Bala – strength – is of great significance in Yoga. But the importance is given to mental strength. Physical strength is very much necessary but should be complemented by mental and spiritual strength.
Mental power, when it manifests through the body, is strength; when it is exercised mentally it is called power.
In addition to mental power certain superhuman powers (vibhuti siddhi) have also been mentioned in Yoga Sutra.
They are all mental powers. When strength/power is channelized towards spiritual achievement it becomes power.
In Patanjali’s Yoga Sutra, the term bala occurs thrice:
Maitryadisu balani (Yogasutra III.23) – By employing samyama on maitri and the rest the practitioner gets strength in these qualities. Thus the qualities gain excellence by yogic practice. In this phrase, Bala means mental power only.
Baleshu Hastyadini (Yogasutra III.24) – One acquires the strength of an elephant by samyama (self control). This is clear indication of the physical strength. But the point to be noted is that strength as such is enormous and tremendous in the human body, but is lying dormant. It has to be awakened by yogic practices. A clear indication of the mental power being the source of physical strength is give by the aphorism.
The physical strength is referred to (Yogasutra III.47) but it is the result of samyama on various aspects of elements (Yogasutra III.47)
The importance of bodily strength is mentioned in other scriptures.
Mundaka Upanishad (3-2.4) states that the self cannot be realized by a weak person.
Chandogya Upanishad (VII 8.1) states: “The world is sustained by power, so acquire power.”