Introduction to Human Biomechanics - External Forces

Introduction[edit | edit source]

Centroid human.png

Human beings are able to produce a variety of postures and movements giving them the ability to move from one place to another, i.e. the locomotive function. This is made possible by our musculoskeletal system that supports body loads and movement of body segments.[1] This function is embedded in the principles of human biomechanics.

Biomechanics is considered to be one of the underpinning principles in physiotherapy practice to provide optimal care for movement-related injuries or conditions.

Image 1: Position of the centre of gravity "T" at different positions of the human body

Biomechanics major applications areas: improving movement performance; reduction of movement impairment; intervention in movement-related injuries or conditions.[2]

Physiotherapists make use of biomechanical principles in eg

  1. Therapeutic exercises: range of motion; active and passive insufficiency; concave-convex rule; Newton's laws of motion.
  2. Ergonomic training and the design of modern orthopaedic devices eg advanced walking aids are based on the application of the biomechanics concept.[3][4]

How Do We Solve Problems in Biomechanics?[edit | edit source]

Biomechanics ortho and xray, .png

Knowing that biomechanical principles have a role in physiotherapy practice it is important to understand how to solve problems relating to biomechanics. Biomechanics provides information for a variety of kinesiology professions to analyze human movement to improve effectiveness or decrease the risk of injury. How the movement is analyzed falls on a continuum between a qualitative analysis and a quantitative analysis.

  1. Quantitative analysis involves the measurement of biome-chanical variables and usually requires a computer to do the voluminous numerical calculations performed. Even short movements will have thousands of samples of data to be collected, scaled, and numerically processed.
  2. Qualitative analysis has been defined as the "systematic observation and introspective judgment of the quality of human movement for the purpose of providing the most appropriate intervention to improve performance". Knudson and Morrison.[5]

Image 2: X-ray, orthopedic performance and biomechanics - Epiphysesis of the knee joints used to stop the growth by fixing the growth plate with metal clamps ( used to compensate for the unequal growth of the length of the limbs).

Basic Biomechanics Terminology[edit | edit source]

Mechanics is a branch of science that deals with forces and the effects produced by these forces.

The application of this science to the biological system is referred to as biomechanics.

  • Human biomechanics focuses on how forces act on the musculoskeletal system and how the body tissue responds to these forces.[6]
  • Using the forces involved in the production of movement and posture, biomechanics can be viewed in the context of either external or internal biomechanics.[7]
    1. External biomechanics describes external forces on body segments and their effect on body movement,[7]
    2. Internal biomechanics are forces generated by the body tissues and their effect on movement.[7] "This included the muscle forces and the forces in bones and joints that result from transmission of the muscle forces through the skeleton".[8]

External Forces (External Biomechanics)[edit | edit source]

Mechanics Domain[edit | edit source]

There are two domains of mechanics (biomechanics):

  1. Static: describes mechanics that analyse the bodies at rest or in uniform motion
  2. Dynamics: the study of conditions under which an object moves.[1]

The dynamics concept can be further discussed under kinematics and kinetics.

  • The kinetics concept: deals with body motion and the forces that cause it to move.[1]
  • Kinematics describes: body motion without regard to the forces that produce that motion.

Kinematics Variables[edit | edit source]

In kinematics, there are five variables of interest:

  1. Type of motion or displacement,
  2. The location,
  3. The direction,
  4. The magnitude
  5. Rate of the motion or displacement[1].

1.Type of motion

Human motion is described as general motion (a complex combination of linear and angular components of motion).[9] Most of the time human motion is analysed as either linear or angular motion as these two types of motion are basically considered “pure” motion.

  • Linear motion (or translatory or translational motion): all parts of the body are moving in the same direction and at the same speed. If this motion occurs along a : straight line it is referred to as linear or rectilinear motion; a curved path it is referred to as curvilinear path.[9] [8]
  • Angular motion is described as a rotation that occurs around a central imaginary line known as the rotation axis.[9]

Pure linear movement in humans, like in walking, running and swimming rarely occur as the orientation of body segments to each other changes continually. [8]

  • In activities like skating and ski jumping there might be brief moments of pure linear motion. [8]

The movement of a multi-segmented body, like the human body, which involves simultaneous linear and angular motion of the segments, is usually referred to as general motion.[8] In humans, whole-body movements are described as general motion, as explained in the following examples

  • When a person walks, the head and trunk movements are fairly linear, but the legs and arms movements are linear and angular simultaneously as the person's body translates forward
  • In cycling, the head, trunk and arms move in a fairly linear fashion but the legs move simultaneously in a linear and angular motion.


