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Introduction to Human Biomechanics - Internal Forces

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

Introduction

Ballerina standing on tiptoe (en pointe). Biomechanics - simplified force analysis of the lower extremity.
Ballerina standing on tiptoe (en pointe). Biomechanics - simplified force analysis of the lower extremity.

When external forces act on a body segment to produce posture and movement, the internal structures of that body segment react by generating specific forces to counteract the external forces and, thus, create the desired posture or movement.[1]

Understanding the mechanical properties of these internal structures enables physiotherapists to assess and prevent movement-related injuries.

Important Definitions

Stress: the internal resistance or internal force per unit area of a material against the deforming effects of an external force or load. The total resulting resistance corresponds to the external load.

Strain: the relative change or deformation of the shape of a structure that accompanies stress. Dense connective tissue can show a wavy configuration under the microscope at rest, called "crimp", which decreases with increasing strain.[2]

Viscoelastic Properties

Dense Connective Tissue: Relatively high proportion of collagen fibres, contains fewer cells and less ground substance than loose connective tissue. A key feature of dense connective tissue is its viscoelastic properties. These properties allow the tissue to temporarily stretch and return to its original length after unloading:

  • Compliance: the ability of dense connective tissue to temporarily stretch
  • Elasticity: the ability of the tissue to return to its original length after unloading

These processes occur at the physiological limit, where the wave-like configuration (crimp) of the corresponding collagen fibre is lost when dense connective tissue is stretched.[2][3] Therefore, it is important to consider this physiological limit when stretching dense connective tissue, as well as the stress-strain curve (see below), which provides insight into the tissue's physiological limit.[2]

For more information on the viscoelastic properties of dense connective tissue, please see: Tendon Biomechanics.

Stress-Strain Curve

The stress-strain curve is a diagram that illustrates how stress changes as connective tissue is subjected to increasing strain (also known as the load-deformation curve).

Based on the force applied to a specific tissue, the stress-strain curve can be explained using four specific regions or phases in the diagram: 1. Toe Region, 2. Elastic Region, 3. Plastic Region, 4. Ultimate Failure Point.[2]

In the toe region of the curve, the crimp of the collagen fibres is initially lost. The fibres gradually smooth and elongate (stretch) proportionally to the applied force during the elastic phase. In the plastic phase, the force applied to the collagen causes a permanent (irreversible) change in the tissue structure. If this force continues beyond the elastic limit, the tissue can tear at its ultimate failure point.[2]

Relevance of the stress-strain curve in rehabilitation practice

Early joint rehabilitation should focus on mobilising joint tissue within the toe region of the stress-strain curve.[4][5] In this phase, the risk of tissue trauma is low, as only the crimp is reduced. Types I-III mechanoreceptors are most active in this phase. If tension on connective tissue increases up to the point where type IV receptors are triggered (i.e. nociceptive pain), the joint mobilisation has exceeded the tissue's physiological limit.[4]

The concept of the stress-strain curve is also important for understanding the physical mechanisms of dense connective tissue, including the interactions between its constituents during stretching and load transfer across different tissue length scales.[5][6][7]

Human Skeletal System

The skeletal system is a biomechanical system of rigid parts (i.e., bones) connected at joints to enable specific movements. The human skeletal system forms the support apparatus or passive locomotor system (in contrast to active components, such as muscles and tendons) and consists of bones, ligaments, and joints. Muscles are attached to bones to generate forces and enable movements. To understand human movement, it is important to be aware of the biomechanical properties of the various elements of the skeletal system.[8]

Biomechanical Properties of Bone

Bone is a highly specialised, metabolically active connective tissue that constantly repairs and remodels.[9] It consists of two types of bone tissue:

  • Compact or cortical bone: forms the outer edge of the bone, is dense (5-30% porous) and rigid[10]
  • Spongy or trabecular bone: occupies the central space within the bone, is less dense (90% porous) than cortical bone, but considerably more malleable[10][11]

Bones have high compressive, tensile, viscoelastic, and shear strength. Due to these biomechanical properties, the human skeleton is well-suited to support load and move body segments.

Biomechanical Properties of Human Joints

A joint is the area where two or more bones meet. Various structures make up human joints, including bones, bursae, capsules, cartilage, ligaments, tendons, and other associated tissues.

