Paired-EMS: Enhancing Electrical Muscle Stimulation (EMS)-based Force Feedback Experience by Stimulating Both Muscles in Antagonistic Pairs

Document Title

Paired-EMS: Enhancing Electrical Muscle Stimulation (EMS)-based Force Feedback Experience by Stimulating Both Muscles in Antagonistic Pairs

Document Information

• Subject Area: HCI (Human-Computer Interaction) focusing on haptic feedback and enhancing virtual reality experiences
• Keywords: Electrical Muscle Stimulation (EMS), haptic feedback, virtual reality, force feedback, user experience, antagonistic muscle pairs, wearable devices, perceived realism, experimental design

Research Background and Problem

• Identified Issues or Challenges:

  • Existing EMS-based haptic feedback technologies typically stimulate the "antagonistic muscle" opposing the user's actively contracting muscle to simulate external forces. This leads to unnatural muscle contractions and significant discrepancies between the perceived haptic sensation and real-world experiences.
  • Current EMS technologies may cause imbalances and disharmony in user experience, especially in scenarios involving high-intensity feedback, such as boxing or sword fighting.

• Significance:

  • For virtual reality (VR) applications, particularly when simulating external forces like punching or pushing a wall, haptic feedback is critical for enhancing immersion and user engagement. The physical inconsistencies of current technologies undermine the immersive experience.

• Motivation and Related Work:

  • EMS technology has been widely applied in VR haptic feedback scenarios, including simulating impact forces (boxing, sword fighting), weight (lifting virtual objects), and counterforces (pushing objects). However, these technologies only stimulate antagonistic muscle groups, leading to experiences that contradict user expectations.
  • The authors propose a method that simultaneously stimulates both the "active muscle group" and the "antagonistic muscle group," referred to as Paired-EMS, to reduce disharmony and enhance simulation fidelity.

Solution

• Proposed Method or Solution:

  • A novel EMS method, Paired-EMS, is introduced to overcome the limitations of existing technologies that only stimulate antagonistic muscle groups. By simultaneously stimulating both active and antagonistic muscle groups, the method enhances the realism and harmony of force feedback in VR.

• Innovations:

  • Introducing stimulation of active muscles, contrasting with traditional EMS technologies that only stimulate antagonistic muscles.
  • Focusing on improving the balance of sensations in force feedback design to enhance the overall harmony of the user experience.
  • Differentiating two types of muscle actions (isometric and isotonic) in experimental design and analyzing user preferences in different contexts.

• Implementation Steps and Key Techniques:

  1. User Preference Study:
     • Conducted a small pre-test (n=8) to calibrate muscle stimulation intensity.
     • Adjusted intensity levels based on experimental results (minimum, medium, and maximum acceptable intensities).
  2. Research Platform and Equipment:
     • Used FDA-certified EMS devices (iSTIM EV-805) in combination with VR headsets (HTC Vive Pro 2) for experiments.
     • Attached electrodes to muscle regions and incorporated vibration feedback devices (e.g., bHaptics Tactosy) to reduce the tingling sensation caused by EMS stimulation.
  3. Main Experiment and User Experience Evaluation:
     • Tested the performance of Paired-EMS versus traditional EMS in a boxing scenario (n=32).
     • Evaluated differences in force feedback experience for isometric actions (blocking punches) and isotonic actions (throwing punches), collecting subjective user ratings on realism, harmony, entertainment, and comfort.

Research Outcomes

• Specific Findings:

  • Compared to traditional EMS, Paired-EMS significantly improved the following four user experience metrics:
    • Realism: Enhanced consistency in perceiving resistance to external forces.
    • Harmony: Significantly reduced the unnatural sensation of haptic feedback.
    • Entertainment: Delivered a more vivid interactive experience.
    • Overall Preference: 78% of participants preferred using Paired-EMS.
  • In terms of comfort, Paired-EMS performed similarly to traditional methods.

• Advantages Over Existing Solutions:

  • Provides a more realistic haptic experience, particularly in high-intensity scenarios (e.g., boxing and shooting games).
  • The design of simultaneously stimulating two muscle groups not only enhances the immersion of haptic feedback but also improves adaptability to high-intensity feedback scenarios.

• Experimental and Evaluation Results:

  • User evaluations showed that Paired-EMS demonstrated significant improvements across different action types (isometric and isotonic) and haptic device configurations (electromagnetic and vibration-based feedback):
    • Realism, entertainment, and overall preference all showed significant improvements (p<0.01).
    • Although the comfort rating for Paired-EMS was slightly lower, its advantages in realism attracted the majority of users to prioritize this method.

• Limitations and Future Directions:

  • Limitations:
    • Paired-EMS increases user calibration time (requiring simultaneous adjustment of stimulation intensities for multiple muscle pairs).
    • Some users may experience slight discomfort during prolonged high-intensity use.
  • Future Directions:
    • Explore further improvements in comfort and expand the stimulation coverage area (e.g., anterior deltoid or upper trapezius) to enhance realism.
    • Develop automated calibration methods combined with magnetic muscle stimulation technologies to reduce the complexity of manual setup and improve usability.
    • Optimize haptic feedback for different game types (e.g., sword fighting, shooting) to make it more broadly applicable to other VR interaction scenarios.