Domain: Healthcare — Human Physiology
Target concept: Biceps reflex

Video generation prompt: A seated subject places the forearm on the thigh with the elbow flexed. The examiner locates the biceps brachii tendon in the cubital fossa, presses the thumb against the tendon, and strikes the thumb once with a reflex hammer. The forearm exhibits an immediate response.

Evaluation guide:
(i) Target concept: Monosynaptic deep tendon reflex elicited by mechanical stretch of the biceps brachii tendon leading to involuntary elbow flexion.
Minimum mechanism: Intact sensory (Ia afferent) and motor (alpha motor neuron) pathways from C5–C6; functional neuromuscular junction; passive mechanical coupling between tendon tap and muscle spindle activation; no voluntary interference or cortical inhibition.
(ii)Storyline: Frame 1–2: Seated subject in stable posture, forearm resting on thigh, elbow flexed; full visibility of anterior elbow/cubital fossa. Frame 3–4: Examiner"s hand enters view, thumb locates and settles over tendon in cubital fossa; anatomical landmarks (brachioradialis, pronator teres, antecubital crease) fully visible and correctly positioned. Frame 5: Reflex hammer strikes thumb; impact moment captured with clear contact and immediate skin deformation. Frame 6–7: Forearm initiates smooth, isolated elbow flexion; motion originates solely at elbow joint, no shoulder/wrist contribution, no delay relative to impact. Frame 8–10: Flexion completes with natural deceleration; thumb remains in place; no secondary motion or recoil. Perspective: Fixed frontal-oblique view centered on cubital fossa and elbow joint; no camera movement; full visibility of thumb, hammer head, tendon region, and forearm segment throughout.

Manual:
Experimental Procedure
1. Subject sits upright, places forearm on ipsilateral thigh, flexes elbow to ~90°.
2. Examiner locates biceps brachii tendon in cubital fossa (medial to brachioradialis, lateral to pronator teres).
3. Examiner presses thumb firmly over tendon, then strikes thumb once with reflex hammer.
Observation of Experimental Phenomena
Forearm exhibits immediate, involuntary flexion at the elbow joint — no wrist/hand movement, no shoulder involvement.
Summary of Experimental Principles
Biceps reflex tests C5–C6 spinal cord integrity via monosynaptic stretch reflex arc: tap stretches tendon → muscle spindle activation → Ia afferent signal → spinal synapse → alpha motor neuron efferent → biceps contraction → elbow flexion.

Prompt Grounding rubric:
level_1: No seated subject visible; no forearm-on-thigh placement; no elbow flexion shown; no examiner present; no reflex hammer visible; no thumb contact with cubital fossa region; no striking action depicted.
level_2: Seated subject present AND forearm placed on thigh OR elbow flexed (at least one correct), but missing ≥3 of: examiner presence, reflex hammer, thumb placement on cubital fossa, striking action, or correct anatomical location of tendon contact.
level_3: Seated subject present; forearm placed on thigh; elbow visibly flexed; examiner present and holding reflex hammer; thumb visibly contacting skin over cubital fossa region; striking action occurs once; all steps in correct sequential order, but thumb placement not confirmed as directly over biceps tendon (i.e., location ambiguous), or hammer strike lacks clear contact with thumb.
level_4: All elements from score 3 present AND thumb is unambiguously placed directly over the biceps brachii tendon in the cubital fossa (visually confirmed by anatomical landmarks: medial to brachioradialis, lateral to pronator teres, distal to antecubital crease), AND hammer strikes thumb once with clear impact motion: no extra strikes, no misplacement, no substitution (e.g., finger instead of thumb, hammer head instead of handle).
level_5: All elements from score 4 present AND the seated subject maintains stable posture without shifting during the entire sequence; the examiner's hand remains fully visible throughout thumb placement and the strike; the thumb, hammer, and cubital fossa region are never blocked from view; the hammer strike reads as one clean, unambiguous impact: a single visible contact against the thumb with no repeated taps, no visible hesitation before contact, and no repositioning or corrective adjustment of the hammer, thumb, or forearm either before or after it lands.