The primary functions of a joint are to enable movement and force transmission. These functions have resulted in unique structural features.[2]

Joint Classification

There are various ways to classify joints. Some of these classifications are based on joint complexity, the number of existing axes, joint geometry, or the degrees of freedom of the joint. Since human biomechanics involves the analysis of body movements and the forces causing these movements (i.e., kinematics and kinetics), it's important here to first classify joints by type.

Joints can be divided into three main categories based on joint type:

  1. Synarthroses, e.g., symphysis pubis, the syndesmosis between the tibia and fibula, and the connections between skull bones in children
  2. Amphiarthroses, e.g., sacroiliac joint, intercarpal joints
  3. Diarthroses, e.g., shoulder joint (a ball joint) or interphalangeal joints (hinge joints)

Joint Surfaces

A joint's movement is largely determined by its structure, particularly the shape of the joint surfaces. The traditional classification of synovial joints by structure includes the following categories: spheroid, trochoid, condyloid, ginglymoid and ellipsoid joint.[4]

Limitations in the Anatomical Joint Description

The anatomical classification of joints is insufficient to explain the finer details of the mechanical properties of a joint. But these details can be highly relevant for rehabilitation professionals.[4]

  • Joint surfaces are not perfect spheres, ovals, or ellipses
  • Each joint deviates from a pure geometric basic form.
  • Example: a diarthrodial hinge joint, such as the humeroulnar joint, does not allow a true hinge movement of flexion and extension, but rather a screw-like (helical) movement involving considerable rotation.

Joint movements have traditionally been described as sliding, angular movement, rotation, and circumduction. However, this description ignores movements that occur between joint surfaces.[4] Therefore, it is important to classify joints according to both the characteristics of joint movements between bones and between the respective joint surfaces. This leads us to the concept of osteokinematic and arthrokinematic movement.

Osteokinematic and Arthrokinematic

The terms osteokinematic and arthrokinematic movement describe different aspects of kinematic movement.[12]

Osteokinematics deals with the angular movement occurring between bones at a joint. Movements are described by their direction, plane, and movement axis.[12] They can be measured with a goniometer and include flexion/extension, abduction/adduction, and internal/external rotation. These movements occur around a mechanical axis of the joint. The mechanical axis refers to a line that runs through the moving bone, touching the centre of the relatively stationary joint surface, and lying perpendicular to it.

Arthrokinematic movements are also called accessory movements. These describe the relative movement of joint surfaces to each other when an osteokinematic movement occurs. Arthrokinematic movements include rolling, gliding, and spinning, and are often summarised under the term "joint play".[12]

Rolling occurs when a joint surface moves in relation to another surface during an angular motion. Rolling happens in the direction of the osteokinematic movement. New points on the rolling surface come into contact with new points on the other surface during rolling.[12]

Gliding/sliding is the translational movement of one joint surface on another joint surface. This means that one point on the moving joint surface comes into contact with multiple points on the other joint surface.[12]

Spinning is a pure rotational movement. During spinning, multiple points of the moving joint surface come into contact with a single point on the stationary joint surface.[12]

For joint integrity, a combination of rolling and gliding typically occurs.[12] When joint surface "A" rolls on the opposite joint surface "B", joint surface "A" moves across different contact points on the corresponding opposite surface "B". For example, a car tyre must roll across different points on the ground for movement to occur. But when only a single point of the moving joint surface touches various points of the opposite surface, this is called gliding. An analogy would be a car tyre skidding on ice. The tire does not rotate but moves relative to the road surface.

Clinical implications: a joint movement that occurs without normal arthrokinematics can lead to either impingement or dislocation.[4] The principles of osteokinematics and arthrokinematics are also important in treating joint mobility restrictions. For instance, treating patients with adhesive capsulitis of the shoulder through mobilisation with targeted angular movements (osteokinematic) has been shown to lead to improvements in shoulder pain, range of motion, and disability.[13] Another study showed that considering osteokinematics in people with lateral ankle sprains can contribute to improving range of motion.[14]

Concave-Convex Rule

An important concept in arthrokinematics is the concave-convex rule. This rule states that:

  • when a concave surface moves on a convex surface, rolling and gliding must occur in the same direction
  • when a convex surface moves on a concave surface, rolling and gliding occur in opposite directions

Clinicians may be able to apply this concept to restore restricted joint movements by considering roll and slide movements alongside the traditional anatomical movement of the affected joint.[4]

