Page 149 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

126 BIOMECHANICS OF THE HUMAN BODY

combinations of movement analysis and biomechanical modeling (e.g., Delp et al., 1996). Gait
analysis can also be used to monitor the progression of the disease and the efficacy of the treatment.
Third, the study of human athletic performance has been revolutionized by motion analysis
equipment and software that make it possible to readily analyze complex three-dimensional
movements. From cricket bowling to figure skating to swimming to pole vaulting, the kinematics
and kinetics have been examined with an aim to improve human performance.
Fourth, there is substantial interest in human movement from those studying ergonomics and
human factors related to military applications. Both the development of human-machine interfaces
for high-tech weapons and the minimization of industrial injuries require knowledge of human
kinematics and kinetics.
Finally, the kinematics of human movement has been studied by animators interested in making
computer-generated characters move in realistic ways. By recording actors while they perform
choreographed dances and movements, it is possible to get complex kinematic data into a computer,
which can then be used to animate a computer-generated image.

6.2 FORWARD VERSUS INVERSE DYNAMICS

There are two fundamentally different approaches to studying the biomechanics of human movement:
forward dynamics and inverse dynamics. Either can be used to determine joint kinetics (e.g., estimate
joint moments during movements).

6.2.1 Forward Dynamics
In a forward dynamics approach to the study of human movement, the input to the system is the neural
command (Fig. 6.1). This specifies the level of activation to the muscles. The neural command can
be estimated by optimization models (Zajac, 1989; Pandy and Zajac, 1991) or from electromyograms
(EMGs). The neural command is the sum of the neuronal signals from the α-motorneurons (that
originate in the spinal cord) to the fibers of each muscle. This can be represented by a single value

Muscle activation Muscle forces Joint moments

External forces
Multijoint dynamics
and moments
Musculo- .. .
Neural α , α , α 3 Musculo- F 1 skeletal T 1 Eqs θ 1 θ 1 θ 1
2
1
..
.
command tendon F 2 geometry T 2 of θ 2 ∫ θ 2 ∫ θ 2
Dynamics F 3 (moment motion
arms)

Sensory
organs

FIGURE 6.1 Forward dynamics approach to studying human movement. This simplified figure depicts the neural command and forces
for three muscles and the moments and joint angles for a two-joint system. See text for details.