Computerized Biomechanical Analysis of Human Performance

Article. Computerized Biomechanical Analysis of Human Performance

A kinetic analysis of human motion, one of the greatest advances in the field of biomechanics, has been expanded by the computer-digitizer complex which allows analysis of total body motion

Mechanics and Sports

Published on Saturday, January 1, 1972 by Gideon Ariel

Reprinted fromMechanics and Sport, AMD Vol. 4Published byThe American Society of Mechanical Engineers345 East 47th Street, New York, N.Y. 10017


FIGURE 3. Velocity Curves


Gideon Ariel
University of Massachusetts


A kinetic analysis of human motion, one of the greatest advances in the field of biomechanics, has been expanded by the computer-digitizer complex which allows analysis of total body motion through utilization of slow motion cinematography, special tracing equipment to convert the data, and the highspeed computer. Appropriate programming results in a segmental breakdown of information of the whole motion including the total body center of gravity, segment velocities and accelerations, horizontal, vertical, and resultant forces, moments of force, and the timing between the body segments. This analysis provides a quantitative measure of the motion and allows for perfection and optimization of human performance applications of biomechanical analyses permit an objective, quantitative assessment of performance replacing the uncertainty of trial and error, eliminating the element of doubt, and provides a realistic opportunity for improved performance.


As early as the fifteenth century Leonardo Da Vinci wrote:

Mechanical science is the noblest and above all others the most useful, seeing that by means of it, all animated bodies which have movement perform all their actions.

Since that time, biomechanics of human motion developed; however, the kinematic and kinetic analyses of the human body lacked specific force analysis. It was only after the combining of high speed photography, anatomical data, and the utilization of man as an integral part of a system, that total motion analysis of human performance was realized. The computer-digitizer complex has reduced the long tedious hours of tracing and hand calculations to a matter of minutes and, thus, complex whole body motion analysis can be practically obtained. This analysis provides a quantitative measure of the motion and allows for perfection and optimization of human performance in industry, sport, and human factors in man-product interactions, as well as,

contributing to equipment design and for the establishment of criteria for standards and safety factors.

The purpose of the present paper is to introduce a quantitative method for analyzing human motion and to discuss some of its applications.


The laws of physics apply to any system in motion regardless of whether the system is a human or machine. The human body may be likened to a machine made up of mechanical members: the joints serve as fulcrums and the contracting skeletal muscles exert forces on the segments. The segments of the human body form a link system consisting of segments such as the foot, shank, thigh, trunk, shoulders, upper-arm, forearm, and hand.

The kinetic analysis involves the following steps:

  1. Obtaining cinematographic data.
  2. Digitizing the data.
  3. Measuring and utilizing anatomical data.
  4. Utilization of the computer program for kinetic analysis and quantifying human performance.
  5. Interpretation of the results.

Obtaining Cinematographic Data

Slow motion cinematography is used to record any desired motion. This technique permits an undetected recording of an individual performance under actual conditions. Most human motions in sport or in industry require camera speeds between 64 and 200 frames per second with 1/4 open shutter to prevent fast-moving segments from becoming mere blurs. Additional information concerning the camera is available in the American Cinematographer Manual (1).

Digitizing the Data

The second step in assessing the data involves a composite tracing of the joint centers of the body. The film is projected on a screen by a Super Sports Analyst Projector (L-W Photo Inc., Model 800) which facilitates location of the joint center for each segment. The model GP-2 Graf-Pen Digitizer (Figure 1) permits precise determination of the coordinates of the joint centers. These X and Y coordinates are stored and then changed into numerical data by a computer program (2).

Anatomical Data

Calculation of forces and moments of force require knowledge of the mss of each segment as well as its center of gravity. These parameters are available in a publication by the Aerospace Medical Research Laboratory (3) with additional anatomical data listed in a monograph by Krogman and Johnson (4). Tables of body segments percentages of total body weight, specific gravity,

as well as, segments lengths as percentages of total height tables may be used when data is not available on the performer. However, there are various methods for calculation of the weight, volume, and the center of mass of segments of the human body when the subject is available (5). In order to calculate the forces it is necessary to know the Radii of Gyration which may be calculated from Dempster’s data on moments of inertia (6).


