The Pali Canon's Sona Sutta, in the Anguttara Nikaya, tells of the monk Sona, who pushed his walking meditation so hard his feet bled, and grew discouraged that he was making no progress despite the effort; the Buddha is said to have asked him whether, before ordaining, he had played the lute, and whether the instrument sounded well when its strings were too taut or too slack, drawing from Sona's answer, neither, only when tuned to a middle tension, the explicit teaching that effort itself must be tuned rather than simply maximised. This story became the standard reference point across the tradition for virya's balanced character, cited whenever commentary needs to distinguish diligence from either laxity or self punishing strain, and it gives the perfection a criterion that is not obviously available from the word energy alone: the right amount of effort is defined relative to what a given practitioner can sustain, not by a fixed external standard applied uniformly.
Mahayana texts extend the same principle to the scale of the bodhisattva path itself, which is described across countless lifetimes rather than one, framing virya less as a sprint toward a single attainable finish line than as the pacing that makes a project of that length survivable, joy in the practice functioning as the renewable resource a multi lifetime undertaking actually depends on.