What?

. No such thing at low heart interval training. Intervals=high HR

 Sorry for being unclear. Normally when I do intervals I can get my heart rate in the 160s-180s depending on the level. Would’ve expected 168 to 178 for this particular work out , but low- mid 150s. Was struggling.
I just could not get my heart rate up. And couldn’t figure out why. Speculate that it was early morning session, muscle fatigue , possibly over training although my hours have been below normal.
Other factors people can think of? Same location, altitude and sport. Cross country skiing, rolling terrain, 6500 feet.

HRM in need of a new battery?

you need to rest

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How was performance? Was pace down?

Best phone app I ever bought: https://www.hrv4training.com/. Measures hr variability. Tracks sleep, training, other stress. Occasionally tells me "Take it easy, stupid!"

HR depression is due to fatigue. I'd take a few days off.

Exercise intensity can be described by heart rate, but it isn't really *defined* by heart rate.

IOW, "low heart rate interval training" is a thing, just as "low heart rate TTing" is a thing (I have done TTs in stage races where my power is normal but my heart rate is ~15 beats/min below expected).

Without knowing pace or power I would be wary of assuming that.

I thought this thread would be about hi-lo or polarized training. Or maybe rest intervals.

He said with that workout he'd expect 20 bpm higher and he was struggling. Seems fairly obvious to me.

He did not mention pace, power or performance. Only that he was struggling to get his heart rate up to the levels he expected.

Was he struggling to perform at the same pace as usual? Heart rate isn't a power meter or a speedometer.

Early morning session likely. Happens to me a lot when I'm not fully awake. No matter how hard I try, I can't even get my HR over 140 when I'm undersleeped or not fully awakened.

I sleep in a little more, or push the workout back a few hours until I'm fully awake, and it's nearmax 170+.

Is power or pace lower though in the mornings? Is performance down?

Thanks everyone.
Nordic skiing -of the toughest sports to standardize despite the fact I’ve been doing it for 20+ years. Extremely variable conditions. Different terrain, different snow, multiplicity of factors has had me ski in a race anywhere from 2:20 to 5:30/km. Versus most easily replicated swimming where if I’m doing hundreds two seconds slower it’s probably me. So it’s hard for me to know about power output. Because although I’ve skied those trails, I haven’t done intervals on those trails in that specific site.

Circadian rhythms-I hate races in the morning. I always did better in evening running or biking races and mid day skiing races. Early morning triathlons slayed me. Why even go in the morning?..sometimes work, sometimes my “lark“ wife shared workouts-important to compromise.

It might be a little of everything. Just wanted to get everyone’s thoughts. I have had a wonky heart rate monitor and strap do that in the past; Don’t think it’s that but haven’t changed the battery so did today.

My approach will be to take a few days off. I’m not a pro, it’s not that big a deal and it won’t hurt me. Common things being common, probably is fatigue.

Thank you for the heartbeat variability app suggestion-I’m going to get it and use it. Jim Galanes does this first beat technology stuff but it’s pricey. This is the poor man’s 90% version from looking it over- at pennies to the dollar.