Scientific and Causal Correctness rubric:
level_1: No observable forearm movement response; or movement occurs without preceding strike; or movement is upward extension of elbow (not flexion); or movement involves wrist/hand rotation only; or response occurs >1 second after strike (visually discontinuous).
level_2: Forearm movement occurs, but direction is incorrect (e.g., extension instead of flexion) OR movement originates from shoulder or wrist rather than elbow joint OR movement is delayed and disconnected from strike timing (visible gap between hammer contact and onset of motion).
level_3: Forearm exhibits flexion at elbow joint immediately after strike: motion begins within one continuous visible motion segment following impact; no other major joints dominate the movement; response is isolated to biceps-driven elbow flexion (not accompanied by supination or wrist motion), but amplitude or speed is inconsistent with physiological reflex (e.g., sluggish or exaggerated beyond natural variation).
level_4: Elbow flexion response begins within the same continuous motion segment as the hammer impact (no perceptible delay), is isolated to the forearm (no shoulder elevation or wrist deviation), and matches expected biomechanics: short-latency, smooth, involuntary, single-phase flexion (no secondary motion, no tremor, no co-contraction artifacts) and no contradictory phenomena (e.g., simultaneous triceps contraction visible as elbow extension).
level_5: All elements from score 4 present AND the flexion response is confined exclusively to the forearm segment (radius/ulna moving as a rigid unit), with no visible deformation, stretching, or unnatural bending of soft tissue; joint angle change is consistent with intact biceps insertion (elbow flexion without concurrent supination); motion terminates cleanly without rebound, oscillation, or correction; zero contradictory neuromuscular signs (e.g., no facial grimace, no voluntary resistance, no postural compensation).

Spatiotemporal Consistency rubric:
level_1: The seated subject, examiner, reflex hammer, or thumb disappears or reappears at a different position between consecutive frames with no visible path connecting the two; the forearm's position on the thigh changes without a visible motion trajectory; the elbow joint appears to bend past its normal range or visibly separates at the joint during flexion; the room or background shifts laterally while the camera view itself stays fixed; the shadow the examiner's hand casts over the cubital fossa jumps to the opposite side of the tendon within a single cut, with no matching change in hammer position or camera angle.
level_2: A major but less total continuity break: the thumb's position jumps to a new spot on the tendon without a visible path between frames; the hammer is shown mid-swing in one frame and already retracted to its starting position in the next with no intervening contact frame; the forearm changes angle at the elbow without any visible joint articulation carrying it there; the cubital fossa and thumb are hidden behind the examiner's hand or hammer for a noticeably long stretch of the strike sequence; the lit side of the forearm swaps between two clearly different directions partway through the clip with no change in camera position.
level_3: Small continuity flaws that a viewer would notice but that do not break the storyline: the thumb or hammer tip is briefly hidden by the examiner's fingers for a moment during the strike; the shadow along the forearm shifts slightly in sharpness or softness as the hand moves past it; the camera view holds an essentially steady frame with only a faint, generalized unsteadiness rather than a locked-off shot; no subject, hammer, or thumb vanishes or relocates without a visible path; no joint bends past its normal range; the light source driving the scene does not swap sides mid-sequence.
level_4: Every motion path reads as continuous and physically connected: the thumb travels visibly into place over the cubital fossa, the hammer swings along one unbroken arc into contact, and the forearm's flexion at the elbow proceeds as one uninterrupted angular sweep; the thumb, hammer, cubital fossa, and forearm are never hidden from view for a noticeably long stretch; the direction and brightness of the shadow the examiner's hand and hammer cast over the tendon region hold steady across the strike rather than swapping sides or suddenly darkening/brightening; the elbow and forearm's shape stays anatomically coherent, with no bulging, warping, or unnatural bending of the skin over the tendon.
level_5: All elements from score 4 present AND the motion reads as physically plausible throughout: no visible stutter, ghosting double-image, or repeated/frozen frame during the swing or flexion; the elbow's pivot point stays in the same place on screen as the forearm rotates around it; the indentation the thumb makes in the skin over the tendon appears continuously before, during, and after the strike rather than popping in or out; the skin's response to the hammer's impact under the thumb appears immediate and proportionate to the force shown, with no delayed reaction and no exaggerated bounce-back.