Three-Dimensional Kinematic Characterization of An Object Pick-Up Task Using Motion Capture

Summary

This protocol presents a standardized full-body motion capture approach for assessing the object pick-up task, illustrating how distinct movement strategies and compensatory patterns can be quantitatively characterized during this common functional activity.

Abstract

The object pick-up task is a fundamental activity of daily living that integrates trunk, lower- and upper-limb coordination, balance, and visuomotor control within a single, ecologically valid movement. Despite its relevance, standardized protocols for quantitatively assessing object pick-up strategies using full-body motion capture remain limited. The goal of this protocol is to demonstrate a standardized method for capturing and analyzing the object pick-up task using full-body three-dimensional motion capture and to illustrate how different movement strategies can be identified and classified. The protocol details participant preparation, comprehensive marker placement, task execution, and representative kinematic analyses. Thirty participants performed the object pick-up task, during which fourteen kinematic variables were collected. Movement strategies were classified into squat-dominant, hinge-dominant, and hybrid patterns representing distinct and intermediate approaches along a continuum of movement strategies. Representative results demonstrate clear differences in joint kinematics between strategy groups, particularly at the knee and ankle, with additional contributions from torso motion. Differences between dominant and non-dominant hand conditions further illustrate the task’s sensitivity to compensatory movement adaptations. This protocol provides a practical and reproducible framework for assessing object pick-up strategies and whole-body coordination during functional movement.

Introduction

Assessment of human movement is central to clinical and research evaluations of musculoskeletal and functional health¹. Traditionally, movement assessment has relied largely on visual observation, which, while clinically valuable, is inherently subjective, influenced by the rater experience, and limited in its ability to consistently capture subtle coordination patterns. Motion capture technologies, including both marker-based and marker less systems, allow detailed and quantitative examination of whole-body movement with increasing accessibility^2,3,4,5.

When translating these tools into clinical and research settings, practical constraints such as available space, assessment time, and ease of task execution become important considerations. Functional tasks that are brief, require minimal setup, and yet provide rich information about movement coordination are therefore particularly valuable. Within this context, the object pick-up task represents a simple and time-efficient movement that captures integrated whole-body function. It is a standard item included in the Berg Balance Scale and related objective balance measures^6,7,8.

In daily living, picking up an object from the floor is a fundamental activity that requires coordinated control of the trunk, lower limbs, and upper extremities, alongside balance and visuomotor integration. The task combines forward bending, controlled lowering, reach and grasp, object handling, and a return to standing within a single continuous action, imposing meaningful mechanical demands on the lumbar spine and major joints of the lower limb while engaging upper-limb function and postural control⁹.

Pick-up strategies have long been central to occupational health and ergonomics, forming the basis of bending and manual handling training, and are closely intertwined with musculoskeletal health, particularly low back pain, through both contributory and adaptive mechanisms¹⁰. Moreover, among items within the Berg Balance Scale and related objective balance measures, the pick-up object task has been shown to most strongly discriminate between fallers and non-fallers in older adults¹¹. However, despite its relevance to everyday function, the pick-up task is often assessed qualitatively or embedded within composite functional tests, limiting its reproducibility and quantitative interpretation across studies¹².

Full-body motion capture enables detailed characterization of coordinated segmental motion during functional tasks, allowing movement strategies to be objectively described rather than inferred¹³. Standardized protocols for capturing and classifying pick-up movement strategies using full-body kinematics remain limited. This article demonstrates a standardized object pick-up task using full-body motion capture and illustrates how distinct movement strategies can be identified and classified based on whole-body kinematics.

While the task has broad potential applications, the present work focuses on demonstrating the protocol and illustrating representative movement strategies rather than evaluating clinical outcomes. In addition, we demonstrate the full-body marker placement used for this task, providing a practical reference for researchers and clinicians seeking to implement gold-standard marker-based motion capture for whole-body functional movement analysis. A marker-based full-body three-dimensional motion capture system with a comprehensive reflective marker set was used to record kinematic data (Figure 1).

Protocol

The study (IRB-2018-04-014) was approved by the institutional review board of Nanyang Technological University, Singapore and was conducted in accordance with the ethical standards set forth in the 1964 Declaration of Helsinki or comparable ethical standards. All participants provided written informed consent prior to participation.

1. Participant Preparation

NOTE: The task involves participants picking up a standardized object from the floor at a comfortable pace, returning to standing, and handing over the object, as described in Section 7.

