Biomechanics: The Science of Human Movement

As the fitness industry grows in popularity and importance, it is of the utmost importance that we as fitness professionals continue to develop a growing knowledge of the exercise sciences to communicate effectively with the established health professions and sciences on “common ground”. The following article, while at times technical, provides an integral part of that knowledge base necessary to facilitate such communication. We encourage you to read (and reread if necessary) this article in its entirety, and in parts, to properly assimilate the presented information. If necessary, please consult with the appropriate health professional.

Physical education has evolved in recent years from physical training and coaching to kinesiology, the study of movement. To the general public this change has gone unnoticed. For those in the fitness professions, kinesiology may seem mysterious. Kinesiology includes a broad range of disciplines such as exercise physiology, sport psychology, sport sociology, motor control, and biomechanics. Here is an opportunity for personal trainers to get an insight into the science of biomechanics.

Biomechanics is the science which applies the laws of mechanics to biological movement. One area of interest to biomechanists is studying the body during sport or exercise situations. Sports biomechanists attempt to answer two basic questions: how to improve performance, and how to make activities safer. To answer these questions, sports biomechanists use two sub disciplines: 1) kinematics, the description of motion, and 2) kinetics, the study of the forces that act on the body.

Biomechanists attempting to improve performance do so in one of two ways. The first is to use kinematics to analyze the motion of a skilled athlete. The assumption is that the skilled athlete has learned the most efficient way to perform the movement, and that others can improve simply by copying the successful athlete. The second way of improving performance is to use kinetics to find some more efficient way of moving which makes better use of the mechanical advantage of the body.

There are also two different approaches to reducing injury. One approach is to use kinematics and kinetics to determine the forces acting on the body, and find ways to lower the forces which cause injury. Another approach is to leave the movement as it is, and design equipment that would reduce the forces acting on the body. An obvious example of this approach is the improvement of shoe design in recent years.

Applying the basic tools described above, sports biomechanists are able to examine some problems of interest in sports and exercise. However, the greatest difficulty facing biomechanists is how to communicate the research that is being done with the people who are actively involved in sports and exercise. There needs to be more communication between all exercise scientists, including biomechanists, and the fitness professionals who are in a position to apply their research. Like many of the disciplines of kinesiology, biomechanists need the input of practitioners for suggesting future research. With this in mind, this article is intended to give an overview of kinematics and kinetics, including a description of some of the basic terminology and concepts.


In kinematics, the limbs or segments of the body are assumed to rotate about the joints, with no translational, or sliding, movement. While this is not strictly correct, it offers a good approximation to what is actually taking place. The joint serves as an axis, and associated with the axis is a plane, perpendicular to the axis, in which the rotational movement takes place.

Borrowing from mathematics, the location of the body is defined in a three dimensional coordinate system. Using the three dimensional coordinate system and anatomic position, the three axis and their associated planes are defined.

Anatomic position is also important in defining several concepts of location with respect to the body. All the segments are said to have a proximal and distal end, with the proximal end of a segment being the end closer to the head, and the distal end being the end further away from the head. The segments also have a medial side, or the side closest to the midline of the body, and a lateral side, or the side furthest away from the midline. Finally, the anterior side of the segment is the side furthest forward, while the posterior segment is to the rear.

The mediolateral axis is an axis which runs side to side through the body. Movements around the mediolateral axis take place in the sagittal plane, which divides the body into left and right sides. The primary movements in the sagittal plane are flexion and extension. Flexion occurs when the distal end of the segment is moved forward as in bending the elbow, while extension occurs when the distal end of the segment is returned to anatomic position as in straightening the elbow. In addition, hyperextension can occur when the distal end of the segment is moved toward the posterior side of the body.

The anteroposterior axis runs from front to back. Movements around the anteroposterior axis are said to occur in the frontal plane, which separates the anterior and posterior sides. The primary movements in the frontal plane are abduction and adduction. Abduction occurs when the distal end of a segment moves laterally as in raising your arm out to your side (shoulder abduction). Adduction occurs as the distal end of the segment moves medially as in returning your arm back to your side (shoulder adduction).

The third axis is called the longitudinal axis, which runs from the top to the bottom of the body. Rotations around the longitudinal axis take place in the transverse plane. The primary movements in the transverse plane are medial and lateral rotation. Medial rotation occurs when the lateral aspect of the segment moves medially, rotating first to the anterior side such as turning the arm inward. Lateral rotation occurs when the rotation occurs in the opposite direction, when the medial aspect of the segment rotates toward the anterior side such as turning the arm outward.

It should be noted that the actions at the joints are always described as if the body were in anatomic position. For example, when the shoulder abducts, the elbow is moved into position where bending of the elbow seems to occur in the transverse plane, indicating that the action of the elbow would be medial and lateral rotation. However, since in anatomic position the motion is in the sagittal plane, the action is still said to be flexion or extension, and still is described as taking place in the sagittal plane. While the primary motions about the axis have been described in some detail, some joints have different names for the motions about the axis. The position of the body in three dimensions can be described using a combination of movements at all of the joints, and moving around all three axis. In addition, the speed of motion around the joints, or angular velocity, and the change in velocity, or angular acceleration can be examined, usually by filming or videotaping a subject performing the motion, but the velocity and acceleration can also be found by using kinetics, as is described in the next section.


