# Angular momentum conservation

Take a guy sitting on a chair and say we ignore all frictional forces in this example.
We set the guy spinning on the chair and he holds two hammer out at full extension. He then tucks his arms in reducing his moment of inertia as mass is concentrated closer to centre of rotation and as you would expect due conservation of angular momentum his angular speed increases.
Now i dont know if im bring stupid but what provides the angular acceleration to increase the angular speed as it cant just happen like that there must be a torque to produce the angular acceleration i just dont know where it comes from in this demo to provide angular acceleration.

Like i said i may be missing something really obvious but was just wondering as in linear momentum its alwasy pretty obvious the impulse that causes change of momentum.
The increase in angular velocity => increase in rotational kinetic energy. This increase in rotational kinetic energy is supplied by work done by the person's muscles