Introduction to Human Biomechanics 1

Original Editor - Tolulope Adeniji Top Contributors - Tolulope Adeniji and Kim Jackson  

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We Human being have a unique characteristics in producing a variety of posture and movement and we have ability to move from one place to another, i.e. the locomotive function. The enabler of these functions is our musculoskeletal system that support the body loads and movements of the body segments. This function is embedded in the principles of human biomechanics. Biomechanics has its major application in the areas of improving movement performance, reduction of movement impairment or intervention in movement related injuries or conditions.[1] In physiotherapy practice, Biomechanics concepts such as principles of range of motion,active and passive insufficiency, concave-convex rule, law of forces, motion and machines are applied in therepaeutic exercises, Ergonomics training and the design of modern orthopedic devices such as advanced walking aids. Therefore, biomechanics is considered to be one of the basic knowledge in physiotherapy practice to provide optimal care for several movement related injuries or conditions. Hence, I will start by introducing us to some of these biomechanics concepts .

How do we solve problems in biomechanics?

Having known that biomechanical principle has its major role in physiotherapy practice it is important for us to know how we solve problem in biomechanics. There are two ways to address biomechanical problems, which are quantitative and qualitative analysis. In Quantitative analysis we need to store the biomechanical variables of the desired problem to solve and do numerical analysis of the generated variables. Knudson and Morrison[2] describes qualitative analysis of biomechanics as using a systematic observation and introspective assessment of the quality of human movements for the purpose of providing the most appropriate intervention to improve performance"

Basic biomechanics terminology

Mechanics is a branch of science that deals with forces and the effects produced by these forces. The application of this science to biological system is referred to as biomechanics. Human biomechanics focus on how forces act on the musculoskeletal system and how the body tissue responds to these forces.[3] Using the forces involve in the production of movement and posture. The biomechanics can be discussed in the context of either external or internal biomechanics.[4] External biomechanics describes external forces on body segment and their effect on body movement, while internal biomechanics are forces generated by the body tissues and their effect on movement. In this aspect of the introduction to biomechanics, I will focus on the external biomechanics.

Mechanics domain

There are two domains of mechanics, therefore, biomechanics: static and dynamic. Static is a branch of mechanics that analyse the bodies at rest or uniform motion and dynamics concept deals with the study of conditions under which an object moves.[5] The dynamics concept division: kinematics and kinetics. Kinematics describes body motion without regard to the forces that produce that motion. Kinetics concept deal body motion and the force that cause it to move. In kinematic, there are five variables of interest: type of motion or displacement, the location, the direction, the magnitude and the rate of the motion or displacement[5].

Kinematics variables

Type of motion

human motion can be regarded as general motion i.e a complex combination of linear and angular components of motion. And most of the time, human motion is analyzed as either linear or angular motion as these two types of motion are basically considered “pure” motion. Linear motion, is also known as translatory or translational motion. In linear motion all parts of the are body moving in the same direction and at the same speed and if this motion occurs along a line is straight it is refered to as linear while along the path that is curve it is called curvilinear. Susan J Hall.,[6] describes angular motion as a rotation that occurs around a central imaginary line known as the rotation axis, and this motion.

Magnitude of motion

For angular motion, its magnitude can be measured in radians or degree by use of goniometer. while the linear motion of a segment is measured by the linear distance that the object covered and this can be evaluated with walking assessment tools like 6-minute walk test.[6]

Rate of motion

Speed or velocity is used to measure the rate of motion and change in velocity is acceleration.

Location of joint motion in space

One common reference system for location joint motion is that of anatomical planes and axes. A plane of motion can be described as a particular dimension of motion that run through an imaginary flat surface of the body and an axis is an imaginary line that the body segment is rotating about.[5] A sagittal plane has its axes as medio-lateral and mediolateral is also known as transverse axes and the frontal and transverse planes have their axes as antero-posterior and longitudinal respectively .

