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Grasp Taxonomy for LABIS Hand Recognition

Date: 2026-05-07 Status: Initial version Authors: Luis Arancibia (compilation), based on clinical sources below.

Purpose

This taxonomy defines the grasp types to be studied in the LABIS hand and grasp recognition research line. Each grasp includes a clinical definition, bibliographic reference, example objects, expected hand landmarks (MediaPipe/OpenPose 21-keypoint model), and an observability criterion indicating how reliably the grasp can be detected from video.

Landmark Reference

Hand keypoint models (MediaPipe Hands, OpenPose Hand) use 21 landmarks: - 0: Wrist - 1-4: Thumb (CMC, MCP, IP, TIP) - 5-8: Index (MCP, PIP, DIP, TIP) - 9-12: Middle (MCP, PIP, DIP, TIP) - 13-16: Ring (MCP, PIP, DIP, TIP) - 17-20: Little (MCP, PIP, DIP, TIP)

Inclusion Criteria

A grasp is included if: 1. It appears in established clinical taxonomy (Kapandji, Napier, Cutkosky, or Feix GRASP taxonomy). 2. It is relevant to activities of daily living (ADL) or rehabilitation assessment. 3. It can be differentiated by observable finger posture from at least one camera angle.

Exclusion Criteria

A grasp is excluded if: 1. It requires proprioceptive or force data not available from vision alone. 2. It is a subvariant indistinguishable from a parent type by keypoints alone.

power_grasp

  • Definition: Full hand wrap around a cylindrical or large object with all fingers flexed and thumb opposed or adducted. The entire palmar surface contacts the object. Napier (1956) classified this as a "power grip"; Kapandji describes it as prehension palmaire involving flexion of all digits around the object axis.
  • Reference: Kapandji AI. Fisiologia Articular, Tomo 1: Miembro Superior, 6a ed., pp. 308-318, 2006. Napier JR. The prehensile movements of the human hand. J Bone Joint Surg Br. 1956;38-B(4):902-913. Cutkosky MR. On grasp choice, grasp models, and the design of hands for manufacturing tasks. IEEE Trans Robot Autom. 1989;5(3):269-279.
  • Example_object: Bottle, cylinder, hammer handle, handrail.
  • Expected_landmarks: All fingertip landmarks (4,8,12,16,20) flexed toward palm; thumb TIP (4) opposed near index MCP (5); wrist (0) aligned with forearm. High flexion at PIP/DIP joints (landmarks 6-7, 10-11, 14-15, 18-19 close together).
  • Observability: HIGH from dorsal and lateral views. Moderate from palmar view (fingers occlude palm). Object occludes finger-object contact zone.

precision_pinch

  • Definition: Opposition of thumb tip to index fingertip, holding a small object between the pulps of the two digits. Other fingers may be flexed but do not participate. Kapandji terms this "pince bidigitale terminale" (terminal bidigital pinch). Napier (1956) classified it as a subtype of precision handling.
  • Reference: Kapandji AI. Fisiologia Articular, Tomo 1, 6a ed., pp. 308-318, 2006. Napier JR. 1956. Feix T, Romero J, Schmiedmayer HB, Dollar AM, Kragic D. The GRASP taxonomy of human grasp types. IEEE Trans Hum Mach Syst. 2016;46(1):66-77.
  • Example_object: Coin, small bead, pen tip, needle.
  • Expected_landmarks: Thumb TIP (4) and index TIP (8) converge (small distance); index PIP-DIP extended or slightly flexed; middle/ring/little fingers (9-20) flexed away. Wrist (0) neutral.
  • Observability: HIGH from dorsal and lateral views. The two approaching fingertips are clearly distinguishable. LOW if object is very small and occluded between fingertips.

spherical_grasp

  • Definition: All five digits spread and flexed around a spherical object, with the palm cupping the object and thumb abducted. Kapandji describes this as requiring maximal thumb abduction and finger spreading. Cutkosky (1989) lists it as a power grasp subtype for round objects.
  • Reference: Kapandji AI. Fisiologia Articular, Tomo 1, 6a ed., pp. 308-318, 2006. Cutkosky MR. 1989. Feix T et al. 2016.
  • Example_object: Tennis ball, apple, baseball, 3D-printed semisphere.
  • Expected_landmarks: All fingertips (4,8,12,16,20) spread and equidistant from object center; thumb (1-4) widely abducted; MCP joints (5,9,13,17) show finger spreading (large inter-MCP distances). Palm faces object.
  • Observability: MODERATE. Dorsal view shows finger spreading clearly. Palmar view heavily occluded by object. Lateral view shows thumb abduction well. Object size relative to hand affects how many landmarks are visible.