1. Ask the participant to wear a singlet and lightweight shorts to allow accurate placement and visualization of reflective markers (Figure 2A).
2. Ensure that the participant is barefoot.
3. Instruct the participant to remove reflective accessories, such as wristwatch.
4. Record participant anthropometrics as required by the motion capture system (e.g., height, weight), and document hand dominance.
   NOTE: For participants with modesty constraints, a full-body motion capture suit may be used instead of a singlet and shorts.

2. Participant Screening and Safety Check

1. Instruct the participant to walk a short distance away from the examiner at a comfortable self-selected pace (Figure 2B).
2. Ask the participant to turn and walk back towards the examiner at the same pace (Figure 2C).
3. Observe the participant during both walking directions to confirm that they can walk, turn, and maintain balance without assistance.
4. Ask the participant to perform the following movements in sequence: extend the neck by looking upward, flex the neck by looking downward, rotate the head to the left and right, and tilt the head to both sides (Figure 2D).
5. Ask the participant to perform the following movements in sequence: bend the body forward, bend the body backward, rotate the body to the left and right, and perform side bending to both sides (Figure 2E).
6. Observe the participant’s ability to follow instructions and perform all movements without discomfort or obvious restriction.
7. If any movement restriction or difficulty is observed, document it prior to proceeding.
8. Once the participant has completed all movements comfortably and safely, instruct the participant to remain standing for subsequent marker placement.
   NOTE: This brief screening is performed to confirm participant safety, ability to follow instructions, and to note any obvious movement restrictions prior to task execution. No motion capture data are recorded during this phase, and the screening is not intended as a diagnostic assessment or outcome measure.

3. Full-Body Marker Placement

1. Instruct the participant to face away from the examiner.
2. Ensure the participant is standing in a relaxed, upright posture with arms resting naturally by the sides.
   NOTE: A total of 50 reflective markers were placed on anatomical landmarks and 10 segment clusters to capture full-body kinematics.
3. Palpate posterior anatomical landmarks and place magnetic bases sequentially over the T4, T8, and T10 spinous processes, the left and right posterior superior iliac spines (PSIS), and the posterior surface of the iliac crest, approximately 2-3 inches superolateral to the PSIS (Figure 3A).
4. After the posterior magnetic bases are positioned, secure them with tape (Figure 3B).
5. Visually verify midline alignment, bilateral symmetry, and participant comfort before proceeding.
6. Ask the participant to turn and face the examiner while standing upright.
7. Palpate the left and right anterior superior iliac spines (ASIS) and place magnetic bases bilaterally.
8. Palpate the xiphoid process and place a magnetic base over this landmark.
9. Secure the anterior magnetic bases with tape.
10. Ask the participant to put the top back on and return to an upright standing posture.
11. Palpate the previously placed magnetic bases through the clothing and attach reflective markers onto the magnetic bases sequentially (Figure 3C).
12. Ensure that all reflective markers are firmly seated on their corresponding magnetic bases and remain visible.
13. Ask the participant to turn away from the examiner and flex the neck forward.
14. Palpate the seventh cervical vertebra (C7) and place a reflective marker over this landmark (Figure 4A).
15. Ask the participant to return to a neutral head position and turn to face the examiner.
16. Palpate the sternum and place a reflective marker over this landmark.
17. Palpate the left and right acromion processes and place reflective markers bilaterally (Figure 4B).
18. Ask the participant to sit comfortably and place the right palm over the left knee.
19. Palpate the medial and lateral epicondyles of the right humerus and place reflective markers over both landmarks.
20. Repeat the procedure for the left elbow (Figure 4C).
21. Ask the participant to place both hands comfortably on the thighs.
22. Palpate the radial and ulnar styloid processes and place reflective markers bilaterally (Figure 4D).
23. Palpate the capitate bone in the hand and place a reflective marker over this landmark on each hand.
24. Place one reflective marker on the thumbnail and one on the ring fingernail of each hand (Figure 5A, B).
25. With the participant seated comfortably, palpate the medial and lateral femoral condyles of the right knee and place reflective markers over both landmarks (Figure 5C).
26. Repeat the procedure for the left knee.
27. Palpate the medial and lateral malleoli of the right ankle and place reflective markers over both landmarks (Figure 5D).
28. Repeat the procedure for the left ankle.
29. Palpate the first, second, and fifth metatarsal heads of the right foot and place reflective markers over each landmark.
30. Palpate the intermediate cuneiform of the right foot and place a reflective marker.
31. Palpate the calcaneal tuberosity of the right foot and place a reflective marker.
32. Repeat steps 29–31 for the left foot to complete placement of foot markers (Figure 6A).
33. Attach marker clusters to the forearm, upper-arm, hand, thigh and shank (leg) segments bilaterally (Figure 6B-D).
34. Secure each cluster firmly using straps or tape.
35. Ask the participant to stand facing the examiner and look straight ahead.
36. Place reflective markers bilaterally on the frontal bone, just above the eyebrows.
37. Place reflective markers bilaterally on the temporal bone (Figure 7A).
38. Visually inspect all markers and clusters to confirm correct placement, symmetry, and visibility.
39. Secure all marker bases with tape to prevent displacement or detachment during task execution.
40. For all markers placed directly on the skin (without magnetic bases), and for clusters positioned on the skin, outline the respective positions using a washable skin marker to document placement and facilitate consistent repositioning if required (Figure 7B).
41. Check for loose markers, cluster movement, or occlusion during standing posture.
42. Correct any placement errors or insecure attachments as needed.
43. If no issues are identified, proceed to the object pick-up task.
    NOTE: The marker placement described above reflects the workflow used in our laboratory for whole-body kinematic capture of the object pick-up task. While different marker sets may be used across laboratories, the principles of anatomical landmark identification, consistency, and verification remain essential.

