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20-Bio-A4 Anatomy and Physiology · Undated paper

Question 3 of 4: Ligament Stress–Strain Behaviour and a Winged-Scapula Case

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exams May 2019 — 04-Bio-A4, Biomechanics. 3 hours, open book (any non-communicating calculator permitted). FOUR (4) questions constitute a complete exam paper; each is of equal value (15 marks).

Question 4's arm figure is read as 300 mm at 60° and 350 mm horizontal. The marking-scheme line for Question 2 prints a stray digit (“c) 5 4 marks”) although Question 2 has only three sub-parts; this solution uses the 5 marks per sub-part that the question text states (5 + 5 + 5 = 15).
Check — subject-identity note: every page of the paper is headed and footed “04-BIO-A4, May 2019” and the cover page itself states the subject as “04-Bio-A4, Biomechanics”. All four questions (knee-joint structure/kinematics, glenohumeral joint statics, ligament mechanical behaviour, upper-limb inverse dynamics) are genuine Biomechanics content. This solution follows the paper's true subject and cites biomechanics references accordingly.

Reference texts: Winter, Biomechanics and Motor Control of Human Movement (4th ed.); Zatsiorsky, Kinematics of Human Motion; Nordin & Frankel, Basic Biomechanics of the Musculoskeletal System (5th ed.).

Question 3: Ligament Stress–Strain Behaviour and a Winged-Scapula Case (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) A ligament loaded along its long axis produces a characteristic non-linear curve with five recognisable regions, low stiffness at small strain rising to a much stiffer near-linear region, then softening again toward failure.

Stress Strain iii) Toe region ii) Elastic (linear) region iv) Yield stress v) Plastic region i) Failure stress
Figure 3 — idealised ligament stress–strain curve along its long axis, with the five requested features labelled i–v.

Reading the curve left to right: the toe region (iii) is the initial shallow, concave-up segment where crimped collagen fibrils straighten under low load and compliance is high; the elastic (linear) region (ii) follows once fibrils are taut and load is carried directly along the collagen, giving a much steeper, near-constant modulus; the yield stress (iv) marks the onset of progressive fibril micro-failure, after which the plastic region (v) shows increasing deformation for little added stress as fibres fail sequentially, ending at the failure stress (i), the peak load the ligament sustains before macroscopic rupture.

(b) The photograph shows both inferior scapular angles protruding posteriorly and medially away from the thoracic wall — classic bilateral winged scapula (scapular winging). Unilateral winging is most often serratus anterior palsy from an isolated long thoracic nerve injury, but true bilateral, symmetric winging in a young patient is the textbook presentation of a primary muscle disease affecting the periscapular stabilisers — most characteristically facioscapulohumeral muscular dystrophy (FSHD), in which weakness of serratus anterior, trapezius and other scapular stabilisers lets the scapulae ride up and wing symmetrically; bilateral serratus anterior/periscapular weakness of another cause (e.g. bilateral traction neuropathy, inflammatory myopathy) is the main differential.

(c) Rehabilitation centres on scapular stabilisation strengthening targeting the remaining serratus anterior, middle/lower trapezius and rhomboid function — progressive closed-chain exercises such as the push-up-plus and wall/incline scapular protraction drills, plus periscapular proprioceptive and postural retraining. A rigid or semi-rigid scapular orthosis (bracing) can mechanically hold the medial border against the thorax during functional activity while strength is built, and a neurology/genetics referral is appropriate to confirm the underlying diagnosis (nerve conduction studies if a nerve lesion is suspected; genetic testing if FSHD is suspected), since the rehabilitation plan and prognosis differ materially between the two.

(d) Loss of the scapula's stable base disrupts normal scapulohumeral rhythm — the coordinated 2:1 glenohumeral-to-scapular rotation needed to keep the subacromial space open during arm elevation. With the scapula unable to upwardly rotate and posteriorly tilt properly, the humeral head migrates superiorly during overhead motion, narrowing the subacromial space. This patient is therefore now at elevated risk for secondary shoulder impingement syndrome (and, with repeated impingement, rotator cuff tendinopathy/tears) on the affected side(s).