Dual spring force couples yield multifunctionality and ultrafast, precision rotation in tiny biomechanical systems

Small organisms use propulsive springs rather than muscles to repeatedly
actuate high acceleration movements, even when constrained to tiny
displacements and limited by inertial forces.  Through
integration of a large kinematic dataset, measurements of elastic recoil,
energetic math modeling, and dynamic math modeling, we tested how trap-jaw
ants (Odontomachus brunneus) utilize multiple elastic structures to
develop ultrafast and precise mandible rotations at small scales. We found
that O. brunneus develops torque on each mandible using an intriguing
configuration of two springs: their elastic head capsule recoils to push
and the recoiling muscle-apodeme unit tugs on each mandible.
 Mandibles achieved precise, planar, circular trajectories up to
49,100 radians/sec (470,000 rpm) when powered by spring propulsion. Once
spring propulsion ended, the mandibles moved with unconstrained and
oscillatory rotation.  We term this mechanism “dual spring force
couple” meaning that two springs deliver energy at two locations to
develop torque.  Dynamic modeling revealed that dual spring force
couples reduce the need for joint constraints and thereby reduce
dissipative joint losses, which is essential to the repeated use of
ultrafast, small systems.  Dual spring force couples enable
multifunctionality: trap-jaw ants use the same mechanical system to
produce ultrafast, planar strikes driven by propulsive springs and for
generating slow, multi-degree of freedom mandible manipulations using
muscles, rather than springs, to directly actuate the movement. 
Dual spring force couples are found in other systems and are likely
widespread in biology.  These principles can be incorporated into
microrobotics to improve multifunctionality, precision, and longevity of
ultrafast systems.