Introduction to Human Biomechanics 2

Original Editor - Tolulope Adeniji Top Contributors - Tolulope Adeniji, Tarina van der Stockt and Kim Jackson  


When an external forces act on a body segment to produce posture and movement the internal structures generate forces to interact with the external forces for desire posture and movement. Understanding the mechanical properties of these internal structures will enable clinicians like physiotherapist to evaluate causes of injury that are movement-related and to prevent it. When an external forces act on a body segment for movement and posture the body tissue also respond by generating force to interact for desire posture and movement.[1] These body tissue response to the external force applied is called stress. Stress is define as the amount of force on an area of the connective tissue. The relative change or deformation in the shape of the structure that accompanies the stress is called strain.[2] This crimpy property of dense connective is peculiar to all biological collagenous tissues that undergo tension.[2]

Viscoelastic properties

Another property of dense connective tissue is viscoelastic properties. This property allows a dense connective tissue fibre to temporarily elongate but quickly recovers to its original length, with the removal of stress. This temporarily elongation of a dense connective tissue is referred to as compliance while the tissue ability to return to original length after the stress removal is called elasticity. This activities occur at physiologic limit and the crimpy property of a relax collagen fibre is removed when a dense connective tissue elongate.[2][3] Thus it is important to consider what is this physiological limit when stretching a dense connective tissue. And stress-strain curve will give us overview of this physiologic limit.[2] Also for more information see tendon biomechanics on Physiopedia page for more information on viscoelastic properties of a dense connective tissue

Stress-Strain Curve

The Stress-Strain Curve is also known as the load-deformation curve when a connective tissue undergoes tension. Based on the force applied to tissue, the stress-strain curve can be explained at level of four area, the toe, elastic , plastic region and ultimate failure point.[2] At the toe region of the curve the crimp straightens with little force collagen fibers are stretched as the elastic region begins. Furthermore, force application to collagen after the elastic region causes a residual change in tissue structure that is referred to as plastic region. If this force persists the tissue may rupture at its ultimate failure point. [2] The stress-strain curve is applicable in manual therapy when early rehabilitation of a joint should focus on the joint proprioception by mobilizing the joint tissue at what we can said to have fall in the toe region of the stress-strain curve.[4][5] This is because at this region the tissue crimp can be removed without any trauma and it is at this region that type 1 to 111 mechanoreceptors are active. But further stress on the connective tissue to the point of eliciting type iv receptor that is a nociceptive pain suggests that the mobilization is no longer at normal physiologic limit.[4] The concept of the stress-strain curve is also essential in understanding the interaction between dense connective tissue constituents and the physical mechanisms such as stretching, sliding that govern material behavior at and between length scales.[5][6] Thus it is essential to understand details of these concept of load transfer and damage from the molecular to the tissue level for delivering effective intervention and safe practice

Human Skeletal System

In relation to mechanics, skeletal system is a rigid links that are connected to each other at joints to allow specific movements. The human skeletal system comprises of bones, tendons, ligaments and soon. Muscles are attached to bones to provide forces and generate movement. It is important to understand biomechanical properties of skeletal system elements to understand human motion. More importantly, the nature of musculoskeletal interconnection enhance human stability, voluntary movement, and robustness to injury[7]

Biomechanical properties of bone

Bone is a highly specialised connective tissue, and a biological composite. It is composed of two types of bony tissues : cortical bone that is also known as compact bone and cancellous bone that is also known as spongy bone. Cortical bone forms the walls of the bone and it is dense (5%–30% porous) and stiff,[8] while cancellous bone occupies the central space within the bone, which is less dense (90% porous) but considerably more malleable.[8][9] Bone has great compressive, tensile, viscoelastic and shear strength and this biomechanical properties of bone make skeleton very well suited for its function to support body load and movement of body segments.

Biomechanical properties of human joints

A joint can be defined as the area where two or more bone meets. A joint's primary function is to allow motion and also in force transferal.These two functions led to interesting structural designs of joint. Human joint materials include bones, bursae, capsules, cartilage, ligament, tendon and other associated materials. And it important to note that the fact that the materials used in human joints are composed of living tissue makes human joints unique. This uniqueness includes the ability of the tissue to respond to environmental changes or functional demands.[2]

Joint classification

Atomists have categorized joints in several ways, some of these classification is based on joint complexity, the number of axes present, joint geometry, or movement capabilities. Because in biomechanics we are more interested in kinesiologic motion that occur in a joint, we will be focusing on joint classification system based on its motion capabilities. From the perspective of movements permitted, there are three major categories of joints: synarthroses (immovable joints)—suture joints in the skull, amphiarthroses (slightly movable joints),--- pubic symphysis and diarthrosis (freely movable joints)...hinge joint and ball and socket joint (see table 1).