Direction of motion

Direction of motion can be described in terms of how the movement occur along the plane and axis. A motion reduces joint angle in sagittal plane it is called flexion and the "extension" motion increases the joint angle. Other common direction of motion in the sagittal plane are dorsiflexion and planter-flexion. Motion to the extremes of the range of motion is often referred to as "hyper," as is the case with hyperextension, and this also occurs in the sagittal plane. The motion of a segment away from the midline in the frontal plane is called “abduction,” while the movement back toward the midline is called “adduction”. Other direction of motion that is common in this plane includes eversion and inversion. Common motion along the transverse plane are internal rotation and external rotation, pronation and supination are also common motion along the transverse plane. There are other directional terms to help describe the position of the body segment relative to the anatomical position, this include the superior and inferior, which describes body position towards the head and the feet, respectively. Also anterior and posterior can be used to describe objects relation to the body as front or back direction to the body, respectively. Parts or motion towards the midline of the body are said to be medial, while motion or position towards the sides of the body is lateral.

kinematic chain

Degree of freedom: Describe number of directions that a joint allow a body segment to move and it is the number of independent coordinates that is used to precisely specify the position of the object in space. Combination of degree of freedom forms kinematics chain and kinematics chain can be opened or closed. In open kinematic chain, one joint can move independently of the others while in closed kinematic chain one end of the chain remains fixed. in closed kinematic chain conditions, movement at one joint automatically generates movement at the other joints of the chain.[5] Levangie and Norkin,[5]elucidated that open and closed park position concept help to describe movements that is taking place under weight-bearing and non-weight-bearing conditions and it is important to take note of these when exercise is to target a single or multiple joint.

Kinetic Concept in Motion Analysis

While kinematic concept describes a segment of a body's motion, the concept of kinetic gives us an idea of the forces associated with that movement. And that will lead us into the kinetic concept of motion analysis, and I'm going to start by defining what the force is in biomechanics. Force is a simple way to represent load in biomechanics and can be defined as an action of one object to another. Force can be external or internal.

External forces are either pull or push on the body that occurs from sources outside the body and internal forces are those forces that act on the structures of the body and are generated by the body tissue. Force can change the shape of an object , and can change the state of motion of the object. Force is also characterized by magnitude, direction and point of application. All these factors determine effect of force on an object. There are multiple forces that act on an object and it is possible to resolve these forces into a single 'resultant' force which has the same effect as all other forces acting together. The process of combining these two or more forces into a single resultant force is known as composition of forces. Having understanding what force is, it is essential to look into some of the laws guiding the force application.[5]

Levangie and Norkin,[5] reiterated that there are three primary rules of forces: 1. A force that acts on a segement must come from something , 2. anything that contacts a segment must create a force on that segment and 3. gravity is considered to have force effect on all objects.

Newton’s law of motion

Newton’s law of motion describes effect of force and motion. The first Newton's law of motion also known as the law of inertia(Inertia is the resistance of the body to change its state of motion), states that an object will remain at rest or uniform motion unless an unbalanced net force act on it. Concept in Newton’s law of inertia shows that the higher the mass of an object the higher the force to move it. For example to wheel an endomorph man on a wheel chair will require much amount of force than to wheel an ectomorph man. Another area in which the first law of motion is applied is in the static analysis. Static analysis is an engineering method for the analysis of forces and moments produced when objects interact.[3] This concept is applied in biomechanics for estimation of unknown forces of muscle and joint reaction in musculoskeletal system.

The second law of the Newton states that a net force will act on an object to change its momentum by causing the object to accelerate or decelerate. Newton's third law states For every action, there is an equal and opposite reaction. One application of this concept is that an athlete will be able to run faster in concrete floor compare to sandy floor due to the opposing ground reaction forces that is required to propel the body.

Contact force

Another form of force is Contact forces . It occurs when two objects are in contact with each other. These forces between them can be resolved into normal reactions and friction. For normal reaction, the force is perpendicular to the surface in which the object is in contact, and friction is the force acting on parallel surfaces. Knowledge of contact force is essential in the design of an athletic shoes or training shoes by introducing a frictional force to improve ground reaction forces.