Kinetic Analysis

Location of the joint centers enables measurement of the segment lengths and angles, while calculation of the segment mass, centers of gravity, and radii of gyration is acquired from the anatomical data. Knowledge of the film speed and the displacement of the joint centers enables calculation of

velocities of the body segments and from the velocities it is then possible to calculate segment accelerations. Segment masses are utilized in the calculation of forces and moments of force. Appropriate programing (7) results in a segmental breakdown of information of the whole motion, including the total body center of gravity; segment velocities and accelerations; horizontal, vertical, and resultant forces; angle of the resultant force application; moments of force, which indicates the magnitude of the muscle action at each joint; the vertical and horizontal forces at the ground contact points; the timing or coordination of motion between the body segments; and the differences due to body builds. This combination of the moments of force, the interrelated patterns of the body segments, and the task performed provides a quantitative measure of the motion and allows for perfection or optimization of the activity.

A kinetic analysis of a world-record javelin throw by Lusis illustrates the present technique. Figure 2 shows the cinematographical data obtained from the film at a speed of 64 frames per second. The joint centers, which are marked by points, were traced by the digitizer to obtain the relative position of each joint center at each position. This data when processed yielded the velocity and acceleration curves which are presented in Figures 3 and 4. The relationship between maximum velocities and accelerations are important in performance technique and gait analysis.



FIGURE l. Schematic representation of Grof-Pen operation.

FIGURE 2. Composite tracing of the Jove/in Throw

From the present figures one may observe that in the best throws of an implement, such as the javelin, the velocity of the last segment was at its maximum just prior to the release rather than at the instant of release. This maximum velocity was achieved by rapid deceleration of other segments prior to release (Figure 4). A good thrower requires a properly timed link system with respect to the coordination of accelerations and deceleration of all body segments in a sequence of action from the fixed point to the last segment. A proper timing of body extremities aids in various human activities by altering vertical and horizontal forces. For example, in the case of a vertical jump, attaining maximum deceleration of the arms just before the take-off will aid the knee extensors in executing the jump. Thus, the timing of the deceleration of one body part may aid the movement of other body segment.

FIGURE 4. Acceleration Curves

The moments indicate the dominant muscle forces and the effect of one segment on the adjoining segment (Figure 5). If muscle action is moving a segment in the clockwise direction, it will be attempting to move the adjoining segment in the counterclockwise direction; thus, in any human performance, one segment may affect the adjoining in a manner that is undetectable by the human eye. At times, the moments of one segment are so large that they will be the dominating muscle force at the next segment, producing a dominant muscle action at this adjoining segment just opposite to that which might be expected. For example, in a deep knee bend, the direction of the moment depends upon the angles of the body segments and the dynamic forces. With a relatively straight trunk, the dominant muscle action at the knee are caused by the extensors. However, if the trunk is bent forward slightly, the knee flexors become the dominant muscles at that joint. These results demonstrate how the dominant force at one segment, the hip extensors, affects the muscle action at the adjoining segment, knee flexors or extensors. However, this muscle action could not have been determined by visual inspection and illustrates the strength of this analytic technique.

After obtaining the forces and the moments of force for each instantaneous position, it is possible to calculate the contribution of each segment

to the total motion. When the segment velocities and accelerations are zeroed in successive order, the contribution of each body part to the whole movement can be ascertained. Figure 6 illustrates percent contributions of different body segments to the moments of a javelin throw. Note that in the release, the shoulders and the front thigh are main contributors. This analysis may aid in interpretation of human motion and allows evaluation of each segment and its contribution to the performance.

Consideration of the location of the center of gravity and its horizontal and vertical displacement suggests patterns of forces and aids in explanations of gait or in athletic performances. The present program yields the’ precise location of the center of gravity of the body at each position for the entire performance. A comparison of center-of-gravity displacement between two shotput throws by the same athlete (Randy Matson, world record holder), serves as an example (Figure 7).

In the better throw (70 feet), the horizontal and vertical displacement of the center of gravity was uniformly continued through the release. If the horizontal displacement of the center of gravity stopped at any instant prior to the release, the throw was less successful as is evident in the pattern of the 65-foot throw. The horizontal displacement of the center of gravity usually stopped when the thrower lost contact with the ground with both feet. The contact with the ground during the entire throw enables the thrower to continue to displace his center of gravity horizontally.

Each human movement may involve different link systems depending on the desired pattern. When more than one link system is used in the same motion, a primary link system is selected and then the forces due to the body segments outside of the selected primary chain are applied as external forces. For example, in a deep knee bend, one link system might be sufficient while 5 link systems would be necessary for calculation of all the kinetic forces in the golf swing.

