Biomedical Engineering Reference
In-Depth Information
The number of bonds formed depends on the degree of overlap between thick and thin fila-
ments and is dictated spatially and temporally due to finite electrical and chemical activation
rates. Asynchrony in bond formation and unequal numbers of bonds formed in each half sar-
comere, as well as mechanical disturbances such as muscle shortening and imposed length
transients, cause small movements of the myofilaments. Since myofilament masses are taken
into account, these movements take the form of damped vibrations with a spectrum of fre-
quencies due to the distributed system properties. When the stress in a bond goes to zero,
the bond detaches. Consequently, myofilament motion and bond stress relaxation lead to
bond detachment and produce relaxation without assumption of bond detachment rate func-
tions. In essence, relaxation results from inherent system instability. Although the model is
built from linear, time-invariant components (springs, dashpots, and masses), the highly
dynamic structure of the model causes its mechanical properties to be highly nonlinear and
time-varying, as is found in muscle fibers and strips.
Sensitivity of the model to mechanical disturbances is consistent with experimental evi-
dence from muscle force traces, aequorin measurements of free calcium ion, and high-
speed x-ray diffraction studies, which all suggest enhanced bond detachment. The model
is also consistent with sarcomere length feedback studies in which reduced internal motion
delays relaxation, and it predicted muscle fiber (cell) dynamics prior to their experimental
measurement.
This model proposes a structural mechanism for the origin of muscle's complex mechan-
ical properties and predicts new features of the contractile mechanism—for example, a
mechanism for muscle relaxation and prediction of muscle heat generation. This approach
computes muscle's complex mechanical properties from physical description of muscle
anatomical structure, thereby linking subcellular structure to organ-level function.
This chapter describes some of the high points of biological tissues' mechanical proper-
ties. More comprehensive references include Fung's
Biomechanics: Mechanical Properties of
Living Tissues
, Nigg and Herzog's
Biomechanics of the Musculo-Skeletal System
, and Mow
and Hayes's
Basic Orthopaedic Biomechanics
. Muscle contraction research has a long history,
as chronicled in the topic
by Needham. For a more comprehensive history of
medicine, see Singer and Underwood's topic. The next two sections apply biomechanics
concepts introduced in Sections 4.2-4.5 to human gait analysis and to the quantitative study
of the cardiovascular system.
Machina Carnis
4.6 CLINICAL GAIT ANALYSIS
An example of applied dynamics in human movement analysis is clinical gait analysis.
Clinical gait analysis involves the measurement of the parameters that characterize a
patient's gait pattern, the interpretation of the collected and processed data, and the recom-
mendation of treatment alternatives. It is a highly collaborative process that requires the
cooperation of the patient and the expertise of a multidisciplinary team that typically
includes a physician, a physical therapist or kinesiologist, and an engineer or technician.
The engineer is presented with a number of challenges. The fundamental objective in data
collection is to monitor the patient's movements accurately and with sufficient precision for
clinical use without altering the patient's typical performance. While measurement devices
for clinical gait analysis are established to some degree and are commercially available, the
