The word 筋疲力尽 is formed by combining two parallel phrases: 筋疲, meaning "tendons are tired", and 力尽, meaning "strength is exhausted". Each phrase pairs a noun (筋 for tendons/muscles, 力 for strength) with a descriptor of depletion (疲 for tired, 尽 for exhausted), and their conjunction creates a compounded emphasis on complete physical fatigue.