To overcome paretic upper limb reduced ability post-stroke, many patients use trunk compensations sometimes leading to proximal arm nonuse, but the reason why is not fully clear. 22 healthy participants held a weight (85% of Maximal Voluntary Contraction) during seated reaching in two conditions: spontaneous arm use (SAU) and maximal arm use (MAU, in which they were asked to voluntarily maximize arm use and minimize trunk use). We found that proximal arm nonuse is a movement strategy that healthy people spontaneously choose to cope with a lower force-over-weight ratio of their antigravity muscle of the upper limb, Yet, this suggests that proximal arm nonuse post stroke is largely due to antigravity muscle weakness, which in turn suggests that antigravity muscle reinforcement may result in less proximal arm nonuse post stroke, especially for patients with high PANU scores.