Clinical example: flexion in the glenohumeral joint is restricted. Clinicians must consider active or passive flexion (angular movement), as well as the inferior glide of the humeral head on the glenoid—reduced inferior glide can contribute to a flexion limitation. Knowledge of the concave-convex rule thus helps the therapist understand in which direction to apply accessory mobilisations.[4]

Closed and Open/Loose-Packed Position

Joint stability is the ability of a joint to resist displacement of the articulating bones. Although most joints have asymmetrical surfaces, there is typically one position where the area of contact is at its maximum. This position is called the closed-packed (or locked) position; this is the position of maximal joint stability and congruence. In this position, the capsule and ligaments are maximally stretched, and there is limited accessory movement (joint play).

Any movement of the bones in the joint away from the locked position leads to a reduction in stability and contact surface.[15] The opposite of the closed-packed position is called the loose-packed (or open) position. The loose-packed position of a joint is the position with the least congruence, where the capsule and ligaments are loose and accessory movement is at its maximum.[16]

Applying the Closed- and Loose-Packed Position Concept

This concept can be applied to joint distraction and compression. Distraction occurs when joint surfaces move apart; compression occurs when there is approximation of opposing joint surfaces. Movement towards a closed-packed position involves an element of compression, while movement away from this position involves distraction.

Hertling et al.[4] note that the concept of joint closed- and loose-packed positions helps clinicians understand joint stability and to know when to apply joint distraction or compression during movement.

Clinical example: the loose-packed position of the knee is 25 degrees of flexion. This means the knee is "unlocked" and tests, such as ligament stress tests, can be performed easily.[16] The closed-pack position of the knee is in full extension.[16]

Joint Flexibility

For clinicians, it is important to understand that joint flexibility is primarily a function of the relative tightness of the muscles and ligaments surrounding the joint. When these tissues are not regularly used through their full length, they tend to shorten.

Methods to improve flexibility include:

Among these approaches, PNF is one of the most effective techniques for stretching muscles and ligaments. This is because all PNF patterns involve a sequence of contraction and relaxation of agonist and antagonist muscles to achieve an effective stretch.

Muscular System

Muscles exert forces on the bone to produce movement at joints.[19] A isometric contraction is when the applied force aims to maintain a constant length. The force used to shorten a muscle is called a concentric contraction. Finally, the force used to lengthen a muscle is called an eccentric contraction.

  • In concentric muscle contractions, positive dynamic work is performed
  • In isometric muscle contractions, static work is performed
  • In eccentric muscle contractions, negative dynamic work is performed

Skeletal Muscle Fibre Type

In a skeletal muscle fibre, the sarcomere is the smallest contractile unit. It is where a muscle generates active tension through cross-bridges between myosin filaments and actin filaments.[20] The rate of tension generated depends on the type of muscle fibre. Skeletal muscle fibres also differ in their fatigue resistance. Muscle fibers are classified into three types based on these differences:

  • Type I / Slow-twitch (ST fibers)
  • Type IIA / Fast-twitch (FR fibers)
  • Type IIB / Fast-twitch (FF fibers)

Mechanical Characteristics of Muscle

There are three mechanical properties of muscle based on variations in muscle force due to differences in speed, length, and time relative to activation:

  • Force-Velocity Relation
  • Force-Time Relation
  • Force-Length Relation

Force-velocity relation[21] explains how the force of a fully activated muscle changes with speed. In a shortening muscle (concentric movement), the force the muscle can generate decreases with increasing speed. In eccentric movements, this relationship is reversed.

Force-time relation describes a temporal delay in the development of tension. This delay can be party caused by a delay in muscle stimulation (active state or excitation dynamics). Repeated activation of muscle fibres can cause a temporal delay in tension development. However, the length of time to develop tension strongly depends on the cognitive effort of the person, training, type of muscle action, and activation history of the muscle group.

Neuromuscular Control of Motion

The mechanical response of muscles also depends on how the muscles are activated. The basic functional unit of neuromuscular control includes the motor unit, recruitment and the firing rate

A motor unit consists of one motor neuron and all muscle fibres it innervates. Recruitment is the activation of different motor units within a muscle, and the firing rate is the repeated stimulation of a specific motor unit over a certain period. A tetanic muscle contraction is a sustained muscle contraction triggered when a motor nerve innervating a skeletal muscle generates an action potential at very high rate.

