Archaic KJV Word
Strength
Modern equivalent: power
What Was Lost
The divine transfer. 'Renew their strength' was not 'take a nap and feel better' but 'exchange their depleted human resources for God's own inexhaustible power.' The word implied a transaction: your emptiness traded for His fullness. Strength was not something you generated but something you received from the source of all power.
Closest Survivor in Modern English
strength (still used but usually means human capacity rather than divine power transferred)
Peak Usage (1611)
KJV Psalm 28:7 -- 'The Lord is my strength and my shield'; Isaiah 40:31 -- 'They that wait upon the Lord shall renew their strength'
Died still used but physicalized (~1800)
Hebrew oz ('strength/might/fierce power/the force of God transferred to humans') reduced to physical muscle power or abstract capability. The divine-transfer dimension -- God's own power entering human weakness -- was lost.
What Replaced It
“power”
Generic capacity; oz was specifically God's own fierce power made available to frail humans
“ability”
Human competence; oz was divine enablement beyond human capacity
“muscle”
Physical only; oz included moral courage, spiritual resilience, and supernatural endurance