Shank and Should…


A kinetic analysis of a whole body motion may be applied in any field where human performance is needed so that the possibilities of application of this analysis are far reaching. Its potential in the areas of therapeutics, industry, athletics, and/or other fields are unlimited. The following applications provide examples of whole body motion analysis.

Athletic Equipment

Currently, research is underway in perfecting golf and ski equipment, as well as, athletic shoes. For example, shoes should be designed relative to performance and playing surfaces. In order to accomplish this, the total body motion analysis yields the magnitude and direction of forces on the shoe for each activity, in addition to the total body center of gravity corresponding to the force data. The resultant information can be incorporated by the manufactures into scientifically-designed products. To date, shoe efficiency in normal life and athletics has been the domain of the designer and engineer. The designer considered only the esthetic qualities of the shoe, while the engineer was concerned with physical components such as the slip equation, strength of the material, and durability. However, it is contended that slipping, durability, shock absorption, etc. cannot be thoroughly researched without taking into account “the man in the shoe.” in fact, the

issue of shoe efficiency and performances are inextricably tied to the biomechanics and the variability of different human activities, such that optimal rather than maximum friction and absorption levels should be considered. These complex considerations can be obtained with the present technique.

Synthetic Playing Surfaces

Conventional methods of characterizing the properties of polymere compounds used in artificial playing surfaces are inappropriate. For example, the durometer hardness (stiffness measurement) may bear no relation to the stiffness of the material under dynamic impact. The present method overcomes this weakness and provides a test system which will allow measurement of properties of compounds under dynamic conditions. Data such as dynamic stiffness, hysteresis or damping efficiency, and usable deformation can be assessed by obtaining forces due to human performance. Optimization of shock absorption, surface stiffness, the recovery rate of the material, and energy absorption are essential information for optimization of human performance and development of synthetic playing surfaces.

Prosthesis and Disability Research

Development of a prosthesis that would duplicate the function of the normal limb in support and locomotion requires forces, vectors, velocities, accelerations, and moments of force. Information concerning the forces involved, as well as, the contribution of each body segment to the desired motion, can contribute to prosthetic-device development based on objective evidence and eliminate the archaic “trial and error” method. Whole body analysis is the only realistic method for obtaining information for utilization in the design of artificial devices.

The potential of analyzing human motion for standardizing percent disabilities is of tremendous interest to those people in the Veterans Administration and insurance companies involved with disability and rehabilitation programs.

Physical Activities

To date we know very little about the sequence in which skills are learned if indeed there is a common sequence. In order to understand this problem, it is essential to determine how much force is being applied and at what point in the movement sequence maximum force occurs for optimum results in activities, such as take-offs for jumping, pole vaulting, hurdles, etc.

Many activities in athletics have been condemned by medical authorities as being potentially harmful. For example, the American Medical Association has condemned deep knee bends as being theoretically harmful to the posterior and anterior cruciate ligaments; however, no research supported this statement. A biomechanical analysis to determine these forces during this activity, under dynamic conditions, may shed light on the credibility of this posture.


Inquiry and research continue to unfold new avenues of potential application for biomechanical analysis of human performance. The use of the analysis as a sound, scientific aid in the improvement of performance helps to remove the element of doubt and the uncertainty of trial and error is replaced by accurate, scientific data — a welcome change by all who seek perfection.


  1. Mascelli, J. V., and Miller, A., American Cinematographer Manual, Hollywood, Cal.: American Society of Cinematographers Holding Corp., 1966.
  2. Ariel, G., “The Effect of Anabolic Steroid (Methandrostenolone) Upon the Voluntary Force of Skeletal Muscle,” Unpublished Doctoral Dissertation, University of Massachusetts, 1973, pp. 110-112.
  3. Clauser, C. E., McConville, J. T., and Young, J. W., Weight, Volume, and Center of Mass of Segments of the Human Body, Wright Air Development Center, AMRL-TR-69-70, Wright-Patterson Air Force Base, Ohio, 1969.
  4. Krogman, W. M., and Johnston, F. E., Human Mechanics: Four Monographs Abridged, AMRL Technical Documentary Report No. ANRL-TDR-63-123, 1963.
  5. Plagenhoef, S., Patterns of Human Motion, 1st Ed., Prentice-Hall, Inc., New Jersey, 1971, pp. 18-27.
  6. Dempster, W. T., Space Requirements of the Seated Operator, Wright Air Development Center TR-55-159, Wright-Patterson Air Force Base, Ohio, 1955.
  7. Computerized Biomechanical Analysis Incorporated, P.O. Box 300, Hanover, N. H. 03755.