Moment of force or Torque

An important area of biomechanics is the moment of force or torque, which is the force acting on an object that can cause it to rotate. The moment of force is a product of force and distance, and also refers to the force of rotation of a segment. Important of this concept is that moments of force is important for the muscle to function effectively in maintaining weight bearing. For example, in the knee, the patella creates an effective moment with the quadriceps around the centre of the rotation of the knee, so that the extension of the knee is maintained enough to carry weight.[3]

Simple machine

Having considered some of the external forces in isolation, it is important to see how these forces combine together for a particular function in the form of a machine. A machine converts energy from one form to another, and that energy is the ability to do work. Work takes place when a force moves an object. In mechanics, machine converts energy from one form to another by performing work, i.e. generating movement. The musculoskeletal system is a set of simple machines that work together to support loads and generate movement. There are only three simple machines in the human musculoskeletal system, the lever, the wheel and the axle, and the Pulley. This simple machine enables three functions, including amplification of force and motion and a change in the direction of the applied force. However, most of these simple machines in the musculoskeletal system, are designed to amplify motion rather than force.[3]

Lever system

When muscles develop tension, it pulls on bone either to support or to move the resistance of the applied load to a body segment. These muscle and bone are functioning mechanically as a lever. A lever is any rigid segment that rotates around a fulcrum. And fulcrum is the point of support, or axis, about which a lever rotate. A lever system exists whenever two forces are applied in a way that they produces an opposing moments. the force that is producing the resultant moment is called the effort force (EF). The other force that is creating an opposing moment, it is known as the resistance force (RF). Based on arrangement of load, effort and fulcrum lever can be classified into first to third class. The common anatomical lever in human body is third class and the reason is that the muscle insertion is usually close to joint of action thus the effort is usually between the fulcrum and the resistance, which is a third class lever. This design is for body to gain motion and speed and thus human musculoskeletal system is designed for speed and range of motion at the expense of force.

Wheel & axle

In the musculoskeletal system, the wheel and axle arrangements provide amplification of both force and motion example of these is shoulder joint medial and lateral rotation.


Anatomical pulley is a modified form of wheel and axle. Pulley main function is to redirect a force to make a task easier. The “task” in human movement is to rotate a body segment. Anatomic pulleys make this task easier by deflecting the action line of the muscle away from the joint axis, thus increasing the Mechanical advantage of the muscle force. Mechanical advantage (M A) is a measure of the mechanical efficiency of the lever and it is a function of effectiveness of the effort force to resistance force).

There are four classes of anatomical pulleys, class I to class IV, that may be of interest to physiotherapist. Class 1 pulley is from external support and it improve muscle action that comes from an external support acting as a pulley. Example of this is patella acting as a pulley to improve the quadriceps function. Class II pulley is formed by the bone, cartilage, and tendon. One example of this is when a bone acts as a pulley when the lateral malleolus of the fibula that acts as a pulley for the peroneus longus muscle. In the third class of pulley, joint acts as a pulley and example is femur epicondyles that gives the gracilis tendon a favourable angle of insertion as the tendon insert on the tibia. Muscle acts as a pulley in class IV and example is biceps muscle, which increases in size as its insertion angle increases.The application of pulley in physiotherapy includes pulley exercise to improve range of motion and co-ordination, especially in shoulder arthritis condition.


Kinetic and kinematic concept is important in understanding human motion and potential implication of force on body segment while moving. In designing supportive and adaptive devices and equipment there is need to consider biomechanics concept of force, friction and machines for the device to aid or improve human motion as the case maybe.


  1. Knudson D. Fundamentals of biomechanics. Springer Science & Business Media; 2007 May 28.
  2. Knudson DV, Morrison CS. Qualitative analysis of human movement. Human kinetics; 2002.
  3. 3.0 3.1 3.2 3.3 Malik SS, Malik SS. Orthopaedic biomechanics made easy. Cambridge University Press; 2015 May 28.
  4. McGinnis PM. Biomechanics of sport and exercise. Human Kinetics; 2013.
  5. 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Levangie PK, Norkin CC. Joint Structure and function: a comprehensive analysis. 4th. Philadelphia: FA. Davis Company. 2005.
  6. 6.0 6.1 Hall S. Basic biomechanics. 4th. McGraw-Hill Higher Education; 2014 Feb 7.