such as stance time, step time and stride time), peak amplitudes of acceleration, and/or frequency compo-nents contained within the signal. With the advances in smartphone sensor technology, applications are now available that utilize the signals from accelerom-eters in a mobile phone. These technologies are now used in research and clinical settings to monitor human activity to extrapolate periods of activity and rest, providing an estimation daily function and energy expenditure. The most common application of accelerometers with respect to measuring shock attenuation is to attach the device to the distal end of the tibia and measure vertical accelerations of this segment during walking or running activities generated as a result of the foot striking the floor.  When the foot strikes the floor, its velocity is reduced to zero in a very short amount of time. The rate of change of the veloc-ity of the foot allows the impact associated with foot-strike during walking and running to be characterized: the rate of change in velocity is acceleration

When measuring accelerations of the tibia, the key difficulty that may be encountered is the attachment of the device sufficiently well to represent the acceleration of the underlying bone. Some researchers have attached the accelerometer to Steinmann pins, which are inserted directly into the tibia under local anaesthesia (such as Lafortune, 1991: Lafortune & Hennig, 1991). Many other studies using non-invasive methods have mounted the accelcromter on the anteriomedial aspect of the tibia with non-stretch tape. Often the skin is stretched prior to placing the accelerometer to reduce soft tissue movement and provide a more rigid attachment between the tibia and the device.