Are your sprint intervals a pain in the ANS?

Sprinting, METCON’s, High Intensity Interval Training (HIIT) – they are the new jogging around the park… sort of.  Just as we went through that period where if a 10km jog 1-2 times per week was good, then 10km per day must be great, so it is with all the high-intensity work.  What’s the WOD?

One would like to think that those who engage in this type of training have a good sense of what is required for the different phases of recovery.  There is the initial metabolic recovery, moving the likes of lactate and other metabolites out of the muscle (or processing them within), the refuelling of glycogen and possibly some takes place with regard to intramuscular fat stores, and longer term, there is the repair of muscle and connective tissue damage, which may take several days to occur.  Some individuals are very good with their recovery strategies and others are completely bloody hopeless at it.

When anyone considers their recovery, how many consider the toll a particular session has taken on their nervous system and how long it might take for recovery to take place?  It is something I have been aware of for a few years having read to odd bit on central nervous system suppression following certain types of workouts.  Keep flattening your nervous system and it doesn’t take much to figure that this might be detrimental to us in the long run.

But just what does this nervous system suppression involve, and if active recovery of it is required, just how long do we need to wait?  A recent paper from the Scandinavian Journal of Medicine and Science in Sports might give us some insight and possibly some pause for thought when we insist on blitzing ourselves 5-6 days per week.  Now I have mentioned this paper in a previous post, buried amongst some other cycling-related information.  I thought it might be worthy of its own post and considered from the perspective of those engaged in several WOD-type sessions per week.

Before we crack into the nuts and bolts of this paper, some background reading for you… come on, us bloggers can’t deliver everything to you all on a platter!

The Autonomic Nervous System

Or… Neuroscience for Dummies Kids

During exercise we see a shift from parasympathetic activity, at rest, to increasing sympathetic activity.  This sharpens our senses, our reactions, opens our airways, gets the heart pumping with more power, increases adrenal gland outputs, and reduces inputs into the digestive system.  Think about it from an evolutionary perspective… you aren’t going to be doing a WOD, you might be going to fight for your food.  What might you require of your body in order to increase the success of that fight?  It is these requirements that the sympathetic nervous system are gearing you up for.  If the hunt is successful, then you get to switch back to the rest and digest mode of the parasympathetic system.

What do we see after the sympathetic nervous system of the ANS has been fatigued?  Some markers can include reduced heart rate variability (HRV), cardiovagal baroreflex sensitivity (BRS) (how efficiently the baroreflex is able to change heart rate in response to alterations in systolic blood pressure), and blood pressure variability (BPV).  If all of these functions are suppressed, what you might see is a heart rate that cannot respond as quickly to changes as it should, and you might experience changes in blood pressure moving from a sitting/lying position to standing up.  Effectively, in a similar way to tired muscles not wanting to “fire”, the ANS doesn’t want to fire up cardiovascular responses to movement… it wants you to sit your ass down, Sunshine.

The researchers here induced a degree of fatigue in subjects by getting them to perform either one 30-second all out sprint on a cycle (Wingate test) or multiple sprint intervals consisting of 4 lots of 30-seconds on, 4 minutes easy pedalling.  Sound familiar?  Regular readers will recognise the 30s:4min protocol from a previous post on interval training sweet spots.  So this study here has particular reference to the type of recovery required from that style of HIIT, though I would suggest it is applicable to other forms of sprints, METCON’s, etc.

The main findings of this study are that HRV and BPV indices indicate decreased vagal tone immediately following both single and multiple supramaximal interval exercise with a return to resting levels by 2 h of supine recovery. In contrast, BRS remained depressed after 2 h of supine recovery from multiple but not single high-intensity intervals and in response to a standing orthostatic challenge following both single and multiple high-intensity intervals. This is consistent with the idea that autonomic balance is not fully restored with 2 h of passive recovery following high-intensity interval exercise.

So – if you have the luxury of lying down for 2 hours immediately after your WOD at the Box in the morning, most of your ANS markers will recover – but not all.  Your ability to adjust your blood pressure to postural changes remains depressed.  Now in the grand scheme of things, the protocol used here isn’t overly strenuous.  If you did double this on the bike, or hammered it a bit harder at the gym, your would likely induce a somewhat deeper state of fatigue than seen in the subjects here.

The relatively lengthy ANS recovery time also brings to consideration that appropriate rest should be allowed between sessions or between training and competition. Studies examining long-term recovery have found that autonomic indices have returned to normal at rest, but remain depressed during an orthostatic challenge 48 h after the cessation of exercise. This suggests that 2 d of recovery is inadequate for complete autonomic recovery following nine-cycle intervals at VO2max interspersed with base exercise.

