Quantitative Assessment of Physical Activity Using Inertial Sensors

Introduction

Athletic and clinical testing for performance analysis and enhancement has tradition-ally been performed in the laboratory, where the required instrumentation is available and environmental conditions can be easily controlled. In this environment, dynamic characteristics of athletes are assessed using simulators such as treadmills, rowing and cycling machines, and flumes for swimmers. In general, these machines allow for the monitoring of athletes using instrumentation that cannot be used easily in the training environment, but instead requires the athlete to remain quasi static, thus enabling a constant field of view for optical devices and relatively constant proximity for tethered electronic sensors, breath gas analysis, and so on. Today however, by taking advantage of the advancements in microelectronics and other micro technologies, it is possible to build instrumentation that is small enough to be unobtrusive for a number of sporting and clinical applications (James, Davey & Rice, 2004). One such technology that has seen rapid development in recent years is in the area of inertial sensors. These sensors respond to minute changes in inertia in the linear and radial directions. These are known as accelerometers and rate gyroscopes respectively. This work will focus on the use of accelerometers, though in recent years rate gyroscopes are becoming more popular as they achieve mass-market penetration, benefiting from increased availability and decreases in cost and device size.

Accelerometers have in recent years shrunk dramatically in size, as well as in cost (~$US20). This has been due chiefly to the adoption by industries such as the automobile industry where they are deployed in airbag systems to detect crashes. Micro electromechanical systems (MEMS) based accelerometers like the ADXLxxx series from Analog Devices (Weinberg, 1999) are today widely available at low cost.

The use of accelerometers to measure activity levels for sporting (Montoye et al., 1983), health and gait analysis (Moe-Nilssen & Helbostad, 2004) is emerging as a popular method of biomechanical quantification of health and sporting activity. This is set to increase with the availability of portable computing, storage and battery power, available due to the development of consumer products like cell phones and portable music players.

Researchers have also used accelerometers for determining physical activity and effort undertaken by subjects. These kinematic systems have been able to offer com-parable results to expensive optical based systems (Mayagoitia, Nene & Veltink, 2002). Rate gyroscopes, a close relative of the accelerometer, measure angular acceleration about a single axis and are also used to determine orientation in an angular coordinate system, although these suffer from not being able to determine angular position, in the same way accelerometers have trouble with absolute position. Additionally, many physical movements, such as lower limb movement in sprinting, exceed the maximum specifications in commercially available units that are sufficiently small and inexpensive for such applications.

This chapter introduces the technological basis of inertial sensor technology, including its construction and theory of application, together with how the data might be acquired, stored and collected by a physical device. A number of applications that have penetrated the mass market are introduced, with the focus being on emerging applications. This range of sporting applications cover physiological event detectors, integration with other sensors, and data mining for complex phenomena such as aerodynamic forces on athletes.