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Introduction to Human Biomechanics - External Forces

Original Editor - Tolulope Adeniji Top Contributors - Tolulope Adeniji, Tarina van der Stockt, Lucinda hampton, Kim Jackson, Jess Bell and Alexandra Kopelovich  

Introduction

Position of the centre of gravity "T" at different positions of the human body.

The human body is capable of producing a wide variety of postures and movements, allowing us to move from one place to another. This locomotive function depends on the musculoskeletal system, which supports body loads and drives the movement of body segments.[1] This function is governed by the principles of human biomechanics.

Biomechanics is considered to be one of the foundational principles in physiotherapy practice, and it underpins optimal care for movement-related injuries or conditions. The major applications of biomechanics are: improving movement performance, reducing movement impairment, and guiding intervention in movement-related injuries or conditions.[2]

Biomechanical principles are applied in a range of clinical areas, including therapeutic exercises, such as range of motion and strengthening programmes, ergonomic training, and the prescription of orthopaedic devices such as advanced walking aids.[3][4][5]

How Do We Solve Problems in Biomechanics?

Biomechanics provides tools to analyse human movement in order to improve effectiveness or decrease the risk of injury. The approach to analysing movement falls on a continuum between qualitative and quantitative analysis.

Quantitative analysis involves the measurement of biomechanical variables. Even short movements generate thousands of data samples that must be collected, scaled, and numerically processed, which typically requires computer analysis.

Knudson and Morrison define qualitative analysis 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".[6]

Basic Biomechanics Terminology

Mechanics is the branch of science concerned with forces and their effects. When these principles are applied to biological systems, the field is known as biomechanics. Human biomechanics focuses specifically on how forces act on the musculoskeletal system and how the body tissue responds to these forces.[7]

Biomechanics can be divided into two categories based on the origin of the forces involved: external and internal biomechanics.[8]

External biomechanics describes forces from outside the body acting on body segments, and their effect on movement.[8]

Internal biomechanics describes forces generated by the body's own tissues and their effect on movement,[8] including "the muscle forces and the forces in bones and joints that result from transmission of the muscle forces through the skeleton".[9]

External Forces (External Biomechanics)

Mechanics Domains

There are two domains of mechanics (biomechanics):

  • Statics: the analysis of bodies at rest or in uniform motion.
  • Dynamics: the study of the conditions under which an object moves.[1]

Dynamics can be further divided into kinematics and kinetics.

  • Kinetics deals with body motion and the forces that cause it.[1]
  • Kinematics describes body motion without considering the forces that produce that motion.

Kinematics Variables

In kinematics, there are five variables of interest: the type of motion or displacement, its location, direction, magnitude, and rate.[1]

1. Type of motion

Human motion is described as general motion — a complex combination of linear and angular components.[10] Most of the time, human motion is analysed as either linear or angular motion, as these two types of motion are considered “pure” motion.

  • Linear motion (or translatory or translational motion) occurs when all parts of the body move in the same direction and at the same speed. If this occurs along a straight line, it is termed linear or rectilinear motion. If it occurs along a curved path, it is called curvilinear motion.[10] [9]
  • Angular motion is rotation around a central imaginary line known as the axis of rotation.[10]

Pure linear movement is rare in human movement. During walking, running and swimming, the orientation of body segments to each other changes continually.[9] Brief moments of near-pure linear motion may occur in activities such as skating and ski jumping.[9]

The movement of a multi-segmented body like the human body, which involves simultaneous linear and angular motion, is usually referred to as general motion.[9] In practice, most whole-body movements are described as general motion.

  • When a person walks, the head and trunk move fairly linearly, but the arms and legs move simultaneously in linear and angular motion as the body translates forward.
  • During cycling, the head, trunk and arms move in a relatively linear fashion, but the legs move simultaneously in linear and angular motion.

[11]

2. Magnitude of Motion

The magnitude of angular motion can be measured in radians or degrees using a goniometer. For linear motion, magnitude is measured by the linear distance covered. This can be evaluated with assessment tools such as the 6-minute walk test.[10]

3. Rate of Motion

The rate of motion is measured as speed or velocity. A change in velocity is known as acceleration.[1]

4. Location of Joint Motion in Space

Joint motion is commonly described using anatomical planes and axes. A plane is an imaginary flat surface that passes through the body. An axis is an imaginary line around which a body segment rotates.[1] There are three planes of motion in the body:

  • The sagittal plane, with a mediolateral (transverse) axis.
  • The frontal (coronal) plane, with an anteroposterior axis.
  • The transverse plane, with a longitudinal axis.[1][7]

5. Direction of Motion

The direction of motion describes how movement occurs along the plane and axis.[1]

  • In the sagittal plane, movement that reduces the joint angle is flexion; movement that increases it is extension.[7] Dorsiflexion and plantar flexion also occur in this plane. Movements to the extremes of range are often prefixed with "hyper" (e.g., hyperextension).
  • In the frontal plane, movement away from the midline is abduction, and movement towards it is adduction. Eversion and inversion also occur in the frontal plane.
  • Common movements in the transverse plane include internal and external rotation, and pronation and supination.[10]

Additional directional terms are used to describe the position of a body segment relative to the anatomical position:

  • Superior refers to a position towards the head; inferior towards the feet.
  • Anterior refers to the front of the body; posterior to the back.
  • Medial refers to a position towards the midline; lateral away from it.[7]

Kinetic Chain

The kinetic chain (also referred to as the kinematic chain) describes how joints and segments are linked together. A kinetic chain can be open or closed.

