Muscles in animals are often found in pairs, because
Amuscles need to be able to both push and pull.
Bmuscles can only push.
Cmuscles can only produce a force as they relax.
Dmuscles can only pull.
Amuscles need to be able to both push and pull.
Bmuscles can only push.
Cmuscles can only produce a force as they relax.
Dmuscles can only pull.