  Dynamic locomotion in legged robots is close to industrial collaboration, but
a lack of standardized testing obstructs commercialization. The issues are not
merely political, theoretical, or algorithmic but also physical, indicating
limited studies and comprehension regarding standard testing infrastructure and
equipment. For decades, the approaches we have been testing legged robots were
rarely standardizable with hand-pushing, foot-kicking, rope-dragging,
stick-poking, and ball-swinging. This paper aims to bridge the gap by proposing
the use of the linear impactor, a well-established tool in other standardized
testing disciplines, to serve as an adaptive, repeatable, and fair disturbance
rejection testing equipment for legged robots. A pneumatic linear impactor is
also adopted for the case study involving the humanoid robot Digit. Three
locomotion controllers are examined, including a commercial one, using a
walking-in-place task against frontal impacts. The statistically best
controller was able to withstand the impact momentum (26.376 kg$\cdot$m/s) on
par with a reported average effective momentum from straight punches by Olympic
boxers (26.506 kg$\cdot$m/s). Moreover, the case study highlights other
anti-intuitive observations, demonstrations, and implications that, to the best
of the authors' knowledge, are first-of-its-kind revealed in real-world testing
of legged robots.