4. Motion Capture System Setup

1. Arrange the motion capture cameras around the capture volume to ensure full-body visibility during forward bending, object interaction, and return to standing.
2. Define a capture volume large enough to accommodate the full pick-up movement without occlusion.
3. Calibrate the motion capture system according to the manufacturer’s guidelines to establish the global coordinate system.
4. Verify that all cameras detect reflective markers clearly and that no unintended reflective objects are present in the capture area.
   NOTE: This system setup is typically performed prior to or in parallel with the participant’s marker placement but another personnel. Adequate coverage of the lower limbs, trunk, and upper limbs is essential to capture the full movement sequence of the pick-up task.

5. Data Acquisition

1. Start motion capture recording before the instructing participant to initiate movement.
2. Stop recording only after the participant has completed the task.
3. Review the recording to ensure all the markers are present in the recording before proceeding to the next recording.
4. Repeat the procedure for the desired number of recordings.
   NOTE: Continuous recording ensures complete capture of all task phases and avoids segmentation errors that may occur if recording is started or stopped during movement.

6. Static trial

1. Ask the participant to stand upright within the capture volume.
2. Instruct the participant to abduct both arms to approximately 45° from the body, with the elbows fully extended and palms facing forward (Figure 8A).
3. Ask the participant to remain still for 2 seconds, while the examiner counts aloud “one thousand, two thousand.”
4. Instruct the participant to abduct both arms further to approximately 90° from the body, with elbows remaining extended and palms facing down (Figure 8B).
5. Ask the participant to remain still for 2 seconds, while the examiner counts aloud “one thousand, two thousand.”
6. Instruct the participant to lower both arms to the sides and stand in a relaxed neutral posture (Figure 8C).
   NOTE: The static calibration trial is performed to establish segment coordinate systems, improve upper-limb joint definition, and verify marker visibility prior to dynamic task execution.

7. Object Pick-Up Task

1. Place a standardized object on the floor at a predefined location within the capture volume.
2. Ask the participant to stand upright in a comfortable, self-selected stance, facing the object (Figure 9A).
3. Provide the following instruction to the participant: “Please pick up the object from the floor, at a comfortable pace, return to a standing position, and hand over the object.”
4. Position yourself comfortably in a seated or kneeling position on the floor to retrieve and reposition the object between trials, ensuring consistent placement.
5. Allow the participant to perform one familiarization trial if needed.
6. Instruct the participant to perform three trials using one hand (Figure 9B, C).
7. Instruct the participant to repeat the task for three trials using the other hand (Figure 9D).
8. Allow brief rest periods between trials as needed to minimize fatigue.
   NOTE: No instructions are given regarding movement strategy to allow natural variation in object pick-up behavior. Although this protocol focuses on kinematic analysis, the task can also be performed on force plates if kinetic data are required. In such cases, participants can be instructed to stand within predefined regions aligned with the force plates, placing one foot on each force plate.

8. Data Cleaning, Labeling & Post-processing

1. Use Qualisys Track Manager (QTM) software to label the markers according to Liang et al¹³.
2. Use trajectory editor functions to interpolate missing marker data from both the static and motion files. Ensure that all markers are 100% filled.
3. Using Automatic Identification of Markers (AIM) functions to generate a model to assist in labelling subsequent trials.
4. Export the cleaned data as .c3d files (Figure 10).
5. Import the .c3d files to Visual 3D (V3D) Professional software.
6. Assign the motion files to the appropriate calibration (static) file.
7. Apply a Butterworth low-pass filter (6Hz) to all raw marker data.
8. Apply the model file (supplemental file 1) to implement a hybrid model to the static file and the assigned motion file¹⁴.
9. Using the Compute Model Based Data function compute required joint angle accordingly.
   NOTE: The model file is provided as Supplementary File 1 to allow readers to replicate our model. To use the model, rename the file extension from “.txt” to “.mdh.” Specific steps for data labeling, model creation, filtering, and joint angle computation may vary depending on the motion capture and biomechanical analysis software used.

