Human Hand Movement Control—Differences in Kinematics Between Functional Object Manipulation and Grasping Actions

Academic Background

The functionality of the human hand plays a crucial role in daily life, particularly through its dexterity, which allows us to use tools to complete various complex tasks. However, millions of people lose hand functionality each year due to conditions such as strokes and amputations, leading to in-depth research on hand movement. Although past studies have primarily focused on coordinated hand movements during grasping and reaching (referred to as “synergies”), research on hand synergies during complex object manipulation remains limited. Understanding these differences is critical for designing more advanced prosthetic and rehabilitation devices, as the goal of these devices is to restore patients’ functional manipulation capabilities, not just grasping ability.

This paper aims to explore two key hypotheses:
1. The number of synergies required for manipulation is the same as for grasping actions;
2. The nature of synergies in manipulation differs from those in grasping actions.
By studying human hand movement during wire harness installation, the authors hope to reveal the differences between functional manipulation and simple grasping actions.

Source of the Paper

This paper was co-authored by A. Michael West Jr. from Johns Hopkins University and Neville Hogan from Massachusetts Institute of Technology. It was published in 2025 in the Journal of Neurophysiology.

Research Details

Research Process

1. Experimental Design:
   The study included two experiments:

   • Reaching and Grasping Experiment: Participants reached for and grasped tools or objects used in wire harness installation.
     
   • Functional Manipulation Experiment: Participants used these tools to install a wire harness on a mock electrical cabinet.
     The subjects were 7 healthy adults (3 women, 4 men, aged 18-28), all right-handed.

2. Data Collection:
   Participants wore CyberGlove, a glove with embedded sensors, to record hand joint motion data. In the grasping experiment, participants reached for and grasped tools (e.g., scissors, zip ties, screwdrivers), each tool grasped four times. In the manipulation experiment, participants completed five steps of wire harness installation, including securing the harness, cutting excess material, and screwing it in place.

3. Data Analysis:
   Singular Value Decomposition (SVD) was used to extract hand movement synergies, comparing the number and nature of synergies between grasping and manipulation actions. The researchers also analyzed the similarity of synergies in the two tasks using Cosine Similarity.

Main Results

1. Number of Synergies:
   Functional manipulation required significantly more synergies than grasping actions. For example, using a screwdriver required eight synergies, while grasping required only one. This indicates that functional manipulation involves more complex hand movements.

2. Nature of Synergies:
   The synergies in grasping and manipulation tasks were similar in lower-order synergies (e.g., the first two synergies) but differed significantly in higher-order synergies. Lower-order synergies typically involved finger flexion (e.g., grasping), while higher-order synergies involved more refined actions such as tool rotation and finger repositioning.

3. Synergy Explanatory Power:
   When manipulation data were projected onto the synergies of grasping actions, their explanatory power for manipulation actions was significantly lower than that of the manipulation synergies themselves. This suggests that the synergies of grasping actions cannot fully describe the complexity of functional manipulation.

Research Conclusions

The study shows that functional manipulation requires more synergies than grasping actions, and higher-order synergies play a more important role in manipulation. This finding emphasizes the importance of functional manipulation capabilities in the design of prosthetics and rehabilitation devices, rather than just the restoration of grasping ability.

Highlights of the Research

1. Revealed the differences in synergies between functional manipulation and grasping actions, providing new insights into the control of human hand movement in complex tasks.
   
2. Emphasized the importance of higher-order synergies, often considered noise or secondary factors, which play a critical role in functional manipulation.
   
3. Provided theoretical foundations for prosthetic design and rehabilitation technologies, indicating that future devices need to better simulate the complex hand movements required for functional manipulation.

Research Value and Significance

The scientific value of this study lies in its systematic comparison of hand synergies in grasping and functional manipulation, revealing the role of higher-order synergies in complex tasks. Its application value lies in offering new directions for prosthetic design and rehabilitation device development, emphasizing the importance of functional manipulation capabilities. By deepening our understanding of the mechanisms of human hand movement control, future technologies can better restore patients’ daily life abilities.

Other Valuable Information

The CyberGlove and data preprocessing methods used in the study ensured the accuracy and reliability of the experimental data. The authors also made the experimental data and analysis code publicly available for reference by other researchers. Furthermore, the research results provide insights for robotic manipulation technology, particularly in control strategies for handling non-rigid objects (e.g., wire harnesses).

Through this research, we have not only gained a better understanding of the mechanisms of human hand movement control but also provided important theoretical support for future rehabilitation technologies and robotic manipulation.