Explain the effect of constraint-solver differences on gait quality

Determine how differences between force-based joint-limit and contact models and solver-based constraint formulations, particularly MuJoCo's constraint solver, produce substantial changes in gait stability and realism despite similar contact-force and joint-moment measurements.

Background

Replacing Bolt's force-based constraints with the MuJoCo constraint solver resulted in unstable sprinting gaits, excessive lateral foot motion, and greater variance in stride frequency and stride length. However, inspection of contact forces and joint moments did not reveal substantial differences between the two formulations.

The authors speculate that the degradation may result from the solver treating joint limits as too stiff, but they do not establish the mechanism. Parameter tuning also proved difficult because softer limits could lead to substantial joint-limit violations, leaving the relationship between constraint formulation and overall gait quality unresolved.