lateral_pinch

  • Definition: Thumb pulp presses an object against the lateral (radial) surface of the index finger, typically at the middle phalanx. Also called "key grip" or "pulp-to-side pinch". Kapandji describes this as "prise laterale" used for turning keys and holding cards.
  • Reference: Kapandji AI. Fisiologia Articular, Tomo 1, 6a ed., pp. 308-318, 2006. Napier JR. 1956. Feix T et al. 2016.
  • Example_object: Key, credit card, thin sheet of paper, coin (edge hold).
  • Expected_landmarks: Thumb TIP (4) adjacent to lateral side of index PIP-DIP segment (landmarks 6-7); thumb flexed at IP; index finger extended or slightly flexed; middle/ring/little fingers flexed into fist. Wrist (0) slightly ulnar deviated.
  • Observability: HIGH from dorsal view (thumb-index contact visible). MODERATE from lateral view (thumb may occlude index). LOW from palmar view. The flat object is typically not occluding.

tripod_grasp

  • Definition: Three-finger precision grip using thumb, index, and middle finger tips to hold a small elongated object. Kapandji describes this as "prise tridigitale" for writing instruments. Also called "dynamic tripod" in handwriting research.
  • Reference: Kapandji AI. Fisiologia Articular, Tomo 1, 6a ed., pp. 308-318, 2006. Feix T et al. 2016. Selin AS. Pencil grip: a descriptive model and four empirical studies. 2003.
  • Example_object: Pen, pencil, stylus, chopstick, small screwdriver.
  • Expected_landmarks: Thumb TIP (4), index TIP (8), and middle TIP (12) converge around object; ring and little fingers (13-20) flexed away; PIP joints of index and middle (6, 10) moderately flexed. Thumb abducted from palm.
  • Observability: HIGH from dorsal view. MODERATE from lateral (middle finger may overlap index). Writing posture is well-studied and distinctive in keypoint space.

hook_grasp

  • Definition: Fingers flexed at interphalangeal joints to form a hook shape, without thumb participation. The object is suspended from the flexed fingers. Kapandji describes this as "crochet digital" used for carrying bags or gripping handles without the thumb. It is not a true prehension but a gravity-dependent hold.
  • Reference: Kapandji AI. Fisiologia Articular, Tomo 1, 6a ed., pp. 308-318, 2006. Cutkosky MR. 1989. Feix T et al. 2016.
  • Example_object: Bag handle, suitcase, pull-up bar, bucket handle.
  • Expected_landmarks: Fingertips (8,12,16,20) curled under object with high PIP/DIP flexion; thumb (1-4) relaxed or adducted, NOT opposing; wrist (0) extended. MCP joints (5,9,13,17) at ~90 degrees flexion. Gravity direction matters.
  • Observability: HIGH from lateral view (hook shape clearly visible). MODERATE from dorsal view (object may hide finger curl). Distinguishable by thumb non-participation.

Summary Matrix

Grasp ID Category Thumb Role Digits Involved Key Discriminant
power_grasp Power Opposed/adducted All 5 Full palmar contact, all fingers flexed
precision_pinch Precision Opposed to index 2 (thumb + index) Two fingertips converge, others retracted
spherical_grasp Power Abducted All 5 Fingers spread, cupping round object
lateral_pinch Precision Lateral press 2 (thumb + index side) Thumb presses against index lateral surface
tripod_grasp Precision Opposed to index+middle 3 (thumb + index + middle) Three tips converge, ring/little retracted
hook_grasp Non-prehensile Not involved 4 (index-little) Hook-shaped fingers, no thumb, gravity-dependent

Bibliography

  1. Kapandji AI. Fisiologia Articular, Tomo 1: Miembro Superior. 6a edicion. Madrid: Editorial Medica Panamericana; 2006. pp. 308-318.
  2. Napier JR. The prehensile movements of the human hand. J Bone Joint Surg Br. 1956;38-B(4):902-913.
  3. Cutkosky MR. On grasp choice, grasp models, and the design of hands for manufacturing tasks. IEEE Trans Robot Autom. 1989;5(3):269-279.
  4. Feix T, Romero J, Schmiedmayer HB, Dollar AM, Kragic D. The GRASP taxonomy of human grasp types. IEEE Trans Hum Mach Syst. 2016;46(1):66-77.
  5. Selin AS. Pencil grip: a descriptive model and four empirical studies. Abo Akademi University Press; 2003.
  6. Simon SR et al. Hand keypoint detection in single images using multiview bootstrapping. CVPR. 2017.
  7. Fan J et al. Upper limb pose estimation: a systematic review. Sensors. 2022.