A pressurized EVA glove resists every finger-joint flex because the suit's internal gas pressure pushes the fabric back toward its inflated, straight shape. A standard simplified biomechanical model gives the resisting joint torque as:
T_resist = k · P · A · sin(θ)
k — glove fabric/joint stiffness coefficient
P — suit pressure differential (kPa)
A — effective joint cross-section area (m²)
θ — joint bend angle (max at mid-flex, θ ≈ 70°)
Higher suit pressure or a more sharply bent joint both increase the torque the astronaut's hand has to fight just to close the glove, before any torque is left over to turn the valve. The net torque delivered to the valve each squeeze is:
T_net = (effort/100) · T_MVC · capacity(fatigue) − T_resist
capacity(fatigue) = 1 − 0.8 · (fatigue/100)
Fatigue accumulates while the muscles are actively squeezing, following the shape of Rohmert's classic %MVC–endurance curve: high-effort grips burn out disproportionately fast (fatigue gain scales with effort² per squeeze), and the hand slowly recovers whenever it isn't actively gripping.
- Suit pressure — sets how hard the glove itself pushes back on every joint flex.
- Grip effort — how much of the astronaut's maximum voluntary contraction is committed to each squeeze; higher effort turns the valve faster but fatigues the hand faster too.
- Task cadence — how often a squeeze is attempted; a faster cadence leaves less time to recover between grips.
- When accumulated fatigue drops available capacity below what the glove resistance demands, net torque goes negative — the squeeze fails and the valve does not turn.
This is the same trade-off EVA suit designers and flight surgeons manage for real spacewalks: NASA's shuttle/ISS suits pressurize around 29.6 kPa (4.3 psi), and hand fatigue from fighting glove resistance is one of the most commonly reported physical limits on EVA duration.