A wearable sensor for strength training lives in a hostile mechanical environment. It gets dropped, slammed, sat on, and occasionally run over by a loaded barbell coming off a rack. Thirty-eight grams and lab-grade sensing accuracy are each achievable on their own. Achieving both at once, in a housing that survives repeated high-g impacts, is a constrained optimization problem. Nicolas led the mechanical design of that housing, and I worked alongside him on the simulation and material selection that had to validate every decision before it shipped.
Defining the load case before drawing anything
Before either of us committed to a geometry, we needed a defensible input: what impact is this thing actually going to see. We didn't guess. We measured drop heights and impact orientations from realistic gym scenarios (a bar rolling off a rack at roughly waist height, a plate stack nudging the mount, a direct vertical drop onto a platform) and used those as the boundary conditions for every simulation that followed.
That matters because an arbitrary safety factor is not engineering, it's a guess with a number attached. Every structural decision downstream needed to trace back to a measured or conservatively bounded load case, not an assumption.
Structural and dynamic FEA
Once Nicolas had a candidate geometry, I ran finite element analysis across several coupled regimes rather than treating this as a single static stress problem:
- Structural, to check peak stress concentrations in the housing and mounting points under the worst-case impact orientation.
- Dynamic, because a barbell impact is a transient event, not a static load. The response of the housing over milliseconds, including how internal components resonate against their mounts, determines whether the PCB or battery sees damaging acceleration even if the housing itself survives.
- Buckling, on the thinner wall sections where minimizing mass pushes wall thickness down to the point where local instability, not yield stress, becomes the governing failure mode.
- Thermo-mechanical, since the housing also has to tolerate temperature cycling (a cold garage in winter, a hot car in summer) without inducing stress at material interfaces that a purely mechanical model would miss.
Running these as coupled multiphysics problems, rather than four separate checks, is what let us find failure modes that wouldn't show up if we'd only run a static structural case at room temperature and called it done. Every time a simulation flagged a governing failure mode, that went back to Nicolas as direct input to the next geometry revision.
Material selection as a joint constraint
The mass ceiling meant the housing material couldn't be chosen on stiffness or impact resistance alone, it had to be chosen against the full set of constraints simulation was surfacing: strength-to-weight ratio under the dynamic load cases, fatigue behavior over repeated impacts rather than a single worst-case event, and stability across the thermal range we'd defined. Nicolas and I went through that selection together, using the FEA results to rule candidate materials in or out rather than choosing on datasheet properties alone, since a material that looks strong in a static pull test can still underperform once you factor in how it behaves under a transient impact or after thermal cycling.
The mass constraint is not negotiable
Thirty-eight grams was not an arbitrary marketing target, it's the point past which the sensor stops feeling like an intrusion on the lift and starts behaving like dead weight that changes the bar's dynamics. Every gram added to the housing had to be justified against that ceiling, which meant the FEA wasn't just checking "does this survive," it was checking "does this survive at the minimum mass that clears every load case with margin."
That's an iterative loop between the two of us: Nicolas proposes a wall thickness or rib geometry, I run the coupled simulation, we identify the governing failure mode together, material or geometry gets adjusted to remove mass everywhere except where that mode requires it, and we repeat.
Why simulation isn't the last step
Simulation tells us where a design should fail first and at what load. It doesn't replace physical validation, it directs it. Every housing revision that passed simulation went through physical drop testing at the same load cases the FEA was built around, specifically to catch anything the model's assumptions didn't capture, like manufacturing tolerances in the actual molded part or adhesive behavior that's hard to characterize analytically.
If a physical test failed at a load the simulation said should be safe, that's not a testing problem, it's a modeling gap, and we go back and figure out what boundary condition or material property was wrong before touching the geometry again.
The standard we hold this to
A number we put on a spec sheet has to survive three separate tests: it has to hold up in simulation, it has to hold up if we derive it by hand on a whiteboard as a sanity check against the model, and it has to hold up when the physical part is actually dropped, impacted, or lifted with. If any one of those three disagrees with the others, the design isn't finished, regardless of how good it looks in the other two.
That's the discipline that gets a 38-gram sensor to survive years of gym use without becoming fragile or over-built, and it's a discipline that only works as a close collaboration between the person shaping the geometry and the person validating it under load.
Léo Fortin Dionne works on the advanced measurement systems and engineering simulation behind ORCA Strength Systems.