The M.U.S.C.L.E. Hard Knockin' Rockin' Ring Wrestling Arena was the only playset in the American M.U.S.C.L.E. line. It was a pretty cool idea... you and a friend put your wrestlers in the ring, and duked it out Rock 'Em, Sock 'Em Robots-style until there was only one M.U.S.C.L.E. standing! Certain M.U.S.C.L.E.s performed better than others because their bodies fit the clamps differently. Unfortunately, the plastic they used for the clamps was relatively brittle, and they broke pretty easily.
As Universal Ruler Supreme discovered at Naochin's Kinnikuman site, the wrestling ring had its origins in the Japanese Kinnikuman line. There were two rings: the Pro-Wrestling Game, which seems to be the source of the MUSCLE ring's "body", and the Battle Game, which has the same joysticks and figure clamps as those of the MUSCLE ring.
The M.U.S.C.L.E. Wrestling Ring cost $9.99 in the 80s (according to scans of the 1986 Sears Wishbook at Action Figure Archive).