  This paper endeavours to bridge the existing gap in muscular actuator design
for ligament-skeletal-inspired robots, thereby fostering the evolution of these
robotic systems. We introduce two novel compliant actuators, namely the
Internal Torsion Spring Compliant Actuator (ICA) and the External Spring
Compliant Actuator (ECA), and present a comparative analysis against the
previously conceived Magnet Integrated Soft Actuator (MISA) through
computational and experimental results. These actuators, employing a
motor-tendon system, emulate biological muscle-like forms, enhancing artificial
muscle technology. A robotic arm application inspired by the skeletal ligament
system is presented. Experiments demonstrate satisfactory power in tasks like
lifting dumbbells (peak power: 36W), playing table tennis (end-effector speed:
3.2 m/s), and door opening, without compromising biomimetic aesthetics.
Compared to other linear stiffness serial elastic actuators (SEAs), ECA and ICA
exhibit high power-to-volume (361 x 10^3 W/m) and power-to-mass (111.6 W/kg)
ratios respectively, endorsing the biomimetic design's promise in robotic
development.