2. Magnitude of Motion

For angular motion, its magnitude can be measured and recorded in radians or degrees with the use of a 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.[9]

3. Rate of Motion[edit | edit source]

Speed or velocity is used to measure the rate of motion and change in velocity is acceleration.[1]

4. Location of Joint Motion in Space[edit | edit source]

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 runs through an imaginary flat surface of the body and an axis is an imaginary line that the body segment is rotating about.[1] There are three planes of motion in the body, namely the sagittal, frontal and transverse planes.

  • A sagittal plane has its axes as mediolateral, also known as transverse axes
  • The frontal (coronal) and transverse planes have their axes as anteroposterior and longitudinal respectively.[1][6]

5. Direction of Motion[edit | edit source]

The direction of motion can be described in terms of how the movement occurs along the plane and axis.[1]

  • When a motion reduces joint angle in the sagittal plane it is called flexion and a motion that increases the joint angle extension.[6] Other common direction of motion in the sagittal plane are dorsiflexion and planter-flexion.
  • Movement to the extremes of the range of motion are 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 directions of motion that is common in this plane include 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.[9]

There are other directional terms to help describe the position of the body segment relative to the anatomical position eg

  • Superior and inferior, which describes body position towards the head and the feet, respectively.
  • Anterior and posterior can be used to describe objects related to the body as the front or back orientation to the body, respectively.
  • Parts or movement towards the midline of the body is called medial, while motion or position towards the sides of the body is lateral.[6]

Kinematic Chain[edit | edit source]

The kinematic chain (also referred to as the kinetic chain in literature). Combinations of the degree of freedoms form kinematics chain - kinematics chain can be opened or closed.

  1. In an open kinematic chain, the degree of freedom describes the number of directions that a joint allows 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.[1][11]
  2. Closed Kinetic Chain (CKC) exercises or closed chain exercises are exercises or movements where the distal aspect of the extremity is fixed to an object that is stationary.

Levangie and Norkin,[1] elucidated that the open and closed park position concept help to describe movements that are taking place under weight-bearing and non-weight-bearing conditions and it is important to take note of these when exercise is to target single or multiple joints.

Baseball pitching motion 2004.jpg

An order of natural kinetic chain in the upper and lower extremity involves an integrated biomechanical task that when impaired results in dysfunctional biomechanical output leading to pain and/or injury.[11][12] [13]

  • Eg in the shoulder, when deficits exist in the preceding links, they can negatively affect the shoulder.[13][14] Therefore, while managing shoulder dysfunction, an attempt should be made to restore all kinetic chain deficits and therapeutic sessions should follow integrated exercises on proprioception, flexibility, strength and endurance with kinetic chain order.[15]

Kinetic Concept in Motion Analysis[edit | edit source]

While kinematic concepts describe a segment of a body's motion, the concept of kinetics gives us an idea of the forces associated with that movement. When discussing the kinetic concept of motion analysis we need to define force in biomechanics. Force is a simple way to represent load in biomechanics and can be defined as the action of one object to another.[16] Force can be external or internal.

  • External forces either pull or push on the body that occurs from sources outside the body
  • Internal forces are those forces that act on the structures of the body and are generated by the body tissue.[17][18]


  • Cam change the shape and the state of motion of an object.
  • Characterized by magnitude, direction and point of application.

All these factors determine the effect of force on an object.[17] There are multiple forces that act on an object and it is possible to resolve these forces into a single 'resultant' force that 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 the composition of forces. Having understood what force is, it is essential to look into some of the laws guiding the force application.[17]

Levangie and Norkin,[17] reiterated that there are three primary rules of forces:

  1. A force that acts on a segment must come from something
  2. Anything that contacts a segment must create a force on that segment
  3. Gravity is considered to have a force effect on all objects.

The principle of understanding the biomechanics of movement is an in-depth understanding of force, Newton’s laws of motion, work and energy.[19]

Newton’s Law of Motion[edit | edit source]


Newton’s law of motion describes the effect of force and motion.

