Question at position 21 Which muscle is the primary flexor of the trunk during movements such as sit-ups? Rectus abdominis Iliocostalis External oblique Transversus abdominis Clear my selection单项选择题
A
Rectus abdominis
B
Iliocostalis
C
External oblique
D
Transversus abdominis

Question

Question at position 21 Which muscle is the primary flexor of the trunk during movements such as sit-ups? Rectus abdominis Iliocostalis External oblique Transversus abdominis Clear my selection

BlackTom AI · Accepted Answer

Rectus abdominis is a long, vertical muscle running from the pubic symphysis to the xiphoid process and the costal cartilages; its fiber orientation and attachments make it the principal muscle that decreases the angle between the thorax and pelvis, i.e., produces trunk flexion.

Relevant knowledge points:
- Muscle actions are determined by origin, insertion, and fiber direction. Rectus abdominis has vertical fibers and segments (tendinous intersections) that pull the rib cage toward the pelvis when it contracts.
- Synergists and antagonists: External obliques act as synergists in flexion and are prime movers in rotation; transversus abdominis is a deep stabilizer, compressing the abdomen; iliocostalis (part of the erector spinae) is a posterior extensor of the spine and thus an antagonist to flexion.

Step-by-step reasoning:
1. Identify the movement: sit-ups require flexion of the lumbar and thoracic spine (bringing the thorax toward the pelvis).
2. Determine which muscle's contraction produces that motion: contraction of rectus abdominis shortens the distance between the rib cage and pelvis, creating trunk flexion.
3. Consider other muscles listed: external oblique can assist (especially when both sides contract) and contributes to rotation; transversus abdominis compresses the abdominal cavity and stabilizes the trunk but does not produce major sagittal flexion; iliocostalis extends the spine, opposing the motion in a sit-up.

Why the correct answer is correct:
Rectus abdominis is anatomically positioned and oriented to generate the primary force for sagittal trunk flexion during movements such as sit-ups, making it the main flexor.

Why the other options are incorrect:
- Iliocostalis: extensor (part of erector spinae), opposes flexion.
- External oblique: assists in flexion but is mainly involved in rotation and lateral flexion; not the primary flexor.
- Transversus abdominis: deep stabilizer that compresses the abdomen, not a primary trunk flexor.

Conclusion: The correct answer is Rectus abdominis, which matches the provided answer.

Question at position 21 Which muscle is the primary flexor of the trunk during movements such as sit-ups? Rectus abdominis Iliocostalis External oblique Transversus abdominis Clear my selection单项选择题

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