Mechanical Properties + Biomechanics of Biological Materials

Question 1

What effect does cross-sectional area have on muscle function?

Answer 1

The cross-sectional area of a muscle is a major determinant of its absolute strength. The greater the cross-sectional area, the more muscle fibres there are, and thus, the greater the potential for force production. This means that muscles with larger cross-sectional areas can generate more force compared to muscles with smaller cross-sectional areas, assuming all other factors are equal. Muscle force is directly proportional to the cross-sectional area. When a muscle contracts, the force it generates is distributed across its cross-sectional area. A larger cross-sectional area means a greater total force output.

Question 2

What is the difference between a cosynergist and a synergist?

Answer 2

Fine control of movement requires the contraction of agonists, antagonists and a range of synergists. Agonists perform the desired movement; antagonists oppose the desired movement. If an agonist has two actions but only one action is wanted, recruit a true synergist to antagonise the unwanted action; on the other hand a cosynergist has a dual function in antagonising the unwanted action AND enhancing the desired movement.

Question 3

A muscle that has a greater moment arm is said to have a mechanical advantage. Explain this statement.

Answer 3

A muscle with a greater moment arm has a mechanical advantage. This means that the muscle is more effective in generating or controlling movement around a joint because it can produce a larger torque (rotational force) for a given amount of muscle force.

Question 4

Describe the composition of the two parts of the intervertebral disc

Answer 4

The nucleus pulposus is a gelatinous mass with a high proportion of water and glycosaminoglycans, which help to maintain the water content through their negatively-charged molecular structure. Note that the position of the nucleus pulposus tends to lie more to the posterior aspect of the intervertebral disc. The annulus fibrosus consists of the traditional fibrocartilaginous tissue structure and surrounds the nucleus pulposus like a cage. The annulus fibrosus
consists of a high density of collagenous fibres which are oriented in multiple directions to help resist bending and torsional loads. The annulus fibrosus is organised into a number of lamellae (layers), with each lamellae having a perpendicular arrangement of collagenous fibres to the adjacent lamella to produce a structure able to resist loads in multiple directions

Question 5

What physiological change explains the force-length relationship of muscles? Why is force production highest at the resting length of the sarcomere?

Answer 5

When a muscle is shortened beyond its optimal resting length, there is less overlap between actin and myosin filaments. This reduced overlap leads to fewer cross-bridge interactions and less force generation. Force production is highest at the resting length of the sarcomere because this length allows for the optimal degree of filament overlap, maximizing cross-bridge formation and thus the number of active cross-bridges in each sarcomere.

Question 6

Which ligament has the largest mechanical advantage in stabilising flexion?

Answer 6

The ligament with the largest lever is able to produce the greatest passive tension and has the greater ability to withstand excessive flexion movements (greatest mechanical advantage in stabilising flexion). Patellar ligament

Question 7

Explain how proteoglycans allow articular cartilage to resist compressive loads.

Answer 7

When compressive stress is applied, water flows out, increased proteoglycan concentration which increases repulsive forces of +ve charged glycosaminoglycans which aid in resistance to compressive forces

Question 8

Why is physical activity essential in maintaining the health of articular cartilage?

Answer 8

Collagen density is highest in the superficial zone where stresses are the greatest with fibres lying parallel to the joint surface in the superficial zone compared to fibres oriented perpendicular to the joint surface in the deep zone (middle zone has random fibre arrangement). Proteoglycans offers resistance to compression also

Question 9

Why are the stress-strain properties different when the cartilage is loaded at a high velocity compared to a slow velocity?

Answer 9

Cartilage is a viscoelastic material, which means it exhibits both viscous (fluid-like) and elastic (solid-like) properties. The mechanical response of viscoelastic materials depends on the rate at which they are loaded. Cartilage deforms differently under dynamic or high-velocity loading compared to slow or static loading.

Question 10

Consider the case of a herniated disc. Why are herniated discs most common in the lumbar region of the vertebral column and why do they occur most commonly in a posterolateral direction? Also consider the consequences of a herniated disc on patient health.

Answer 10

A herniated disc (slipped disc) results from a degeneration or trauma to the annulus fibrosus which often leads to a full tear in the annulus fibrosus and movement of the nucleus pulposus. This often occurs in a posterolateral direction which may cause direct impingement on spinal nerve roots.

The lumbar spine bears a significant portion of the body’s weight and is subject to constant mechanical stress and movement. This increased load and movement make the lumbar region more vulnerable to disc herniations.

Regarding the posterolateral direction of herniated discs, this is often due to the orientation and anatomy of the lumbar discs. The nucleus pulposus, which is the gel-like centre of the disc, is often located toward the posterior aspect of the disc. When there is a tear or weakness in the annulus fibrosus (the outer fibrous ring of the disc), the nucleus pulposus can push through this opening, typically in a posterolateral direction.

Question 11

Which part of the long bone has the highest proportion of cortical bone tissue?

Answer 11

Diaphysis

Question 12

Which part of the long bone has the highest proportion of trabecular bone tissue?

Answer 12

Epiphysis

Question 13

Which muscle is considered ‘strongest’ in moving the glenohumeral joint?

Answer 13

Deltoid

Question 14

Which muscle is a co-synergist for biceps brachii in elbow flexion?

Answer 14

Brachialis

Question 15

Which muscle is a synergist for biceps brachii in supination?

Answer 15

Supinator muscle

Question 16

Which muscle is a synergist for biceps brachii in elbow flexion?

Answer 16

Brachioradialis

Question 17

In what position of the forearm is elbow flexion normally the strongest? Why?

Answer 17

Supinated as it uses biceps brachii, brachialis, brachioradialis

Question 18

What is the advantage of having multiple muscle fibres in series (eg. pennate muscle such as rectus femoris) rather than a single long muscle fibre extending the length of the muscle belly (eg. parallel-fibred muscle such as sartorius)?

Answer 18

Pennate generate more force per unit as more fibres are packed parallel at an angle that creates mechanical advantage. Parallel muscles can create great ranges of motion

Question 19

Which muscle group is strongest at the talocrural joint? Justify your answer.

Answer 19

Tibialis anterior

Question 20

Consider the function of sesamoid bones in relation to muscle mechanics. In a patient with a patellectomy, how much more force would the quadriceps femoris muscle have to produce in order to produce the same moment vector?

Answer 20

33-50% more force needed. This is because it is attached by a ligament to the tibia which increases leverage afforded to quadriceps femoris muscle thus increasing efficiency of extension