Joint Classification.png

Joint surfaces

The nature of movement at any joint is largely determined by the joint structure, especially the shapes of the joint surfaces. The traditional classification of synovial joints by structure includes the categories of spheroid, trochoid, condyloid, ginglymoid, ellipsoid joint and so on.[4]

Limitations in the anatomical joint description

The anatomical classification of joints is deficient in explaining finer details of mechanical properties of a joint structure and this details might be useful to clinicians that are considered movement specialist such as Physiotherapist.[4] For instance, joint surface is neither truly spheres, ovals, nor ellipses. But, any joint surface can be thought of as being part of an ovoid surface, that is, resembling the surface of an egg. Another example, an hinge joint under diarthrosis, such as the humeroulnar joint, does not allow a true hinge motion on flexion and extension but rather a helical movement involving considerable rotation. Also traditional classification of joint movement such as sliding, angular, rotation and circumduction ignore movement occurring between joint surfaces meaning that when movement is defined what happens at that joint surface is often ignored.[4] Therefore, it is essential to classify joints into specifics of joint movement occurring between bones and between respective joint surfaces. This leads us to the concept of osteokinematic and arthrokinematic.

Osteokinematic and Arthrokinematic

Osteokinematic deal with movement occurring between two bones. Two types of movement occur in osteokinematics: spin and swing, which occurs about a joint mechanical axis. Mechanical axis is a line that passes through the moving bone, touching the centre of the relatively stationary joint surface and lying perpendicular to it.

Spin occurs when a bone moves on a stationary mechanical axis. While pure swing allows a bone to move on several points of an opposing joint surface. The two movements that occur in swing are roll and slide (glide) and this led to the concept of arthrokinematic.

Arthrokinematic describes movement at the joint surface. If joint surface “A” roll on the opposing joint surface “B”, the joint surface “A” will move at different points of contacts on the corresponding opposing joint surface “B” at the same intervals. For example, the tire of a car needs to roll on different points on the ground before motion takes place. If, however, only one point on the moving joint surface contacts various points on the opposing surface, slide is said to have taken place. This is analogous to a tire on a car that is skidding on ice; the tire is not turning but is moving relative to the road surface. One clinical implication of these is that a joint movement that occurs in the absence of normal arthrokinematic will cause either impingement or dislocation.[4] More so, principles of osteokinematic and arthrokinematic are essential in relieving adhesive joints. Recent findings note that when a patient with shoulder adhesive capsulitis was managed with angular joint mobilization, a form of osteokinematic approach, there was an improvement in the shoulder pain, ROM, and disability.[10] Another paper showed that consideration of osteokinematic in lateral ankle sprain is essential to improve its ROM[11]

Concave-convex rule

An important concept from arthrokinematic is concave-convex rule. The concave-convex rule states that if a concave surface moves on a convex surface, roll and slide must occur in the same direction; if a convex surface moves on a concave surface, roll and slide occur in opposite directions. Clinicians like Physiotherapist may need to apply this concept in the restoration of restricted joint motion, by considering roll and slide motion alongside the traditional anatomical motion of the joint that is involved. [4] For example, if flexion of the humerus is restricted, in addition to active or passive motion into flexion to increase this movement, one must also consider that inferior glide of the head of the humerus on the glenoid cavity as it may be restricted as well. Thus, by knowing the concave-convex rule, the therapist knows in which direction to apply joint slide mobilizations to increase any restricted swing of a bone.[4]

Close and Loose-packed Position

Joint stability is the ability of the joint to resist displacement of the articulating bones. Although most joints have asymmetrical surface but there is typically one position of best fit in which the area of contact is maximum. This is known as the close-packed position, and it is in this position that joint stability is usually greatest. Any movement of the bones at the joint away from the close-packed position results in a loose-packed position, with reduction of the area of contact.[12]

Application of close and loose-packed position concept

This concept is applied to joint distraction and compression. A distraction occurs when there is separation of joint surfaces while approximation of opposing surfaces is compression. The principle here is that a movement toward the close-packed position has an element of compression, whereas movement out of this position involves distraction. Hertling et al [4]noted that the concept of close and loose-packed position of a joint will help clinician like Physiotherapist to understand joint stability and to know when to apply joint distraction and compression when exercising a joint.

Joint Flexibility

It is important for clinicians to know that joint flexibility is primarily a function of the relative tightness of the muscles and ligaments that span the joint. And If these tissues are not stretched, they tend to shorten. Approaches for increasing flexibility include active versus passive stretching, static versus dynamic stretching and PNF. Among this approach, PNF is among the most effective procedure for stretching muscles and ligaments. This is because all PNF patterns involve some pattern of alternating contraction and relaxation of agonist and antagonist muscles for effective stretching.

