Healthy limbs, bodily strength, and available tools provide a general capacity. A healthy person being able to write is different from that person actually writing at this moment.
Muscular strength is a helpful example of readiness before the act. It cannot simply be identified with every specific relation of power involved at the instant of performing the act.
A healthy person may choose not to act, or an impediment may intervene despite an intention to act. These cases distinguish general fitness, intention, and actual performance as three different aspects.