  1. In an open kinetic chain (OKC), one joint can move independently of the other joints in the chain.[1][12]
  2. In a closed kinetic chain (CKC), movement at one joint produces movement at the other joints in the chain.

The distinction between open and closed kinetic chains is clinically relevant when planning weight-bearing and non-weight-bearing exercises, and when targeting single or multiple joints.[1]

Upper and lower limb kinetic chains involve an integrated movement sequence that, when impaired, can lead to pain and/or injury.[12][13] [14]

For example, problems at the glenohumeral joint can be caused by issues in the preceding links in the kinetic chain, such as the shoulder girdle and trunk.[14][15][16] When managing shoulder dysfunction, clinicians should therefore aim to restore all kinetic chain deficits, including in the thoracic spine and scapula.[17][18][19]

Forces in Human Movement

Force is a simple way to represent load in biomechanics, and it can be defined as the action of one object on another.[20] Force can be external or internal.

  • External forces are pushes or pulls on the body from sources outside the body.
  • Internal forces are generated by the body, and they act on the body's own structures.[21][22]

Forces can change the shape and state of motion of an object. They are characterised by their magnitude, direction and point of application, all of which determine the effect of the force on an object.[21] When multiple forces act on an object, they can be resolved into a single 'resultant' force that has the same effect as all other forces acting together. This process is known as the composition of forces.[21]

There are three primary rules of forces:[21]

  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.

Newton’s Laws of Motion

Newton’s laws describe the relationship between force and motion.

1. Newton's First Law of Motion (Law of Inertia)

Inertia is the resistance of a body to changes in its state of motion. The first law states that an object will remain at rest or in uniform motion unless acted on by an unbalanced net force. The greater the mass of an object, the greater the force required to move it.[21] This means that a change in resultant force is required to create a change in movement.[23]

Examples:

  • Wheeling a person with a larger body mass in a wheelchair requires more force than wheeling a lighter person.[21]
  • When a football player kicks a football, the kick changes the resultant force on the ball, causing it to move.[23]
  • A passenger in a car moves at the same speed as the car. If the car brakes suddenly and the passenger is not wearing a seatbelt, they will continue moving forward at the car's original speed.[23]
  • To lift a heavy object, a person must produce an upward force greater than the weight of the object, or it will not move.[23]

The first law also underpins static analysis — an engineering method for analysing forces and moments when objects interact.[20] In biomechanics, this is applied to estimate unknown muscle forces and joint reaction forces in the musculoskeletal system.

2. Newton’s Second Law of Motion (Impulse–Momentum Principle)

Newton's second law states that a net force acting on an object will change its momentum, causing it to accelerate or decelerate.[20] This principle has wide applications in sport, where performance often depends on how much force can be applied and for how long. For example, in shot put, the focus is on applying force over a longer duration to maximise distance.[23]

3. Newton's Third Law (Action-Reaction)

Newton's third law states that for every action, there is an equal and opposite reaction. For example, an athlete can run faster on a concrete surface than on sand, because the rigid surface returns greater ground reaction forces to propel the body forward.[20]

Contact Force

Contact forces occur when two objects are in physical contact with each other. The forces between them can be resolved into two components.

  • Normal force: the force is perpendicular to the contact surface.
  • Friction: the force is acting parallel to the contact surface.

An understanding of contact forces is essential in applications such as athletic shoe design, where frictional properties are engineered to improve ground reaction forces.[7]

[24]

Moment of Force or Torque

The moment of force, or torque, is a force that causes an object to rotate. It is the product of force and the perpendicular distance from the force's line of action to the axis of rotation. This concept is clinically relevant because adequate moments of force are necessary for muscles to function effectively during weight-bearing. For example, the patella increases the moment arm of the quadriceps around the knee's axis of rotation, improving the efficiency of knee extension during weight-bearing.[7]

Simple Machines

A machine converts energy from one form to another by performing work (i.e., generating movement).[7] The musculoskeletal system is a set of simple machines that work together to support loads and generate movement.

There are three simple machines in the human musculoskeletal system: the lever, the wheel and axle, and the pulley. These serve three functions: amplifying force, amplifying motion, and changing the direction of applied force. However, most simple machines in the musculoskeletal system are designed to amplify motion rather than force.[7]

Lever System

When a muscle develops tension, it pulls on the bone either to support a load or move a body segment against resistance.[7][8] The muscle and bone function 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 when two forces produce opposing moments.
  • The effort force (EF) is the force producing the resultant moment.
  • The resistance force (RF) is the force creating the opposing moment.

