enable( key, enabled ) {

			if ( this.enabled[ key ] === undefined ) {

				throw new Error( 'THREE.MMDAnimationHelper.enable: ' + 'unknown key ' + key );

			}

			this.enabled[ key ] = enabled;

			if ( key === 'physics' ) {

				for ( let i = 0, il = this.meshes.length; i < il; i ++ ) {

					this._optimizeIK( this.meshes[ i ], enabled );

				}

			}

			return this;

		}
Enabes/Disables an animation feature.

@param {string} key
@param {boolean} enabled
@return {MMDAnimationHelper}