9. Definition of Movement Phases

1. Apply a custom developed pipeline written within V3D to segment each of the six repetitions into three distinct phases: 1) Descent, 2) Ascent, 3) Object handover (Table 1) (Figure 11).
2. Define the right and left object pick up by assessing wrist marker trajectories and incorporate into the custom developed pipeline (Figure 12A, B).
3. Within the same pipeline, export peak sagittal plane torso, hip, knee, and ankle joint angles using the previously identified descent and ascent events, and separate these outcomes by side (left vs right; e.g., left hip flexion, right hip flexion) and by dominant versus non-dominant hand during the pick-up task.
   NOTE: Marker displacements and joint kinematics are recommended as signals for segmentation of the distinct phases. Acknowledging movement variations in task execution, selecting signals with large displacements is encouraged to overcome noise.

10. Classification of Pick-Up Strategies

1. Examine trunk and lower-limb kinematics during the descent phase.
2. Classify pick-up strategies into the following categories based on the relative contribution of trunk flexion and lower-limb joint motion:
   1. Squat-dominant (Figure 13A)
   2. Hinge-dominant (Figure 13B)
   3. Hybrid (Figure 13C)
      NOTE: Although the object pick-up task has the potential to support analysis of multiple biomechanical, neuromotor, and coordination-related outcomes, this protocol focuses specifically on demonstrating how pick-up strategies can be identified and classified based on trunk and lower-limb movement patterns, providing a normative reference.

Results

Thirty participants performed the object pick-up task, during which 14 kinematic variables were collected. Participants were subsequently classified into three strategy groups representing distinct and intermediate approaches along a continuum of movement strategies, interpreted as squat-dominant (Group 1), hinge-dominant (Group 2), and hybrid strategies (Group 3). No statistically significant differences were observed between groups for age, sex distribution, height, or weight (Table 2). All participant...

Discussion

This protocol demonstrates a standardized approach for capturing and analyzing an object pick-up task using full-body three-dimensional motion capture and illustrates how different movement strategies can be quantitatively characterized. Participants were purposefully selected and grouped to represent distinct and intermediate movement strategy patterns observed within the sample. Visual inspection of joint kinematics indicated that knee flexion magnitude was the primary differentiating feature, with overlap observed in ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Box for Pick up task	N/A	N/A	70mm x 70mm x 70mm (Lx W x H) 3D-printed using polylactic acid (PLA) filament (eSUN; Shenzhen Esun Industrial Co., China).
Cluster markers	Qualisys, AB	· 160146 Upper body · 160145 Lower body · 160149 SuperWrap for cluster	Clusters included two upper-body clusters (upper arms) and four lower-body clusters (two on the thighs and two on the shanks).
Cluster markers (custom)	N/A	N/A	Custom-designed cluster markers for tracking the wrist (RFA, LFA) and hand (RHMC, LHMC) were 3D-printed using polylactic acid (PLA) filament (eSUN; Shenzhen Esun Industrial Co., China).
Computer workstation	Generic	NA	Intel Core i9 processor, 32 GB RAM, 10 TB Harddisk, NVIDIA RTX2080
Double-sided adhesive skin tape	Qualisys, AB	160188 Double-sided adhesive tape	For attaching marker base to skin
Force plate	Kistler	Type 9260AA6	Two plates (600 × 500 x 50 mm) embedded underneath the flooring, one for each foot.
Hypoallergenic adhesive skin tape	3M	Micropore
Marker base	Optitrack	M4 Marker base
Motion capture system	Qualisys, AB	Miqus Video Aqus A12	x2, Full HD / 2 MP, Resolution 1920 × 1080 x12, 12 MP 300fps, Resolution 4096 × 3072, FOV 54 × 42 °(motorized)
Neodymium Magnets	Generic	N/A	Diameter 15mm, Thickness 1mm
Qualisys Track Manager (QTM)	Qualisys, AB	Version 2019	The software is used to record movements during the experiment and perform data post-processing.
Retroreflective markers	Optitrack	12.7 mm (1/2) M4 Markers
Visual3D	HAS-Motion	Version 2021	The software is used to do biomechanical modelling and produce the relevant joint angles