תהלים, פרק ק״ט, פסוק כ״ד

Physical and emotional suffering often intertwine, ultimately breaking down a person's body and leading to total exhaustion. In times of severe distress, the sheer toll of wandering, fasting, and anguish can drain away every ounce of a person's strength. The experience of knees buckling from fasting expresses extreme physical weakness and an inability to walk after enduring numerous fasts. The purpose behind this fasting is understood in several ways. One approach suggests that these fasts were observed while fleeing, intended to awaken God's mercy so that He might see the sufferer's affliction and deliver him from his enemies [רד״ק, אבן עזרא]. Another perspective views this fasting as a profound process of repentance and the cleansing of sins [מלבי״ם]. Taking a different angle, it is suggested that because the individual previously lived a life of luxury and comfort, even a single fast was enough to cause his knees to give out [אלשיך].

This physical collapse extends to severe weight loss and bodily wasting [מצודת ציון]. The primary approach among commentators is that the fat and natural moisture that once filled his body completely disappeared as a result of the fasting, intense suffering, and the hardships of the road [רד״ק, אבן עזרא, מאירי, מצודת דוד, ביאור שטיינזלץ]. Building on this, a specific distinction is made regarding how the body deteriorated. Typically, when a person loses weight, it is the fat that melts away rather than the flesh itself. In this case, however, the individual was once so soft and delicate that his very flesh was saturated with fat. Consequently, as the fat vanished, the flesh itself became visibly depleted and thin [אלשיך].