Domain: Healthcare — Human Physiology
Target concept: Triceps Reflex

Video generation prompt: A seated subject relaxes the arm with the elbow flexed. The examiner places the reflex hammer above the olecranon and strikes the triceps brachii tendon once. The forearm exhibits a reactive movement. Focus on the clear hand position, the striking action, and the resulting arm movement.

Evaluation guide:
(i) Target concept: Monosynaptic stretch reflex mediated by triceps brachii muscle spindle activation leading to involuntary elbow extension.
Minimum mechanism: Intact C6–C7 spinal segments, functional radial nerve, viable triceps musculotendinous unit, and passive mechanical coupling between tendon strike and elbow joint rotation.
(ii)Storyline: [Phase 1: Baseline] Seated subject with relaxed, flexed arm held steady; visible olecranon and tendon region exposed. [Phase 2: Intervention] Hammer approaches and contacts tendon at precise anatomical location; contact must be singular, unambiguous, and spatially anchored above olecranon. [Phase 3: Response] Elbow extension initiates immediately after contact; movement must be isolated to elbow joint, originate from rest, follow a single arc, and terminate cleanly. [Perspective constraint: Fixed frontal/slightly oblique view showing full arm from shoulder to hand, with unobstructed view of posterior elbow and tendon region throughout all phases.]

Manual:
Experimental Procedure
1. Subject sits upright, arm relaxed at side, elbow flexed to ~90°, forearm supported or hanging naturally.
2. Examiner positions reflex hammer tip directly above the olecranon, aligning with the triceps brachii tendon (a narrow vertical band just proximal to the bony tip).
3. Examiner delivers one firm, quick strike to the tendon.
Observation of Experimental Phenomena
Forearm extends rapidly at the elbow joint, with no concurrent shoulder, wrist, or hand movement; hand remains neutral; movement is singular, smooth, and terminates without rebound.
Summary of Experimental Principles
The triceps reflex tests the C6–C7 spinal cord segments via the radial nerve; tapping the triceps tendon stretches the muscle spindle, triggering monosynaptic reflex arc resulting in involuntary elbow extension.

Prompt Grounding rubric:
level_1: No seated subject visible; no relaxed arm with elbow flexed; no reflex hammer present; no strike action shown against the arm; no forearm movement of any kind observed.
level_2: A seated subject and an arm are visible, but the elbow is not held in a relaxed, flexed posture (e.g., the arm is straight, the fist is clenched, or the shoulder is raised); a reflex hammer is present but is never brought to the back of the elbow, or it contacts an unrelated body part such as the knee or wrist; no forearm movement follows, or an unrelated movement occurs instead (e.g., a shoulder shrug or a finger tap).
level_3: A seated subject with a relaxed, flexed elbow (roughly 90°) is shown; a reflex hammer is brought to the back of the upper arm and makes one contact near the elbow; some forearm movement follows, but the hammer's contact point is not clearly above the olecranon (for example it lands on the point of the elbow itself or too high on the upper arm), or the movement afterward is too brief or ambiguous to tell that it starts at the elbow.
level_4: The subject's arm is fully visible with the elbow flexed near 90° and relaxed; the hammer tip is placed just above the olecranon, over the region where the triceps tendon runs, and delivers a single clear tap there; a forearm movement follows the tap without hesitation, and no other joint (shoulder or wrist) visibly moves at the same time.
level_5: Everything described for a 4 is true, and in addition the hammer's contact point sits on the narrow tendon band just above the bony point of the elbow rather than on the bone itself or lower on the forearm; the strike is a single crisp tap rather than a push or repeated taps; the forearm's movement begins essentially at the moment of contact, follows one continuous path, and the hand's palm-up/palm-down orientation stays the same as it was before the tap.

Scientific and Causal Correctness rubric:
level_1: No forearm response follows the tap, or the response contradicts the triceps reflex (e.g., the elbow bends further instead of straightening, the wrist drops, the fingers curl, or the shoulder pulls inward); the subject's arm looks braced or actively resisting rather than relaxed.
level_2: The forearm moves after the tap, but the movement is not an elbow-extension reflex: it originates at the shoulder or wrist rather than the elbow, the elbow fails to straighten at all, the movement is jerky or repeats back and forth, or the subject visibly tightens the upper-arm or hand muscles on purpose (e.g., a bulging bicep, a shrugging shoulder, a clenched hand) before responding.
level_3: The elbow does extend after the tap, but the movement is not clean: it is accompanied by extra motion at the shoulder, wrist, or fingers, or a visible forearm twist; alternatively, the arm looks relaxed beforehand but the subject visibly tenses the arm just before or during the response, suggesting a voluntary flex rather than an involuntary reflex.
level_4: The elbow extension is isolated to the elbow joint and looks passive throughout, with no visible tightening of the shoulder, upper-arm, or hand muscles at any point; the arm stays relaxed both before and after the response, with no bracing, no shrug, and no correction beyond the single extension.
level_5: Everything described for a 4 is true, and in addition the extension's size and timing look physiologically right for a stretch reflex: the forearm moves from its flexed starting angle to roughly straight without snapping past straight into hyperextension, the movement stops cleanly without bouncing back, and it begins essentially without any pause after the tap; the hand's orientation does not change during the movement.

Spatiotemporal Consistency rubric:
level_1: The subject or arm disappears and reappears between frames, or jumps to a different position on screen; the hammer appears already in contact with the arm with no visible approach; the room's lighting or the arm's shadow changes abruptly partway through the clip; the elbow appears to bend in an anatomically impossible way, such as folding backward.
level_2: There is a visible jump cut or skip during the swing or the forearm's response; the hammer's position jumps from one spot to another with no visible path connecting them; the elbow angle changes back and forth non-monotonically (extending, then flexing, then extending again) instead of moving in one direction; the background shifts or the camera zooms for no reason tied to the action.
level_3: Only small continuity issues appear: the hammer's path toward the arm has a brief jitter, the moment of contact is briefly blurred, or a hand or sleeve momentarily blocks the view of the tendon area before the view clears again; the forearm's movement pauses briefly but still continues in the same direction overall.
level_4: The hammer's approach, the tap, and the forearm's response play out as one continuous, physically believable sequence with no reversal, freeze, or overshoot in the arm's motion; the tendon and elbow area are never hidden from view during the tap or the response; the room's lighting stays the same in direction and brightness for the whole clip, with no sudden change tied to the action.
level_5: Everything described for a 4 is true, and in addition the entire strike-and-response sequence reads as one smooth, uninterrupted action at normal viewing speed: there is no freezing, stutter, or sudden speed change in the hammer's swing or the forearm's movement, the elbow stays in roughly the same place in the frame throughout the response, and the moment of hammer-tendon contact is clearly visible rather than smeared into a blur.