I do 4x4 minutes as one of my regular interval sessions. Although I prefer running on a track I do most of these on a treadmill during my lunch break. Recently I have made an interesting observation: If I run all 4 intervals at the same speed, say 11 mph, this is noticeably more taxing relative to the distance covered than running at variable speed 10.9, 11.0, 11.1 and 11.2 even though the variable speed set is faster. In the single speed set I can see that run efficiency is decreasing while in the variable speed set efficiency is almost constant for the last three intervals. I record HR at the end of each interval and pair that with a calculated distance traveled per 4 minutes. My proxy for run efficiency is distance traveled divided by a normalized HR at the end of the interval. I have done enough of these that I can be sure that the difference is not good day - bad day thing.
My first hypothesis on the improved performance during the variable set was that I was able to increase the speed and my stride length with only a minimal cost increase as I got warmed up.
=> Warm muscles => Longer stride at ~same cost => Better run efficiency late in the interval set
When I do 4x4 intervals on track my end HR in the last two intervals are close. The last one 1- max 2 beats higher than the third if the time/speed was the same. When I do 4x4 treadmill intervals at a single speed I seem to have more HR drift than on the track. If I do treadmill 4x4 at variable speed I seem to have relatively less drift if I take into account that I am running faster.
My latest hypothesis is that on an outdoor track and at variable speed treadmill set I recruit a larger variety of motor units than the single speed treadmill set. In the single speed set some motor units get tired and drop out, forcing me to recruit less efficient motor units hence causing the larger drift in HR.
=> Variable speed => Distributing workload on more motor units allowing better muscle endurance => Better run efficiency late in the interval set
Any idea which of the hypothesis are more correct?
The big question: Of the two treadmill interval sessions, single vs. variable speed, which is best at improving my overall running? I want to improve my running from 5K to half marathon.
I’d like to hear what you think will produce the best results and why:
Weekly 4x4, 3 min jog recovery, all at same speed (10 sek per mile faster than 5k pace)
Weekly 4x4, 3 min jog recovery, progressive speed (5K pace -7sek, -10sek, -13sek, -16 sek)
#1 may be more challenging for my muscles (see second hypothesis) while #2 is run at a higher average speed. Based on HR both have about the same metabolic cost (OK, I am using terms that I do not completely master here!). If I choose one of the above for the next 8 weeks I will never know if the other would have produced a better result so I’d like to do an educated choice.
Speed in each session will increase with time based on HR obtained in previous session(s). Seems like 0.5-1% improvement of distance travaled in 4 min per session is sustainable over a long time (!), granted that this is not the only run workout I do each week.
Sindre
Can I ask a few questions:
Are your intervals based on a previous 5k time (5:25-5:55 min/mile avg)? I assume these intervals are approximately 5K pace minus 10sek/mile. Two months ago I ran 18:00 and then I did these intervals at 10.6 mph (5:40/mile)
What incline do you train at if any at all? Right now I have zero incline (leg speed). This winter I did mostly 4-5%. Every treadmill is different so I use the same treadmill every time and match my treadmill data to track workouts so I can convert indoor workouts at different angles to outdoor performance.
How does your turnover compare between the two different sessions? I assume cadence is very close at these speeds, but I have to admit I have not counted.
What kinda PR are you looking for? I’m stretching the rubber band as far as it will go. I’ve got a half marathon in October I’d like to do well. Gotta swim and bike as well though so I won’t be maximizing run performance.
Ive had similar experiences, I favor changes in speed than a steady pace as well. For me if anything, its more of a mental hump to get over. Keau
Being that the time intervals are the same and the difference between 10 and 16sec neg split, too close to call.
If #1 is more challenging in your mind, do #1.
From a 4x4, will you be increasing the duration as the race gets closer (i.e 3x6, 1x12, etc).
I use some incline (1.0-3.0), cadence doesnt not change much and it mimics actual road running more. I feel the neglect in my stabilizers at 0% and it is much easier for me to stay in a comfort zone keeping up with the belt.
Have you had problems with interval recovery?