Often the children of God cannot rise up to answer the Lord's call to service simply because, though their physical condition is good, their feelings are low, cold, and reluctant. Or even when their emotions are quite high, passionate, and willing, they find themselves unable to serve the Lord because now the body reacts lazily. The disciples found themselves in precisely that situation in the Garden of Gethsemane: "the spirit indeed is willing, but the flesh is weak"
Watchman Nee (2009). “The Spiritual Man”, p.356, Christian Fellowship Publishers