No single move provides as much upper-body strength as this exercise. Strong arms are vital for class IV (and up) scrambling and for transferring weight onto hiking poles (and off of stiff joints).
Grip a chin-up bar with hands shoulder-width apart. Use either an over- or underhand grip, but be sure to mix it up. From a fully extended hanging position, pull your chin above the bar. Lower your body back to starting position.
Until you can’t do any more, twice
Beginner: Start with negative chins. Stand on a chair to get your chin above the bar; lower on an 8-count.
Intermediate: Max out on chin-ups; switch to negative chins until your muscles quit.
Advanced: Do chin-ups with a weighted pack.