Creating a joint based stab motion(going nuts)

So this is what I have been working on for two days. Everything I try seems to fail THis picture shows the model in a fixed form. It plays like this, with the spear unmoving. No real anomalies. https://www.facebook.com/LSPressworks/photos/a.687161491363909.1073741828.666253256788066/688073274606064/?type=3&theater

However I need it to stab. The arm should rotate, which it currently is not, even though rotation is free. The spear should stab back and forth. Any attempt to liberate that spear, whether by applying torque to the arm, or force to the spear head causes the joints(even configurable) to fly apart and become erratic. The cow itself moves and rotates in space to give chase, albeit not very fast. When the bull is perfectly stationary I can get it to go back and forth with addRelativeForce, but the instant it turns or moves.....bam, major disintegration of joints.

Can anyone please help me? This is such a simple thing, and I can't seem to get it.