Postural stability limits fingertip force production

Compliant fingers are essential for fine manipulation. Like a slender column, a compliant finger is prone to buckle when exerting fingertip forces. But, healthy human fingers rarely buckle. So, we investigated how the neuromuscular system stabilizes finger contacts. Stability may be a by-product of activation-dependent stiffness of muscle or the muscular coordination pattern may be governed by the need to stabilize the finger. We probed these alternatives by using maximal force measurements when the index finger’s posture is externally stiffened using a splint. Upon externally stabilizing the finger with a splint, the maximal force (23 volunteers) increased by 62% (p< 0.05) and the EMG of the finger flexors increased by 63% (p< 0.05). Humans use scaled versions of the maximal force muscle coordination pattern for everyday submaximal tasks. Therefore, our results imply that muscle coordination patterns in fine manipulation are primarily driven by the need for stability against buckling.