Levers are classified into a first, second or third class based on the arrangement of the fulcrum, effort and resistance. Third class levers are most common in the human body, because muscles typically insert close to the joint. This places the effort between the fulcrum and the resistance.[25] This arrangement favours speed and range of motion at the expense of force.[7][26]

Wheel and Axle

In the musculoskeletal system, wheel and axle arrangements amplify both force and motion. An example is medial and lateral rotation at the shoulder joint. The same concept is applied in wheelchair design and manual propulsion.[27][28][29]

Pulley

The anatomical pulley is a modified form of a wheel and axle. Its main function is to redirect a force to make a task easier. In human movement, the “task” is to rotate a body segment. Anatomical pulleys achieve this by deflecting the muscle's line of action away from the joint axis, thereby increasing the muscle's mechanical advantage. Mechanical advantage (MA) is a measure of the mechanical efficiency of a lever, reflecting the relationship between the effort force and the resistance force.[7][8]

There are four classes of anatomical pulleys that may be of interest to physiotherapists:[7]

  • Class I: An external structure acts as a pulley to improve muscle action. For example, the patella acts as a pulley to improve quadriceps function.
  • Class II: The pulley is formed by bone, cartilage and tendon. For example, the lateral malleolus of the fibula acts as a pulley for the peroneus longus muscle.
  • Class III: A joint acts as a pulley. For example, the femoral epicondyles give the gracilis tendon a favourable angle of insertion on the tibia.
  • Class IV: A muscle acts as a pulley.

Conclusion

The kinetic and kinematic concepts discussed in this page are fundamental to understanding human movement and the effects of force on body segments. When designing supportive and adaptive devices, it is important to consider the biomechanical principles of force, friction and simple machines to ensure that the device aids or improves human motion.

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 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. ↑ Aronis G, Kurz M, Wimmer F, Hackl H, Angeli T, Gföhler M. Maximum shoulder torque and muscle activation during standing arm flexion: reference data for biomechanical and ergonomic applications. J Funct Morphol Kinesiol. 2025 Dec 30;11(1):20.
  6. ↑ Knudson DV, Morrison CS. Qualitative analysis of human movement. Human kinetics; 2002.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 Malik SS, Malik SS. Orthopaedic biomechanics made easy. Cambridge University Press; 2015 May 28.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 McGinnis PM. Biomechanics of sport and exercise. Human Kinetics; 2013.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 Watkins J. Fundamental biomechanics of sport and exercise. Routledge; 2014 Mar 26.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Hall S. Basic biomechanics. 4th. McGraw-Hill Higher Education; 2014 Feb 7.
  11. ↑ reference. BertecHQ. Movement Sciences Explained: Kinetics and Kinematics. Available from: https://www.youtube.com/watch?v=4Ntexwf2XQM
  12. ↑ 12.0 12.1 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.
  13. ↑ Sciascia A, Cromwell R. Kinetic chain rehabilitation: a theoretical framework. Rehabilitation research and practice. 2012 Jan 1;2012.
  14. ↑ 14.0 14.1 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.
  15. ↑ 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.
  16. ↑ Salles FLP, Pascoal AG. Enhancing shoulder kinetic chain rehabilitation using elastic resistance in kneeling positions. A kinematics study. 2025. JOSPT Open;3(4):449-57.
  17. ↑ 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.
  18. ↑ Barramuño-Medina M, San Martín-Zurita S, Silva-Correa V, Téllez-Camilo T, Valdés-Badilla P, Bascour-Sandoval C, Gálvez-García G. Kinetic chain modifies muscle activation in adults with shoulder pain: a randomized cross-over trial. J Shoulder Elbow Surg. 2025 Jun;34(6):e468-e476.
  19. ↑ Yu S, Chen S, Yang Z, Ma X, Huang J, Yang L. Effectiveness of thoracic spine manual therapy in treating subacromial impingement syndrome: a systematic review and meta-analysis. Arch Phys Med Rehabil. 2025 Dec;106(12):1886-1898.
  20. ↑ 20.0 20.1 20.2 20.3 Malik SS, Malik SS. Orthopaedic biomechanics made easy. Cambridge University Press; 2015 May 28.
  21. ↑ 21.0 21.1 21.2 21.3 21.4 21.5 Levangie PK, Norkin CC. Joint Structure and function: a comprehensive analysis. 4th. Philadelphia: FA. Davis Company. 2005.
  22. ↑ Knudson DV, Morrison CS. Qualitative analysis of human movement. Human kinetics; 2002.
  23. ↑ 23.0 23.1 23.2 23.3 23.4 Watkins J. Fundamental biomechanics of sport and exercise. Routledge; 2014 Mar 26.
  24. ↑ Jennifer Cash. Normal Force. 2016. Available from: http://www.youtube.com/watch?v=1pbGP-MRN-0
  25. ↑ Elvan A, Ozyurek S. Principles of kinesiology. In Comparative Kinesiology of the Human Body .2020 Jan 1 (pp. 13-27). Academic Press.
  26. ↑ 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.
  27. ↑ Kukla M, Kończak M, Wieczorek B, Warguła Ł, Rybarczyk D, Zharkevich O. Characteristics of factors affecting axle load and friction forces in a wheelchair. Disabil Rehabil Assist Technol. 2025 Aug;20(6):1874-1883.
  28. ↑ 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.
  29. ↑ 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.
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