Neuromuscular movement control is a key consideration when designing exercise for neuromuscular impairments. For example, it has been found that people with anterior shoulder dislocation and mechanical deficits and proprioceptive impairment can achieve improvements with neuromuscular exercises for shoulder instability.[22]

Similarly, neuromuscular control exercises can improve pivoting instability of the lower extremities in patients with movement disorders such as cerebral palsy, stroke, and incomplete spinal cord injury.[23]

Proprioception of Muscle Action and Movement

The term "proprioception" is derived from Latin and can be translated as "self-perception". Sir Charles Sherrington coined the definition of proprioception as "the perception of joint and body movement as well as the position of the body, or body segments, in space".[24]

In a muscle, muscle spindles are proprioceptive stretch receptors. They are responsible for the muscle stretch reflex (self-reflex, myotatic reflex). Stretching within the muscle triggers a muscle contraction as a response. Muscle spindles are sensitive to slow muscle stretching but respond most strongly to rapid stretches.[24]

Physiotherapists can introduce interventions that focus on proprioception, such as Proprioceptive Neuromuscular Facilitation (PNF), to help restore balance control and improve postural dysfunction in patients with neurological and/or musculoskeletal conditions.[25][26] PNF uses the proprioceptive system to "facilitate or inhibit muscle contraction".[27]

Reciprocal Inhibition

In reciprocal inhibition, the contraction of the agonist muscle leads to the relaxation of the antagonist muscle. This relaxation of the antagonist contributes to efficient movement. For example, when the biceps brachii contracts, there is inhibition of the triceps brachii to enable efficient elbow flexion.

Active and Passive Insufficiency

Another important concept of movement is active and passive insufficiency.

Active insufficiency occurs when a multi-joint muscle shortens across all joints simultaneously, reducing its ability to generate optimal tension and force.

Passive insufficiency occurs when a multi-joint muscle lengthens at all the joints it crosses. This limits the range of motion at each joint as a muscle is typically not long enough to allow full range of motion simultaneously at each joint it crosses.

Conclusion

Understanding internal biomechanics is particularly important for assessing the causes and prevention of movement-related injuries.

