you are driving the y directions since you measureorganizer is 1, in my memory the node used in the linear measure you drive has y axis upwards so pointing against the skull, is this what you want?
the driver has reactions swicthed on meaning this driver will carry loads… not the muscles.. to switch it of write
Reations.Type={Off};
To make the barbell start higher either change you data or change the measure you driver. To change the measure simply pick a different node on thorax for example ThoracSeg.ij which is at the sternum.
you add one constraint to the model, remember also to remove an equivalent constraint to keep the DOF count in balance.