The nervous system is incredibly complex. Any signal sent or received is produced or read by multiple cells. Any given motion is encoded into electrical pulses and sent through numerous cells to the muscles. Each muscle is generally innervated by several axons. These axons are in a tree-like system with the trunk being the spinal column.
To produce any given motion, multiple axons need to be stimulated. Unfortunately, the fast-twitch muscles (those that fatigue most quickly) are the first to respond. But by accessing and stimulating individual or very small groups of axons, slow-twitch muscles can be stimulated first.
This requires very high selectivity. A nerve cuff has perhaps eight electrodes per nerve. Each nerve consists of several fascicles and each fascicle consists of 2000 - 3000 axons. Therefore, a nerve cuff has very poor selectivity. Surface electrodes are even worse.
The best (contestably, but this is my opinion) electrode for selectivity is an array implanted into a nerve. These arrays consist of up to 100 electrodes allowing a large range of access to individual axons.
Research is on-going to learn the best way to select and stimulate axons to produce graceful movement with an array.