You re most efficient when relaxed, with all of your muscular energy directed into the task at hand. Proper form while skiing or during fitness training will allow you to perform more effectively and build endurance on the slopes. Working out at the proper intensity will increase your strength and endurance and allow you to enjoy your skiing for longer periods of time. If you perceive that you are working too hard at a level that you cannot sustain, back off the pace. Incorporating rest into your training schedule will benefit you in the long run.
Rest helps both physically and mentally.
Physically, your muscles and your cardiovascular system have time to recover. Mentally, you stay fresh and focused. These exercises are designed for a high number of repetitions with an appropriate weight. You should experiment with different weights to find one that's challenging, but allows you to work in the suggested range of repetitions. A good place to start is two to three sets of 12 to 15 reps each.