  This study introduces an innovative design for a Variable Stiffness 3 Degrees
of Freedom actuated wrist capable of actively and continuously adjusting its
overall stiffness by modulating the active length of non-linear elastic
elements. This modulation is akin to human muscular cocontraction and is
achieved using only four motors. The mechanical configuration employed results
in a compact and lightweight device with anthropomorphic characteristics,
making it potentially suitable for applications such as prosthetics and
humanoid robotics. This design aims to enhance performance in dynamic tasks,
improve task adaptability, and ensure safety during interactions with both
people and objects. The paper details the first hardware implementation of the
proposed design, providing insights into the theoretical model, mechanical and
electronic components, as well as the control architecture. System performance
is assessed using a motion capture system. The results demonstrate that the
prototype offers a broad range of motion ($[55, -45]${\deg} for
flexion/extension, $\pm48${\deg} for radial/ulnar deviation, and $\pm180${\deg}
for pronation/supination) while having the capability to triple its stiffness.
Furthermore, following proper calibration, the wrist posture can be
reconstructed through multivariate linear regression using rotational encoders
and the forward kinematic model. This reconstruction achieves an average Root
Mean Square Error of 6.6{\deg}, with an $R^2$ value of 0.93.
