Page 101 - Handbook of Biomechatronics

Model-Based Control of Biomechatronic Systems 97

The biomechanics of human movement can be simulated in computers
through inverse and forward dynamics simulations. The natural flow of
human motion starts from the motor-neuron spikes in the CNS (i.e., includ-
ing the brain and spinal cord) leading to the production of muscle twitches
and a force pulling the bones to reach the desired position. A forward
dynamic simulation can properly capture these neuromuscular dynamics
since it follows the same natural flow. Equations of motion are integrated
forward in time to obtain motion trajectories from neuromuscular inputs.
In contrast, an inverse dynamics approach processes information in the
opposite direction: the measured joint trajectories and limb motion and
external loads from a motion capture system and force sensors are the sim-
ulation inputs, and the muscle twitches are the simulation outputs. While an
inverse dynamics approach is useful for clinical decision making, it cannot
explain the underlying cause-and-effect relationships between motor
neuron-spikes and system kinematics. The forward dynamic simulation
can also be used to simulate what-if scenarios such as what happens if the
stiffness of a foot-ankle orthoses increases? The biomechanical model
parameters can be adjusted to represent different individuals with various
physical abilities and disorders.

1.2.1 Inverse Dynamic Simulation
To study the biomechanics of a task, one can measure the kinematics
(motion) and perhaps a portion of kinetics (e.g., external loads) of that par-
ticular task in the laboratory. The kinematics can be measured using optical
movement-monitoringsystemswithactiveorpassivemarkers(e.g.,Optotrak
and Vicon motion capture systems, respectively) or with a markerless system
(e.g., Microsoft Kinect), or using other movement assessment tools such as
electro-goniometers and inertial measurement units (e.g., MVN suit). Force
sensors can measure external loads applied to the body (e.g., foot-ground
reaction forces during walking). Knowing the kinematics and external forces
acting on the system, one can compute the required generalized forces
(e.g., net joint torques and forces) to perform the given task by means of
an inverse dynamic simulation. Before an inverse dynamic simulation can
be performed, the equations of motion representing the task should be
extracted using a dynamic modeling method:

ð
_ xtðÞ ¼ fx tðÞ, TtðÞ, FtðÞÞ (1a)
gx tðÞ, TtðÞ, FtðÞÞ ¼ 0 (1b)
ð