|View from the top of Liberty Bowl, North Cascades|
Incorporating hills into your weekly routine accomplishes several things. First, and perhaps most important, is the strength component. Either as a skier, runner or cyclist, you're pushing uphill with powerful big mover muscles that aren't otherwise being loaded in a similar fashion on gentler terrain. For running, there's an explosive component. Even ultra-marathon distance runners will appreciate the benefit of Type-IIA (fast-twitch aerobic) muscle fibers, which assist in the power generation needed to spring off each step, against gravity, up a trail. As a Nordic ski racer, uphills not only the strength akin to running, but also a strength in stability and core to drive the gliding leg against gravity and then to plant the foot and set the wax.
Over the past few years my coach Scott has incorporated some innovative uphill training ideas into my own schedule. Most notable amongst these are weighted uphill ski walks or hiking. The principle is simple enough: find a steep climb and load a pack with 5-10% of body weight with water. The additional load will exert a greater stress on your working muscles without over-taxing the aerobic system. This means that you will feel a strong fatigue in your legs long before your breathing passes an aerobic threshold (nose-breathing). The bonus in carrying water (using milk jugs is easiest, and easiest to measure in weight) is that you can empty the jugs out at the top of the climb and descend unweighted.
Another option: ski touring. The spring is a great season to get some variable strength training into the schedule, and skinning up mountainsides takes the cake. The load which comes from driving boot, ski, and body weight + pack up a 10%+ grade is unrivaled for training benefit. Plus you get some incredible views and some sweet corn to carve on the way down.
Take-home message? Head uphill!