Image:Paolozzi's statue in the courtyard of the British Library in London, seeks to capture the idea that with Newton's equations and his laws of motion and gravity, the Universe became a more deterministic and measured place

1. Newton's first 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 acts on it. The concept in Newton’s law of inertia shows that the higher the mass of an object the higher the force to move it.[17] This means that a change in resultant force is required to create change in movement. [19] Examples:

  • To wheel an endomorph man (someone with a high percentage of body fat) on a wheelchair will require a greater amount of force than to wheel an ectomorph (slender) man.[17]
  • When a soccer player kicks the soccer ball, he changes the resultant force on the ball, to get it to move. [19]
  • A passenger in a car moves at the same speed the car is moving and when the car suddenly brakes, the passenger, if not wearing a seatbelt, will continue to move forward at the same speed as before the car braked. [19]
  • To lift a heavy object, the person lifting must produce an upward force greater than the weight of the object, otherwise, it will not move. [19]

Another area in which the first law of motion is applied is in static analysis. Static analysis is an engineering method for the analysis of forces and moments produced when objects interact.[16] This concept is applied in biomechanics for the estimation of unknown forces of muscle and joint reaction in the musculoskeletal system.

2. Newton’s second law of motion relates to the impulse of a force. This law states that a net force will act on an object to change its momentum by causing the object to accelerate or decelerate. [16] It is also called the impulse-momentum principle and has an array of applications in sport. Sports performance is concerned with increasing and decreasing the speed of movement of the human body or the sporting equipment. This principle leads to the improvement of sport technique on how the amount of force can be applied for longer for example in shot put. [19]

3. 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 on a concrete surface compared to a sandy surface due to the opposing ground reaction forces that are required to propel the body.[16]

Contact Force[edit | edit source]

Contact force is another type of force. It occurs when two objects are in contact with each other. The forces between them can be resolved into normal force reactions and friction.

  • Normal force - the force is perpendicular to the surface in which two objects are interacting. Watch the video below to learn more.
  • Friction - the force acting on parallel surfaces.

Knowledge of contact forces, for example, is essential in the design of athletic shoes or training shoes by introducing a frictional force to improve ground reaction forces.[6]

Moment of Force or Torque[edit | edit source]

An important area of biomechanics is the moment of force or torque - 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. The importance of this concept is that the moment 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.[6]

Simple Machine[edit | edit source]

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, machines convert energy from one form to another by performing work, i.e. generating movement.[6] 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 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.[6]

Lever System[edit | edit source]

When muscles develop tension, it pulls on bone either to support or to move the resistance of the applied load to a body segment.[6][7] The muscles and bone are functioning mechanically as a lever.

  • A lever is any rigid segment that rotates around a fulcrum.
  • A fulcrum is a point of support, or axis, about which a lever rotates.
  • A lever system exists whenever two forces are applied in a way that they produce 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, is known as the resistance force (RF).

Based on the arrangement of load, effort and fulcrum lever can be classified into first to third class. The common anatomical lever in the human body is third class and the reason is that the muscle insertion is usually close to the joint of action thus the effort is usually between the fulcrum and the resistance, which is a third-class lever.[21] This design helps the body to gain motion and speed and thus the human musculoskeletal system is designed for speed and range of motion at the expense of force.[6][22]

Wheel and Axle[edit | edit source]

In the musculoskeletal system, the wheel and axle arrangements provide amplification of both force and motion. An example of this is shoulder joint medial and lateral rotation. The concept is also applied in wheelchair design and its manual propulsion [23][24]

Pulley[edit | edit source]

The anatomical pulley is a modified form of wheel and axle. The pulley's 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 (MA) is a measure of the mechanical efficiency of the lever and is a function of the effectiveness of the effort force to the resistance force.[6][7]

There are four classes of anatomical pulleys, class I to class IV, that may be of interest to physiotherapists.

  • Class 1 pulley is from external support. It improves muscle action that comes from external support acting as a pulley. An example of this is the patella acting like 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, this is illustrated by the lateral malleolus of the fibula that acts as a pulley for the peroneus longus muscle.
  • Class III pulley is when the joint acts as a pulley. An example is the femur epicondyles that gives the gracilis tendon a favourable angle of insertion as the tendon inserts on the tibia.
  • Class IV is when the muscles act as a pulley. An example is the biceps muscle, which increases in size as its insertion angle increases. The application of pulleys in physiotherapy includes pulley exercise to improve range of motion and coordination, especially in shoulder arthritis condition.[6]

Conclusion[edit | edit source]

Calisthenic Movement.jpeg

The kinetic and kinematic concepts are important in understanding human movement and the implication of force on body segments while moving. In designing supportive and adaptive devices and equipment there is a need to consider the biomechanical concept of force, friction and machines for the device to aid or improve human motion.

