Improving Human Diving Endurance with a Field-Deployable, Untethered Exoskeleton

Technical Summary: Improving Human Diving Endurance with a Field-Deployable, Untethered Exoskeleton

Problem Statement
Human underwater locomotion is fundamentally constrained by high energetic demands required to overcome water drag and the finite supply of self-contained breathing gas. Unlike marine animals adapted to aquatic environments, humans exhibit inefficient swimming mechanics, requiring substantial muscular effort and breath to generate thrust, particularly during the flutter kick. This physiological burden is exacerbated by the limited capacity of high-pressure air tanks carried by divers. While exoskeletons have successfully reduced metabolic costs in terrestrial locomotion (walking and running), their potential to enhance endurance in underwater diving remains largely unexplored. Existing underwater prototypes face significant barriers to widespread adoption, primarily due to a lack of untethered, field-deployable systems capable of adapting to the variable pace of real-world diving kicks. Furthermore, the absence of comprehensive biomechanical data on human diving locomotion (specifically muscle activation patterns and joint kinetics) hinders the development of effective assistance strategies.

Methodology
The authors present DiveMate, a field-deployable, untethered exoskeleton designed to improve diving endurance through adaptive hip assistance.

• Hardware Design: DiveMate is a modular, symmetrical system weighing 4.86 kg, integrating seamlessly with standard Scuba equipment. It features dual-sided joint actuators (brushless motors with 9:1 gear reduction, rated at 22 Nm) mounted on a waist structure, connected to thigh structures via carbon fiber rods. The system is waterproofed using lip seals and O-rings to prevent ingress in turbid waters. A portable lithium polymer battery (24V, 3000mAh) powers the system, allowing for approximately one hour of continuous diving.
• Control Framework: A hierarchical control strategy was developed to adapt to variable kicking paces:
  • High-Level: Utilizes IMU data from the thigh to construct a phase portrait of bilateral hip motion (angle difference vs. angular velocity difference). A two-class detection algorithm identifies the "Moving" state versus the "Stationary" state using polar radius and linear regression of the polar angle. It also detects specific kick events (Maximum Hip Extension and Maximum Hip Flexion) to trigger assistance.
  • Middle-Level: Generates desired assistance torque profiles based on a predefined curve scaled to the estimated duration of the upcoming kicking cycle (using a weighted average of the previous three cycles). This ensures synchronized bilateral hip assistance to maintain balance.
  • Low-Level: Implements a PID closed-loop torque controller to execute precise torque delivery at 20 kHz.
• Experimental Protocol: Seven participants performed underwater diving experiments in an indoor swimming pool under two conditions: ExoAssist (DiveMate active) and NoExo (DiveMate passive with unfastened harnesses to maintain weight/buoyancy consistency). Participants swam a predefined octagonal path at a self-selected comfortable speed until a fixed volume of gas (approx. 150 bar) was consumed.
• Data Acquisition: The study measured travel distance, dive duration, real-time gas consumption (via a modified pressure gauge), hip-related muscle activation (EMG of rectus femoris, semitendinosus, and biceps femoris), and kinematic parameters. User surveys assessed sense of agency, comfort, and usability.

Key Results
The study demonstrates that DiveMate significantly enhances human diving endurance and efficiency:

• Endurance Improvement: When consuming a fixed volume of breathing gas, DiveMate increased travel distance by 42.9% and extended dive duration by 54.9% compared to the unassisted condition.
• Energy Economy: The cost of transport was reduced by 31.4%. The net gas consumption rate (accounting for resting metabolism) decreased by 47.0%, while the gross gas consumption rate decreased by 35.5%.
• Physiological Impact: Muscle activation was significantly reduced. The root mean square (RMS) of the hip flexor (rectus femoris) decreased by 28.2%, and hip extensors (semitendinosus and biceps femoris) decreased by approximately 27–28%. Peak muscle activity for the rectus femoris also dropped by 28.2%.
• Kinematics and Regularity: While the thigh range of motion (ROM) showed a non-significant increasing trend, the peak downward and upward kicking velocities increased significantly (29.9% and 23.9%, respectively). Crucially, the coefficient of variation (CV) for kinematic features (ROM, cycle duration, and duty factor) decreased, indicating improved regularity and stability in the kicking motion.
• User Experience: Participants reported a strong sense of agency (average score 6.5/7), indicating they felt in control of their movements. The system received high marks for comfort and usability (System Usability Scale score of 78.9).

Significance and Claims
The authors claim this study represents the first instance of enhancing human diving endurance via exoskeleton technology in real-world underwater environments. The significance of the work is twofold:

1. Bridging a Knowledge Gap: It addresses the lack of field-deployable, untethered underwater assistive devices and provides the first quantitative evidence that exoskeleton assistance can effectively extend human diving endurance by reducing metabolic cost (gas consumption).
2. Foundational Biomechanical Insight: The study provides a quantitative characterization of human underwater diving locomotion, including spatiotemporal parameters, gas consumption, and concurrent biomechanical responses. This data fills a critical void in understanding how humans move underwater, which is necessary for designing future assistive devices.

The paper posits that the substantial improvement in endurance (47.0% reduction in net metabolic cost) surpasses the best-reported reductions for terrestrial walking (24.3%), likely due to the unique properties of the aquatic environment where buoyancy offsets the added mass of the device, and the device converts a static load (weight) into a source of propulsion. The study concludes that applying exoskeleton assistance is beneficial for improving human diving endurance and augmenting the ability to explore the aquatic world, offering a reference for the design of future underwater assistive devices.