What is a synergist or the effects of synergists in producing a particular motion or function such as ascending stairs, throwing a ball, foot pronation during weight bearing, etc.?

Synergistic action of muscles in producing purposeful motion refers to the agonistic and antagonistic collaboration of paired muscle groups in producing smooth movement. For example, for most movements involving either the upper or lower extremities, one muscle group (the agonist) contracts while another muscle group (the antagonist) relaxes and vice versa in order to produce ideal motion.

For example, when climbing stairs, the hip flexors and hamstrings (in this case, the agonists) contract to flex the knee and raise the referenced lower limb off the floor and onto the stair. While this is happening, the quadriceps muscle of the same limb (the antagonist) is relaxed to allow for this motion to occur. Co-contraction of these muscle groups at the same time will not permit smooth movement. It is this synergy of muscle groups that allows for purposeful movement and is present in most human movement including the motions of throwing a ball and pronation.