References[edit | edit source]

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Levangie PK, Norkin CC. Joint Structure and function: a comprehensive analysis. 4th. Philadelphia: FA. Davis Company. 2005.
  2. Knudson D. Fundamentals of biomechanics. Springer Science & Business Media; 2007 May 28.
  3. Tröster M, Wagner D, Müller-Graf F, Maufroy C, Schneider U, Bauernhansl T. Biomechanical Model-Based Development of an Active Occupational Upper-Limb Exoskeleton to Support Healthcare Workers in the Surgery Waiting Room. International Journal of Environmental Research and Public Health. 2020 Jan;17(14):5140.
  4. Jayaraman C, Hoppe-Ludwig S, Deems-Dluhy S, McGuire M, Mummidisetty C, Siegal R, Naef A, Lawson BE, Goldfarb M, Gordon KE, Jayaraman A. Impact of powered knee-ankle prosthesis on low back muscle mechanics in transfemoral amputees: A case series. Frontiers in neuroscience. 2018 Mar 22;12:134.
  5. Knudson DV, Morrison CS. Qualitative analysis of human movement. Human kinetics; 2002.
  6. 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 Malik SS, Malik SS. Orthopaedic biomechanics made easy. Cambridge University Press; 2015 May 28.
  7. 7.0 7.1 7.2 7.3 7.4 McGinnis PM. Biomechanics of sport and exercise. Human Kinetics; 2013.
  8. 8.0 8.1 8.2 8.3 8.4 Watkins J. Fundamental biomechanics of sport and exercise. Routledge; 2014 Mar 26.
  9. 9.0 9.1 9.2 9.3 9.4 Hall S. Basic biomechanics. 4th. McGraw-Hill Higher Education; 2014 Feb 7.
  10. reference. Swing Catalyst. Chapter 2: Kinematics and Kinetics Introduction. 2015. Available from:
  11. 11.0 11.1 Svoboda Z, Janura M, Kutilek P, Janurova E. Relationships between movements of the lower limb joints and the pelvis in open and closed kinematic chains during a gait cycle. Journal of human kinetics. 2016 Jun 1;51(1):37-43.
  12. Sciascia A, Cromwell R. Kinetic chain rehabilitation: a theoretical framework. Rehabilitation research and practice. 2012 Jan 1;2012.
  13. 13.0 13.1 Ellenbecker TS, Aoki R. SStep by Step Guide to Understanding the Kinetic Chain Concept in the Overhead Athlete. Current Reviews in Musculoskeletal Medicine. 2020 Mar 14:1-9.
  14. Richardson E, Lewis JS, Gibson J, Morgan C, Halaki M, Ginn K, Yeowell G. Role of the kinetic chain in shoulder rehabilitation: does incorporating the trunk and lower limb into shoulder exercise regimes influence shoulder muscle recruitment patterns? Systematic review of electromyography studies. BMJ Open Sport & Exercise Medicine. 2020 Apr 1;6(1):e000683.
  15. Borms D, Maenhout A, Cools AM. Incorporation of the Kinetic Chain Into Shoulder-Elevation Exercises: Does It Affect Scapular Muscle Activity?. Journal of Athletic Training. 2020 Apr;55(4):343-9.
  16. 16.0 16.1 16.2 16.3 Malik SS, Malik SS. Orthopaedic biomechanics made easy. Cambridge University Press; 2015 May 28.
  17. 17.0 17.1 17.2 17.3 17.4 17.5 Levangie PK, Norkin CC. Joint Structure and function: a comprehensive analysis. 4th. Philadelphia: FA. Davis Company. 2005.
  18. Knudson DV, Morrison CS. Qualitative analysis of human movement. Human kinetics; 2002.
  19. 19.0 19.1 19.2 19.3 19.4 19.5 Watkins J. Fundamental biomechanics of sport and exercise. Routledge; 2014 Mar 26.
  20. Jennifer Cash. Normal Force. 2016. Available from:
  21. Elvan A, Ozyurek S. Principles of kinesiology. In Comparative Kinesiology of the Human Body .2020 Jan 1 (pp. 13-27). Academic Press.
  22. Cavallone P, Bonisoli E, Quaglia G. Prototyping of manual wheelchair with alternative propulsion system. Disability and Rehabilitation: Assistive Technology. 2020 Nov 16;15(8):945-51.
  23. Requejo PS, Mulroy SJ, Ruparel P, Hatchett PE, Haubert LL, Eberly VJ, Gronley JK. Relationship between hand contact angle and shoulder loading during manual wheelchair propulsion by individuals with paraplegia. Topics in spinal cord injury rehabilitation. 2015 Nov;21(4):313-24.
  24. Leving MT, Vegter RJ, de Vries WH, de Groot S, van der Woude LH. Changes in propulsion technique and shoulder complex loading following low-intensity wheelchair practice in novices. PloS one. 2018 Nov 9;13(11):e0207291.