I am not a doctor, but I can tell you what I have been doing. I finished my ultra trail racing season in late summer at Broken Arrow. after that race I needed to shift to Skimo race prep. I needed 2 things that are generally not in my ultra plan: 1) ME strength 2) high intensity intervals. I work on these things both indoor and outdoor:
Indoors on treadmill
1. 10 min warmup
2. 20 mins interval at 10% grade
3. 3×15 single leg box steps, 3×15 single leg lunges: repeat 3x(180 reps per leg, 540 total reps after 3 complete sets)
4. repeat 2 and 3 three times with no breaks. takes about 1:45 with short cool down jog
my HR is slow to get elevated, but by the mid-point of my 2nd run interval my HR holds steady about 88-90% of my max( max established as highest recorded rate during shorter races, which is slightly above 220-age)
by the end of the workout, I have 40 mins+ in my target HR range. my target range is the point just below LT. I understand my LT HR to be about 90% of max HR based on hours of training. above that HR, I feel lactic build-up. Note: my perceived effort during intervals is about a 7.5. I can comfortably hold the HR for extended periods
outside
I do laps of my local ski resort. there is a section of trail that is .73 miles with 697ft of elevation gain = avg grade of 18%.
1. warmup lap
2. 4x laps at LT. takes about 15 mins to go up at a PE of about 8. down takes about 10 mins at slow jog
3. if I am feeling frisky, I may throw in some leg bounding for 30 secs, once per lap
4. cool down jog back to car
I apologize for not being as technical as some of you guys when it comes to talking about training. and after writing up this workout, I realize that it may actually be closer to a Z4 workout( I have the UA book, but I am not fluent in all the terminology)
what I can say is that after 7 weeks of doing the indoor workout 2x per week, and the outdoor workout 1x per week: my ME leg strength has improved significantly. I have had to increase my indoor pace by .4 mph to hit and hold my target HR. I take 2 rest days per week, an do low intensity running on the other day(s). 2 important notes: while 3x/week hard workouts seems like too much, because I ran ultras all season, my current weekly volume is much, much less than it was during ultra season. I am a Masters level competitor, and I need 2 full days between ME strength workouts. also note, at week 5 I had a 5 days business trip and could only workout once. on my return, I did the outdoor workout and in 4 laps bested my per-lap PR by 1 min per lap …which. tells me that the extended rest was a good thing…but that was intentional because I knew the trip was coming and would afford a rest