FlexGuard: A Design Space for On-Body Feedback for Safety Scaffolding in Strength Training

TL;DR

This work introduces FlexGuard, a design space that integrates sensing triggering and feedback delivery to scaffold safety during strength training. It is grounded in nine co design workshops with trainer trainee pairs and validated through speed dating with storyboard scenarios and a proof of concept intrinsic feedback prototype. The contributions include a two dimensional design space for on body feedback, empirical insights into feedback semantics and movement continuity, and a PoC that demonstrates phase aware intrinsic feedback. The findings point to adaptable, interpretable, phase specific feedback that can transform wearables into interactive training partners with potential impact on rehabilitation and education as well as adult fitness.

Abstract

Strength training carries inherent safety risks when exercises are performed without supervision. While haptics research has advanced, there remains a gap in how to integrate on-body feedback into intelligent wearables. Developing such a design space requires experiencing feedback in context, yet obtaining functional systems is costly. By addressing these challenges, we introduce FlexGuard, a design space for on-body feedback that scaffolds safety during strength training. The design space was derived from nine co-design workshops, where novice trainees and expert trainers DIY'd low-fidelity on-body feedback systems, tried them immediately, and surfaced needs and challenges encountered in real exercising contexts. We then evaluated the design space through speed dating, using storyboards to cover the design dimensions. We followed up with workshops to further validate selected dimensions in practice through a proof-of-concept wearable system prototype, examining how on-body feedback scaffolds safety during exercise. Our findings extend the design space for sports and fitness wearables in the context of strength training.

Figures (13)

• Figure 1: Technology probe provided to participants for creating low-fidelity feedback prototypes. (A) Default setup on shoulders and elbows, using braces with inflatable air wedges and elbow straps. (B) Alternative configurations with attachable waist straps using Velcro for customization. (C) Air wedge simulating on-demand extrinsic actuation. (D) Rigid strap providing non-elastic feedback. (E) Stretchy strap providing elastic and variable feedback when stretched. (F) Props available for prototyping, including additional air wedges, straps, elastic bands, a mini-mannequin, and duct tape. (G) Example of participant N2 wearing the probe during the study with trainer E2.
• Figure 2: A sample worksheet for the dumbbell incline fly exercise, annotated during co-design workshops. The illustration captures multiple phases of the movement, with participant feedback highlighting design considerations such as strap support, anchoring for improved positioning, and adjustments to increase comfort throughout the lift.
• Figure 3: Example co-design prototyping flow from S1. (A) After performing a dumbbell incline fly, participant N1 identified the need to enforce a safe ROM. (B) Trainer E1 configured the probe by adding a rigid strap. (C) N1 repeated the exercise to experience the implemented feedback with the low-fi prototype.
• Figure 4: Sensing & Triggering dimensions of the FlexGuard design space. Trigger Metrics capture what is being monitored, including joint angle, movement dynamics, inter-limb coordination, muscle activation, and motion path. Trigger Policies determine when feedback is actuated or released, either through expert-based knowledge (e.g., recommended ROM, standard joint angles) or through personalized conditions (e.g., fatigue level, body type, or individual muscle engagement). Trigger Locations describe where feedback is sensed, such as the target region (primary muscles), compensation region (unintended muscles taking over), stabilizing region (muscles supporting posture), or moving region (segments guiding trajectory).
• Figure 5: (A) Progressive support: E6 pumped additional air into the wedge as N6’s arms drifted out of plane during a dumbbell front hold, simulating stronger feedback over time to help complete the repetition. (B) Targeted activation: N9 demonstrated arching the back during a dumbbell incline fly to ensure chest muscles were activated.