Strong like Bull - By Pete Vordenberg For SkiPost in 2001
The area where most racers can make grand strength gains is in specific strength. Specific strength is carried out on rollerskis and on skis. It is doing long distance efforts as well as shorter (interval like) repeats using the upper-body only.
Classical distance workouts of 30mins to 2+ hours where one double-poles only or double-poles with a kick only are very important to developing upper-body strength (Every Thursday Olympic contender Barb Jones does a 20 mile double-pole only workout - ten miles all up hill with some very steep sections and then 10 miles back down.) Skate distance workouts where one uses only the V2 technique accomplishes a similar thing.
It is best to take on challenging terrain so that a variety of tempos, and techniques (within double-poling, V2, etc) can be used, even if that means struggling over the tops of a few hills here and there. Shorter, repetitious sessions are also very important, and for most skiers who are comfortable at a medium intensity, doing harder repetitions will absolutely help increase speed as well as strength. There are a number of on ski/rollerski exercises to practice in a specific strength session. The first is double-pole sprints of 15seconds to 2minutes. Use both flat terrain and gradual to very steep up-hills.
The second is single-pole drills where one uses the arms much as one would while classical striding, only using the trunk a bit more actively and not using the legs at all. As with double-poling, efforts can range from an all out 15seconds to a more race like 2-3minutes.
The third is simple double-pole with a kick for sprints and longer intervals as well as distance sessions.
Other exercises are Nerds (standing stalk-up right, locking the elbows at the side and using the triceps only - so called because they look nerdy). Stomach only (double-pole, but lock your arms next to your body and crunch down with your stomach - only really good on a very steep up-hill - and even nerdier looking than nerds). Back only (stay up-right, lock your arms out fairly straight and push the arms through, in a stiff double-pole motion, without compressing with the upper-body at all).
Personally I only do double-pole, single-pole, double-pole with a kick, some classical sprints and perhaps a few nerds - the other exercises are not to my liking, but some people do them.
In Sweden (I trained in Sweden for a year after high school) all we did for specific strength was double-pole sprints and distance double-pole sessions - 15 x 15second all out double-pole sprints and then an hour hard, fast double pole home - and that seemed to do the trick.
An example specific strength workout: warm up for 20 minutes classical rollerskiing. Pick a long gradual hill; do 5x 15seconds double-pole only, then 5x 15second single-pole, then 5x 15 double-pole w/kick - all of them with the throttle wide open. Follow that with 5x 1-2min of each exercise. Recovery between repetitions should allow you to go full out on the 15second sprints and faster than race pace on 1 to 2 minute repetitions on all repetitions.
This is only an example and as is the case with all training start with a manageable, but challenging quantity and build from there.
At Northern Michigan University, a school that turns out fast ski racers year after year, they do a program like the example above. In the early fall they start with about 4km total work and built to 9km total work by snowfall, and continue with specific strength into the winter until a week or two before the big races rolled around.
Pete Vordenberg 2001