Kinematics, while useful in describing motion, tells us nothing about the forces which created that movement. To examine the forces acting on the body, the sub discipline of kinetics is used. Kinetics involves looking at all forces, including external resistance, gravity, and muscle forces using the laws of mechanics discovered by Sir Isaac Newton in the 1600s.

The forces acting on the body are divided into two categories. The first is external forces. External forces are those forces that arise outside the body. Gravity is one example of an external force, being the force exerted by the earth on the body. Other examples include wind resistance and the use of outside weights to create a resistance. Internal forces are forces which arise from inside the body.

Internal forces include muscle forces, the forces exerted by ligaments, and bone-on-bone forces inside the body. In order to create a movement, internal forces must be generated in the proper amounts to overcome the external forces acting on the body. Every force has four qualities: 1) magnitude, or quantity of force, 2) direction, 3) point of application of the force, and 4) a line of action indicating where the force is going. The force accelerates the segment (limb), changing either the speed or direction of the segment, along the line of action of the force according to Newton’s second law of motion:


where F is the force applied, m is the mass of the segment, and a is the resulting acceleration.

If the force does not act directly on a joint it is called an eccentric, or off center, force. An eccentric force, in addition to accelerating the body or limb it is acting upon, will also cause the body or limb to rotate about its axis. The rotating force is called a moment of force, sometimes referred to as a torque. The moment of force, or simply the moment, about each axis has two properties: 1) magnitude, and 2) direction, positive or negative. By convention, a moment which causes a clockwise motion is called negative, while counterclockwise moment is called positive. The magnitude of the moment is determined by the equation:


where M is the magnitude of the torque, F is the force applied, and d is called the moment arm.

The moment arm is equal to the shortest distance from the axis (joint) to the force acting on the limb, determined by the line of action of the force. The moment arm is calculated using the equation: d=rsinq where r is the distance from the axis (labeled 0) to the point of application of the force and q is the angle between the segment and the force. Only in rare cases does only one force, and therefore only one moment act on a segment. A typical case occurs when the force of the biceps, the weight of the forearm and hand, and the weight of a dumbbell, referred to as F(Resistance), are acting on the forearm and hand. When more than one force is acting on a limb, the moments are added together to determine the total moment acting on the limb. The moment equation of this system is:

M= Fmdm – Wdw – Fede

The acceleration of the segment depends on the moment of the system. The segment is accelerated in the direction (positive or negative) of the moment of the system. So if the moment is positive, the segment is accelerated in a counterclockwise direction. The magnitude of the acceleration is given by:


where a is the angular acceleration of the segment and I is the moment of inertia, which represents the weight distribution of the segment.

The description of the muscular contraction is based on the motion of the segment. If the segment is not moving and the muscle is contracting, the contraction is called an isometric contraction. If the system is such that the segment is moving in the direction of the muscle force, the muscle is said to be going through a concentric contraction. If the situation is reversed, and the segment is moving away from the direction of the muscle force, the muscle is said to be going through an eccentric contraction, often called the negative by weight lifters.The moment arm of the biceps muscle is small when compared to the moment arms of the other two forces. This is the situation which normally occurs in the body. A comparison of the moment arms of the forces is made when mechanical efficiency is calculated. The mechanical advantage of a system is:

Mechanical Advantage=IA/EA

where IA is the moment arm of the internal forces and EA is the moment arm of the external forces.

Since the moment arms of the muscles in the body tends to be smaller than the moment arms of the external forces, the mechanical advantage is usually less than 1. The small mechanical advantage means that large forces must be applied to overcome the external forces. Despite this disadvantage, the small mechanical advantage is still helpful since it has the property of allowing a large range of motion for small amounts of muscle shortening.

The Fitness Professional Perspective

While some of the above information does not seem directly applicable to the development of a standard fitness program, the principles presented comprise the underlying basis of all movements performed in sport, exercise, and daily activities. It is the understanding of these concepts which, in part, allow the fitness professional to effectively apply research in program designs to favor progress and minimize injury as well as communicate results of such implementation appropriately.

Suggested Reading

Adrian, M. and Cooper, J. The Biomechanics of Human Movement. Indianapolis, Benchmark Press, Inc., 1989.

Alexander, R.M. The Human Machine. New York, Columbia University Press, 1992.

Beer, F. and Johnston, E.R. Vector Mechanics for Engineers: Statics and Dynamics. New York, McGraw-Hill, 1988.

Kreighbaum, E. and Barthels, K.M. Biomechanics: A Qualitative Approach for Studying Human Movement. New York, Macmillan Publishing Company, 1985.

Norkin, C. and Levangie, P. Joint Structure and Function: A Comprehensive Analysis. Philadelphia, F.A. Davis Company, 1992.

Spence, A. and Mason, E. Human Anatomy and Physiology . Menlo Park, Benjamin/Cummings, 1987.


These resources are for the purpose of personal trainer growth and development through Continuing Education which advances the knowledge of fitness professionals. This article is written for NFPT Certified Personal Trainers to receive Continuing Education Credit (CEC). Please contact NFPT at 800.729.6378 or [email protected] with questions or for more information.