Instrumented Analysis of Human Movement

 

Shaw Bronner PT, MHS, EdM, OCS

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  Table of Contents

 

v    I.         Introduction

v    II.         Motion Systems

v    III.        Data Acquisition

v    IV.        Data Processing

v    V.         Data Analysis

v    VI.        Data Interpretation

v    VII.       Applications

v    VIII.      Definitions

v    IX.        References

v    X.         Questions and Comments

 

 

I. Introduction

 

            Modern studies of human motion can be traced to pioneering publications at the end of the 19^th century by Muybridge (1887), Marey (1873), and Braune and Fisher (1895) (Cappozzo, 1975; Ladin, 1995) . Perhaps the most widely known is Eadweard Muybridge’s successive-exposure photography to study horse locomotion (1878) (Chronophotographical Projections, 2003; Eadweard Muybridge, 1998) . Components included 12 cameras, an electrically-controlled mechanism to operate the cameras' special shutters, and wires laid underground along a track at 21-inch intervals to release the shutter of each camera as the wheels of a horse carriage made contact. The 12 pictures, taken in a period of 0.5s, proved railroad baron and patron Leland Stanford’s theory that during a horse's running stride, there is a moment of suspension where no hooves are touching the ground (Figure 1). Photographic series such these were shown on a Zoopraxiscope, a primitive motion picture developed by Muybridge.

 

Figure 1. Muybridge’s galloping horse series.

 

            Early cinemagraphic motion analysis, generally limited to one plane, was extremely laborious and time consuming. Manual digitizing, frame by frame, provided information on change in position of limb segments. The development of powerful computer systems with high-speed photography in the late 20^th century permitted faster routines for marker recognition, frame grabbing, and processing of three-dimensional (3-D) data. The ability to accurately determine temporal and spatial information from 3-D data, in turn, allowed more sophisticated analyses such as determination of the center of mass and forces through inverse dynamics calculations.

            The purpose of this review is to provide information on the instrumented analysis of human movement. This review will include an outline of the steps involved in conducting instrumented motion analysis research, the issues involved in acquiring good data, and describe what information quantitative motion analysis affords us. The major focus of this review will be on video imaging systems.

Why study human movement? The study of human movement allows us to understand how people move. We can gain insight into the effect of: maturation and development on motor learning (the child learning to walk), training on skill development (mastering a tennis serve); the effect of peripheral or central nervous system injury on activities of daily living (walking or rising from a chair following stroke); and whether our interventions are effective rehabilitation techniques (gait training for an individual with a below-knee leg amputation).

            Quantitative movement analysis falls within the domain of biomechanics (Figure 2), the science involving the study of biological systems from a mechanical perspective (Hall, 1999; Schombert, 2003) . [See section VIII. Definitions] Mechanics (the analysis of the actions of forces), in turn, encompasses statics (the study of systems that are in a state of constant motion (either at rest or moving at a constant velocity) and dynamics (the study of systems in which acceleration is present). In particular, movement analysis focuses on kinematics, which is the study of motion of bodies without reference to the forces that cause the motion. Kinematics describe the pattern and temporal aspects of motion such as positions, angles, velocities, and accelerations of body segments and joints during motion. Quantitative motion analysis also permits the calculation of kinetics, when the anthropometric characteristics of subjects are applied to inverse dynamic computations. Kinetics is the study of forces and moments acting on a body (which cause the motion of the body).

 

Figure 2. Biomechanics diagram.

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II. Motion Systems

 

In biomechanics, the human musculoskeletal system is viewed as a series of linked segments (Figure 3A), which are defined as rigid bodies (Ladin, 1995; Rash & Campbell, 2002) . We use two frames of reference in discussing human motion or kinematics. There is the Newtonian or global frame of reference, which divides the laboratory space into 3-planes (Figure 3B and C). Each body segment can be described as having up to 6-degrees of freedom (DOF), which can describe the location and orientation of that segment in space. In other words, we can discuss motion with respect to the global frame of reference, which includes moving forward or backward in the sagittal plane, side to side in the frontal plane, or inward or outward in the transverse plane.

 

A.                          B.	C.
Figure 3. A. Linked body segments, B. 3-D axes for global frame of reference, C. Planes of Motion: 1 = Frontal plane, 2 = Sagittal plane, 3 = Transverse plane.

            Figure 3A illustrates a body with 17 linked segments. Figure 3B demonstrates the X, Y, and Z axes which define the Newtonian or absolute frame of reference. In Figure 3C, the plane formed by YZ is the frontal plane (1); XZ is the sagittal plane (2); and XY is the transverse plane (3).

There is also a body-centered or local frame of reference, which can describes body segment with respect to another. By convention we discuss movement of the distal segment with respect to the proximal segment. For example, when angular displacement of the knee is reported, it may be described as the (distal) leg rotating with respect to the thigh (Figure 4).

Figure 4. Knee angular displacement: 120˚ flexion.

            In order to record the dynamic motion of the joints and segments of the body for analysis, several types of equipment have been developed. These include:

1)    electrogoniometers,

2)    accelerometers and gyroscopes,

3)    electromagnetic systems, and

4)    imaging systems.