Muscular System

Muscles exert forces on the bone to produce movement and if the force exerted is to maintain a constant length this is called isometric action and the force exerted is to contract the muscle for shortening it is called concentric action, and the action is lengthening it is called an eccentric contraction. In concentric muscle action, there is positive mechanical work that is being done while in isometric muscle action there is no mechanical work being done by isometric muscle action. And negative mechanical work is done by eccentric muscle action.

Skeletal muscle fibre type

Biomechanics of sports and exercise: In a skeletal muscle fibre, the sarcomere is the fundamental structure and within it, the muscle generates active tension by cross-bridges of myosin filaments to actin filaments.[13] The rate of tension generated depends on the type of muscle fibre and Skeletal muscle fibres differ in terms of their fatigue resistance and rate of tension development. The muscle Fibres are classified into three types according to these differences: type I, or slow-twitch oxidative (SO); type IIA, or fast-twitch oxidative-glycolytic (FOG), and type IIB, or fast-twitch glycolytic (FG)

Mechanical Characteristics of Muscle

There are three mechanical characteristics of muscle that are based on the variations in muscle force because of differences in velocity, length, and the time relative to activation. The Force–Velocity Relationship explains how the force of fully activated muscle varies with velocity. In the force-velocity relationship of a muscle that is shortening i.e concentric action, the force the muscle can create decreases with increasing velocity and vice versa for eccentric actions. Force–Time relationship describes a temporal delay in the development of tension. This temporal delay can be partly caused by a delay in the rise of muscle stimulation (active state or excitation dynamics ). Repeated firing of muscle fibres can cause a temporal delay in the time to develop tension. However. length of time to develop tension depends strongly on the cognitive effort of the subject, training, kind of muscle action, and the activation history of the muscle group.

Neuromuscular control of motion

The mechanical response of muscles also strongly depends on how the muscles are activated. The basic functional unit of neuromuscular control includes motor unit, recruitment and firing rate. A motor unit is one motor neuron and all the muscle fibres it innervates. While Recruitment is the activation of different motor units within a muscle and firing rate is the repeated stimulation of a particular motor unit over time. A tetanic muscle contraction occurs when there is a sustained muscle contraction induced when the motor nerve innervates a skeletal muscle produces action potential at a very high rate.

Neuromuscular control of motion is one of the concepts in designing of neuromuscular exercise for neuromuscular impairments. A recent study had shown that in patients with anterior shoulder dislocations with shoulder mechanical loss and proprioceptive impairment improves with shoulder instability neuromuscular exercise.[14] Another recent paper also noted that neuromuscular control exercises are essential in pivoting related injuries and it improves lower limb functions among patients with pivoting related movement disorders such as cerebral palsy, stroke, and incomplete spinal cord injury.[15]

Proprioception of muscle action and movement

What is proprioception? Charles Sherrington first used the term proprioception to mean “the perception of joint and body movement as well as the position of the body, or body segments, in space”.[16] In a muscle, muscle spindles are a proprioceptive stretching receptor, and it is responsible for stretch reflex or myotatic reflex. Static reflex is a muscle contraction in response to stretching within the muscle. Also, muscle spindle is sensitive to slow stretching of the muscle, but provide the largest response to rapid stretches.[16]Physiotherapists are saddles with the responsibility of restoring balance control and postural dysfunctional corrections in patients with neurological and or musculoskeletal impairments such as hemiparesis due to stroke, nerve impairments and soft tissue injuries. One means to achieve a significant functional improvement is through the use of proprioceptive neuromuscular facilitation, PNF, whose core includes the use of proprioception for activating muscle action and movement. A recent paper had shown that PNF with transcutaneous electrical nerve stimulation improves pain, ROM and balance in patients with ankle sprains[17] The PNF techniques have also proven to be effective in stroke rehabilitation as well.[18]

Reciprocal inhibition

In reciprocal inhibition, the muscle spindles act in such a way to inhibit an opposing muscle action when the muscle of interest is shortening. This relaxation of an opposing muscle when the muscle of interest is shortening in action contributes to the efficient movement of the muscle that is shortening. For example when you contract your biceps then triceps are inhibited to allow efficient elbow flexion.

Active and Passive Insufficiency

Another important concept of motion is active and passive insufficiency. Active insufficiency is the decreased tension of a multiarticular muscle when it is shortened across one or more of its joints. This concept implies that Muscle shortening leads to low tension. In passive insufficiency, there is a limitation of joint motion when multiarticular muscles are stretched across multiple joints.[19] This concept is important in generating tension in muscle or explain the limitation of a joint range of motion when multiarticular muscles like hamstrings and quadriceps are to be exercised.


Internal biomechanics knowledge is important particularly to evaluate causes and prevention of movement-related injuries.


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