Page 287 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

264 BIOMECHANICS OF THE HUMAN BODY

Injuries to the head usually involve diffuse or focal brain lesions either with or, commonly,
without skull fracture. The former consists of brain swelling, concussion, and diffuse axonal injury,
that is, mechanical disruption of nerve fibers; the latter consists of localized internal bleeding and
contusions (coup and contrecoup).
The most common neck injury is caused by rearward flexion and forward extension (“whiplash”),
which may result in dislocation or fracture of the cervical vertebrae, and compression of the spinal cord.

11.2 PHYSICAL MEASUREMENTS

The complexity of a living organism, and its ability to modify its mechanical properties (e.g., in
response to mechanical or physiological demands or muscle tension), necessitates the careful design of
experiments. There is a large variability in response between individuals. Also, the direct attachment of
vibration and shock sensors to soft tissues produces a mechanical load that influences tissue motion.
With appropriate measurement methods and instrumentation (Mansfield, 2005; ISO 8041, 2005),
mechanical responses to vibration can be determined for tissues, body segments, and the whole body.

11.2.1 Methods and Apparatus
Tissue Vibration. Noncontact methods are preferred for measuring the surface motion of tissues.
Laser vibrometers are commercially available with sufficient bandwidth and resolution for most
studies. A direct mass load on the skin, together with the skin’s elasticity, forms a mechanical
low-pass filter (see “Simple Lumped Models” in Sec. 11.3.1). If a device is to be mounted directly
on the skin, it must be of low weight (e.g., <3 g) and possess a comparatively large attachment area
2
(e.g., >5 cm ), in order for vibration to be recorded without attenuation of the motion at 80 Hz. An
upper frequency limit of 200 Hz is theoretically achievable (−3dB) with a transducer and skin mount
2
weighing 3 g and an attachment area of 1.8 cm (von Gierke et al., 2002).
The measurement of hand-transmitted vibration requires a bandwidth extending to at least 1250
Hz, which is unobtainable by localized, skin-mounted transducers. Attempts to strap the transducer
to a bony prominence (e.g., by a “watch strap” at the wrist) have demonstrated that the upper
frequency limit for this method of measurement is about 200 Hz (Boileau et al., 1992). A distributed
sensor, such as a pressure-sensitive lightweight film, would permit the motion of a large skin area to
be monitored with acceptable mass loading, and could respond to vibration at higher frequencies.

Interface between Body and Vibrating Surface. Devices have been developed to measure the
motion of the interface between the skin and a source of vibration in contact with the body, such as
a vehicle seat pan or tool handle. The former consists of a flexible disk of rubberlike material thick-
ened at the center, where an accelerometer is mounted. The dimensions have been standardized (ISO
7096, 1982). Attempts have been made to design transducer mounts for the palm of the hand to measure
the vibration entering the hand from tool handles (see, for example, ISO 10819, 1997), and also the
static compressive force (i.e., the combined grip and thrust forces exerted by the hand on the tool handle).
The frequency response of one such device extends to more than 1 kHz (Peterson et al., in press).
Structures in Contact with the Body. The vibration of structures in contact with the body, such as
seats and tool handles, is conveniently measured by accelerometers rigidly attached to a structural
element. Accelerometers are designed to respond to the vibration in a given direction and may be
grouped to record simultaneously accelerations in three orthogonal directions. They are commercially
available in a wide range of sizes and sensitivities, and may be attached by screws or adhesives to
the structure of interest (Mansfield, 2005).

Orientation of Sensors. Tissue-, interface-, and structure-mounted sensors may be aligned as
closely as possible with the axes of a biodynamic coordinate system (see, for example, Fig. 11.2),