1. Electrogoniometers (Figure 5A) are electronic versions of the standard goniometers (Figure 5B) used in the clinic to measure joint range of motion or angular displacement. Available in unixial and biaxial form, the electrogoniometer consists of one or two potentiometers or strain gauges between two bars. Placed across the joint to be measured, the potentiometer produces a varying voltage output depending on the angle of motion. Advantages of electrogoniometry include ease of set up and processing, relatively low cost, and, with small data loggers, portability for collection in the workplace or other sites. These dataloggers permit the collection and storage of large quantities of data over a prolonged period such as a workday (Anderson & Lyons, 2001; Hansson, Asterland, & Kellerman, 2003) . Disadvantages include the lack of data with respect to the global reference system and 6-DOF, errors due to alignment of the axes of rotation, difficulty in monitoring joints surrounded by large amounts of soft tissue (such as the hip), and cross talk between potentiometers. Electrogoniometers are usually employed for inexpensive and approximate quantification of specific joint motion outside the lab (e.g. on the worksite for ergonomic analysis, etc). Currently, the most commonly used flexible-cable system is manufactured by Penny and Giles (Biometrics Ltd., Blackwood, Gwent, UK) (Ladin, 1995; Penny and Giles electrogoniometers, 2003; Rash & Campbell, 2002) .

A.	B.
Figure 5. A. Electrogonimeter, B. Goniometer.

2. Accelerometers and gyroscopes work on the principal of inertia. A single axis accelerometer (Figure 6) consists of a sensor comprised of a known mass suspended from a strain gauge in a housing. Deflection of the strain gauge with motion is translated into an electrical signal. The advent of piezoresistive devices has permitted miniaturization of the sensors and development of triaxial accelerometers measuring 3-D acceleration (Lugne, Alizon, Collange, & Van Praagh, 1999; Luinge, 2002) . Benefits of accelerometers include measurement of rotational segmental motion, broad frequency range (0– 1,000 Hz), small size, and relatively low cost. Disadvantages include signal “drift” which creates increasing artifact over time and the need to determine the following to accurately calculate a segment’s acceleration and velocity: the segment’s initial position and velocity values, the effects of gravity, and identification of the segment’s rotational DOF. Piezoresistive accelerometers include Entran (Entran Devices, Fairfield, NJ), ICSensors (ICSensors, Milpitas, CA) (Entran accelerometers, 2003; ICSensors, 2003) .

Figure 6. Accelerometer.

Gyroscopes (Figure 7) are based on the principal of a vibrating mass undergoing an additional vibration caused by the coriolis effect (Luinge, 2002) . The Coriolis effect, described by Gustave-Gaspard Coriolis in 1835, is an inertial force related to the motion of the object, the motion of the Earth, and the latitude (Schombert, 2003) . The coriolis effect is the apparent deflection of the path of an object that moves within a rotating coordinate system (such as the earth). Gyroscope components include a mass and a piezoelectric element within a housing. Displacement caused by the coriolis force is proportional to the angular velocity and used as a measure of angular velocity. Similar to accelerometers, miniaturization has permitted the manufacture of triaxial gyroscopes. Benefits of gyroscopes include the direct measurement of rotational motion that is not influenced by gravity and small size. Disadvantages include increasing error of several degrees per second caused by gyroscope offset and noise. Small orientation errors lead to larger integration errors (calculation of angular displacement or position from angular velocity) over time. Therefore, gyroscopes have not been employed regularly for human movement measurement because they were not reliably accurate for periods longer than one second. Recent solid-state gyroscopes claim to have eliminated the drift by using a single crystal quartz element, producing long-term calibration stability. Solid-state gyroscope manufacturers include Motus (Motus Bioengineering, Inc, Benicia, CA) (Mayagoitia, Nene, & Veltink, 2002; Motus system, 2003) .

Figure 7. Gyroscope.

3. Electromagnetic systems (Figure 8) are based on low-frequency magnetic coils that permit real time 6-DOF tracking of segments by sensors placed on the segments (Eckhouse, Penny, & Maulucci, 1996) . Limitations include cabling to connect sensors that can inhibit movement, slippage of the sensors, number of sensors that can be tracked at one time (usually up to four), and cost. Interference from metallic objects or other magnetic fields will degrade performance. Benefits include the elimination of marker dropout from the camera field of view (which can occur in videography), real time 6-DOF data, and accuracy. Electromagnetic systems include Liberty (Polhemus, Colchester, VT) and Flock of Birds (Ascension Tech. Corp., Burlington, VT) (Flock of birds electromagnetic tracking system, 2003; Polhemus Liberty electromagnetic tracking system, 2003) .

A.	B.
Figure 8. Electromagnetic system with A. Large, and B. Small sensors.

4. Imaging systems are divided into cinematography, optoelectronics, and videography systems.

4.1 Cinematography, the earliest imaging system developed, provides a high quality image, but lacks automated systems for data reduction. Therefore, it is costly and time consuming.

4.2 Optoelectronic systems (Figure 9) employ active markers that are usually light emitting (infrared) diodes (LED’s) placed on the segments or joints (Ferrigno & Pedotti, 1985; Ladin, 1995) . The LED’s are triggered and pulsed sequentially by a computer, permitting automatic identification of each marker. Advantages include automated marker tracking hence no marker merging or misidentification. However, optoelectronic systems require wire connection to the LED’s, which makes measurement cumbersome and limits them to the laboratory environment. More than one unit may be required to obtain adequate marker coverage. Commercial systems include OptoTrak (Northern Digital, Inc., Waterloo, Ontario), CODA mpx30 (Charnwood Dynamics Ltd, Leicestershire, UK), and Selspot (Innovision Systems, Inc., Columbiaville, MO) (CODA, 2003; OptoTrak, 2003; Selspot) .

A.	B.	C.
Figure 9. Optoelectronic system. A. Position sensor, B. Active marker, C. Instrumented hand.