References

  1. ↑ McGinnis PM. Biomechanics of sport and exercise. 3rd. Human Kinetics; 2013.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Levangie PK, Norkin CC. Joint Structure and function: a comprehensive analysis. 4th. Philadelphia: FA. Davis Company. 2005.
  3. ↑ Barrett JM, Callaghan JP. A one-dimensional collagen-based biomechanical model of passive soft tissue with viscoelasticity and failure. Journal of Theoretical Biology. 2020 Sep 12;509:110488.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Hertling D, Kessler RM. Management of common musculoskeletal disorders: physical therapy principles and methods. Lippincott Williams & Wilkins; 2006.
  5. ↑ 5.0 5.1 Zitnay JL, Weiss JA. Load transfer, damage, and failure in ligaments and tendons. Journal of Orthopaedic Research®. 2018 Dec;36(12):3093-104.
  6. ↑ Freedman BR, Rodriguez AB, Leiphart RJ, Newton JB, Ban E, Sarver JJ, Mauck RL, Shenoy VB, Soslowsky LJ. Dynamic loading and tendon healing affect multiscale tendon properties and ECM stress transmission. Scientific reports. 2018 Jul 18;8(1):1-3.
  7. ↑ Lin AH, Slater CA, Martinez CJ, Eppell SJ, Yu SM, Weiss JA. Collagen fibrils from both positional and energy-storing tendons exhibit increased amounts of denatured collagen when stretched beyond the yield point. Acta Biomater. 2023 Jan 1;155:461-470.
  8. ↑ Murphy AC, Muldoon SF, Baker D, Lastowka A, Bennett B, Yang M, Bassett DS. Structure, function, and control of the human musculoskeletal network. PLoS biology. 2018 Jan 18;16(1):e2002811.
  9. ↑ Choi IA, Umemoto A, Mizuno M, Park-Min KH. Bone metabolism - an underappreciated player. NPJ Metab Health Dis. 2024 Jul 1;2(1):12.
  10. ↑ 10.0 10.1 Malik SS, Malik SS. Orthopaedic biomechanics made easy. Cambridge University Press; 2015 May 28.
  11. ↑ Walker J. Skeletal system 1: the anatomy and physiology of bones. Nursing Times. 2020 Feb;116(2):38-42.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 12.6 Eric Shamus, Arie J. van Duijn. Manual Therapy of the Extremities [Internet]. Burlington, MA: Jones & Bartlett Learning; 2017
  13. ↑ Kim Y, Lee G. Immediate Effects of Angular Joint Mobilization (a New Concept of Joint Mobilization) on Pain, Range of Motion, and Disability in a Patient with Shoulder Adhesive Capsulitis: A Case Report. The American journal of case reports. 2017;18:148.
  14. ↑ Medina McKeon JM, Hoch MC. The ankle-joint complex: a kinesiologic approach to lateral ankle sprains. Journal of Athletic Training. 2019 Jun;54(6):589-602.
  15. ↑ Hall S. Basic biomechanics. 4th. McGraw-Hill Higher Education; 2014 Feb 7.
  16. ↑ 16.0 16.1 16.2 Importance of Open & Closed packed Positions. TSPT. 2015. Last Accessed 1 March 2021
  17. ↑ Yin Y, Wang J, Lin Q, Luo Y, Liu Y, Sun J. Effect of proprioceptive neuromuscular facilitation on patients with chronic ankle instability: A systematic review and meta-analysis. PLoS One. 2025 Jan 9;20(1):e0311355.
  18. ↑ Park KJ, Seo TB, Kim YP. Effects of proprioceptive neuromuscular facilitation and both sides up ball exercise on pain level, range of motion, muscle function after total knee arthroplasty. J Exerc Rehabil. 2024 Feb 21;20(1):17-23.
  19. ↑ Bonanno M, De Pasquale P, Fonti B, Gjonaj E, De Salvo S, Quartarone A, Calabrò RS. Neural control meets biomechanics in the motor assessment of neurological disorders: a narrative review. Front Neural Circuits. 2025 Jun 27;19:1608328.
  20. ↑ Zatsiorsky VM, Prilutsky BI. Biomechanics of skeletal muscles. Human Kinetics; 2012 Apr 10
  21. ↑ Rivière JR, Morin JB, Bowen M, Cross MR, Messonnier LA, Samozino P. Exploring the low force-high velocity domain of the force-velocity relationship in acyclic lower-limb extensions. Sports Med Open. 2023 Jul 13;9(1):55.
  22. ↑ Eshoj HR, Rasmussen S, Frich LH, Hvass I, Christensen R, Boyle E, Jensen SL, Søndergaard J, Søgaard K, Juul-Kristensen B. Neuromuscular Exercises Improve Shoulder Function More Than Standard Care Exercises in Patients With a Traumatic Anterior Shoulder Dislocation: A Randomized Controlled Trial. Orthopaedic Journal of Sports Medicine. 2020 Jan 30;8(1):2325967119896102.
  23. ↑ Lee SJ, Zhang LQ. Learning Patterns of Pivoting Neuromuscular Control Training–Toward a Learning Model for Therapy Scheduling. IEEE Transactions on Biomedical Engineering. 2018 May 30;66(2):383-90.
  24. ↑ 24.0 24.1 Han J, Waddington G, Adams R, Anson J, Liu Y. Assessing proprioception: a critical review of methods. Journal of Sport and Health Science. 2016 Mar 1;5(1):80-90.
  25. ↑ ALAHMARI KA, Samuel PS, Ahmad I, Reddy RS, Tedla JS, Kakaraparthi VN, Rengaramanujam K. Effectiveness of Low-Frequency Stimulation in Proprioceptive Neuromuscular Facilitation Techniques for Post-Ankle-Sprain Balance and Proprioception in Adults-a Randomized controlled trial.
  26. ↑ Chaturvedi A. Effect of proprioceptive neuromuscular facilitation in functional recovery of patients with stroke–a review. J Neurol Neurosci. 2017;8(5):220.
  27. ↑ Guiu-Tula FX, Cabanas-Valdés R, Sitjà-Rabert M, Urrútia G, Gómara-Toldrà N. The efficacy of the proprioceptive neuromuscular facilitation (PNF) approach in stroke rehabilitation to improve basic activities of daily living and quality of life: a systematic review and meta-analysis protocol. BMJ Open. 2017 Dec 12;7(12):e016739.
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