Domain: Healthcare — Human Physiology
Target concept: Reflex of the patellar tendon (knee-jerk reflex) upon percussion with a reflex hammer

Video generation prompt: A subject sits on the examination table with the left leg naturally placed over the right leg, while another person rapidly strikes the patellar tendon below the left knee with a rubber hammer.

Evaluation guide:
(i) Target concept: Monosynaptic deep tendon reflex mediated by L2-L4 spinal segments.
Minimum mechanism: Intact sensory (Ia afferent) and motor (alpha motor neuron) pathways; functional neuromuscular junction; passive muscle-tendon unit with normal tone.
(ii)Storyline: [Phase 1: Stable Setup] Subject seated, left leg crossed over right, knee extended ~10–30°, patellar tendon exposed. [Phase 2: Stimulus Delivery] Hammer approaches tendon region in straight-line trajectory, contacts tendon surface perpendicularly, no pause before impact. [Phase 3: Reflex Execution] Within same continuous motion sequence, isolated knee extension begins, visible as forward swing of foot, centered at knee joint, without hip flexion or ankle motion. [Phase 4: Termination] Leg extension halts cleanly; no recoil, tremor, or sustained contraction; subject maintains upright posture and neutral expression. Perspective: Fixed frontal/slightly oblique view capturing full left knee, hammer, and lower leg; no occlusion of tendon strike zone or knee joint axis.

Manual:
Experimental Procedure
1. Subject sits upright on examination table.
2. Left leg is placed relaxed over right leg, exposing anterior left knee.
3. Examiner holds rubber reflex hammer and strikes patellar tendon just below left patella.
Observation of Experimental Phenomena
Left lower leg extends rapidly and briefly at the knee joint; no movement at hip or ankle; upper body remains still.
Summary of Experimental Principles
Patellar tendon tap stretches quadriceps muscle spindle → triggers monosynaptic reflex arc → causes involuntary quadriceps contraction → knee extension.

Prompt Grounding rubric:
level_1: No seated subject appears; the left-over-right leg crossing is absent; no reflex hammer is visible anywhere in frame; the region just below the left kneecap is never shown or is fully covered by clothing or hands.
level_2: A seated subject is shown, but the legs are not crossed left-over-right (e.g., legs parallel, right-over-left, or uncrossed); or a hammer appears but is never brought near the left knee; or the region just below the left kneecap is not visible when the hammer approaches.
level_3: Subject sits with the left leg crossed over the right leg and the region just below the left kneecap exposed; a rubber-headed reflex hammer is held and brought down to strike that exact region (not the kneecap itself, not the thigh muscle, not the shin).
level_4: All elements of 3-score are present AND the subject's crossed-leg posture and upper body stay motionless in the moments before impact (no shifting, flinching, or leg repositioning as the hammer approaches); the left knee stays extended or nearly so, not bent more than about 45°; the hammer's head meets the tendon region square-on rather than at a shallow glancing angle.
level_5: All elements of 4-score are present AND the seating, leg-crossing, hammer approach, and moment of contact play out as one continuous, uninterrupted shot with the left knee, hammer, and lower leg all kept in frame and unobstructed by hands, clothing, or camera angle from start through the strike; whether the leg subsequently moves is not scored at this level.

Scientific and Causal Correctness rubric:
level_1: No leg movement follows the strike (the leg stays completely still); or the leg movement shown is physiologically wrong for this reflex, e.g., the hip flexes, the ankle flexes or dorsiflexes, or the leg withdraws rather than extends.
level_2: The leg moves after the strike, but the response is inconsistent with a knee-jerk reflex: onset is delayed by roughly a second or more rather than immediate, both legs move, or the knee extension is accompanied by visible hip or ankle motion.
level_3: Within the same continuous shot as the strike, the left lower leg extends forward at the knee with no perceptible delay after contact; the movement is confined to the knee joint, with the hip and ankle visibly uninvolved.
level_4: All elements of 3-score are present AND the extension is a single brief kick rather than a slow push or an exaggerated fling, and the leg returns toward its resting position afterward without additional jerks, bounces, or oscillations.
level_5: All elements of 4-score are present AND nothing suggests the subject is voluntarily assisting or resisting the movement (no visible muscle bracing, grimace, or hand moving toward the leg); the kick begins and ends cleanly with no tremor, sustained contraction, or rebound afterward, consistent with a brief involuntary contraction rather than a deliberate motion.

Spatiotemporal Consistency rubric:
level_1: The subject, hammer, or leg jumps to a different position or shape between frames without continuous motion connecting them; the scene's lighting flips or changes abruptly partway through the shot; the left knee or lower leg bends or bulges in a way a real joint cannot.
level_2: A visible cut or gap interrupts the hammer's path between its approach and the moment it reaches the knee; the direction of the light and shadows on the leg or hammer changes partway through the clip with no light source moving to explain it; part of the leg or hammer flickers in and out of view where nothing should be blocking it.
level_3: The subject, hammer, and both legs stay in consistent relative positions throughout (the crossed-leg posture does not shift on its own); the hammer's path from raised position to contact is shown as one continuous motion; the light source and the resulting shadows on the leg and hammer keep the same direction and strength across the whole clip.
level_4: All elements of 3-score are present AND the hammer's swing speeds up and slows down the way a real swung object would (not a constant-speed glide or a sudden jump into contact); if the leg extends, its arc follows a natural pivot at the knee rather than sliding in a straight line or bending at an impossible point.
level_5: All elements of 4-score are present AND the camera framing stays fixed with no visible drift, wobble, or unexplained repositioning across the whole shot; the hammer never appears to pass through or behind the leg when it should be in front of it; the subject, hammer, and leg all appear to occupy consistent depth and scale relative to one another throughout.