You're thinking about this wrong. Wolverine's forearm muscles only retract the claws they do not extend them. You're right in that the claws are attached to strong flexible tendons. These tendons are what Wolvie's arm muscles use to retract the claws back in. Inside the arm there are tiny bone protrusion that then hook the tendons and lock the in place in a state of constant tension. When the claws are needed again, the muscles simply lift the tendons off the latch and *snikt*. There you go.