It is known that the human muscles can be controlled both intentionally and by automatic responses. However how exactly the neural signals received by the muscles are produced is still unknown. One of the concepts created to answer this question are the muscle synergies. This concept however, doesn’t take into account whether the neural signals are of voluntary or involuntary origin. Most researchers apply this concept to analyze exclusively automatic responses or exclusively voluntary movements or both without distinguishing between them. We propose an extended synergy model that explicitly accounts for both voluntary and involuntary neural signals and try to verify it experimentally.We examine reaching movements with and without constraints that provoke automatic responses. The general goal of this research is the creation of a measure of recovery level in upper limb rehabilitation after brain stroke, as well as, a rehabilitation assisting device. We introduce the synergy stability index and with experiments we show that the synergy stability is lower for movements with disturbance that provoke an involuntary movement.