If the same holds for sprint interval exercise with 4W, then training every second day would not allow for complete ANS recovery between sessions. In such an instance, competition performance may be hindered, or for certain populations, it may further compromise their health. However, it should be noticed that trained subjects are able to recover more quickly from exercise than sedentary subjects.

So again, given that there seems to be many people in Paleo/Crossfit-land hammering themselves with this short(ish) duration, high-intensity training, 4-5 days per week, we are likely seeing relatively deep levels of ANS fatigue induced within these people.  It isn’t uncommon to hear people say that it takes them longer some days to warm up and get into it, or that they feel a bit spacey and go a bit dizzy when moving from sitting to standing.  Are these people suffering from nervous system suppression – with a nervous system that just needs some time off?

If we are looking at potentially 3 days minimum before the ANS can take another pasting, then we are looking at only two of these sessions per week.  As the study mentioned, those who are adapted might be able to handle a bit more than those who are only just starting down this pathway, though I would suggest that those who are well-adapted to HIIT-type training are able to push themselves so much deeper per session that they will still require the same level of recovery.  The newb at the box might only need one relatively light session to cook them for a week.  But how many gyms advise their new clients of that I wonder?

The take home – your central nervous system can become fatigued too, and requires recovery just like the other bits of your body.  Yes intervals are good for you and can do some spectacular things for your health and fitness.  But they can also be, and frequently are, overdone.  Once again, I think Mark Sisson is pretty close with his prescription of move fast once in a while.  Or did he write beat yourself like a dog down at the box everyday?  I can never remember which it is.

18 thoughts on “Are your sprint intervals a pain in the ANS?

  1. Pingback: Are your sprint intervals a pain in the ANS? « Thor Falk's Reading List

  2. Thor Falk

    Excellent post, Jamie. It is great to get some feel for optimal recovery times. By the way, I am not sure that advanced athletes are necessarily faster to recover: certainly their recovery is better, but they are also more capable of beating their bodies up. In the pure strength world the net effect is usually the other way round: novice lifters recover well on a one-on-one-off basis, and more advanced lifter take a week or longer to recover, Rippetoe / Kilgore being of course one good reference for this

  3. Pingback: Sprinting + Strength training in the same day | Mark's Daily Apple Health and Fitness Forum page

  4. Chris Sturdy

    This is excellent advice. I only hope some will read and take it to heart. I have seen so many personal fitness summaries on facebook, blogs, etc. that astound me at the sheer volume of very high intensity training per week. I regularly read people reporting doing crossfit four times a week. This type of a schedule doesn’t seem like it would allow for adequate recovery, yet I read this time and again.

      1. Emily Deans MD

        Most sensible boxes do plan a good recovery/cardio/gymnastic workout twice a week, and heavy strength alternating body parts other days. The 5X a weekers note they are much less sore going often compared to me (I go 2-3X a week). However the volume and type of exercise will lead ultimately to plateaus in strength and breaks need to be had every few months or so.

        Honestly I think the main problem with crossfit is the heavy emphasis on overhead and shoulder exercises. Shoulders are burned almost daily, and there are more shoulder injuries than anything else. No need to be a hero on the push press or the overhead squats when you have snatches and pull-ups the next day…

        1. thatpaleoguy Post author

          Muscle soreness isn’t going to be a good reflection of what is happening at a nervous system level. It could be that those who are training 5x per week are lifting with relatively less loading per session than those lifting 2-3 times per week, in an effort to conserve energy across the week. I know I would drop my loads per session if I were deadlifting 5x vs. 3x.

          Some boxes might be good at varying the stimulus, but sometimes no stimulus is needed. I’m yet to see any CF programming where they kick you out of the gym for 1-2 weeks for a forced lay-off.

          Shoulders – agreed… too many OHP’s, OHS, wall balls, swings, and so on… And don’t get me started on kipping! Shoulders take a hammering at the best of times, let alone with no end of pushing, kipping, and hyperextension.

  5. Victoria

    I remember getting the shakes towards the end of my first ever WOD and when I got to my car later thinking “I’m not really sure I should be driving…”. My nervous system was definitely not happy with me. I’m generally a 2 WOD per week person, and I’m amazed at those that are 2 WODs per day, though I’m sure the style and type of workout makes a difference. Of course, exercise induced ANS fatigue is probably only part of the story for many people (it definitely is for me), with a number of other stressors probably causing equal (or perhaps worse) effects. I don’t think it’s all necessarily bad, but adequate recovery and rest from all types of stress seems important, though is frequently overlooked.

    1. thatpaleoguy Post author

      Yes I have had the “I really shouldn’t be driving” moments too. If I had to dial my brain up for something it would have been like “The number you have called is outside the coverage area. Please try again later.”

  6. Jeff Merryman

    I started crossfit almost three years ago but as I have aged (I am about to turn 55) the three on one off cycle has not allowed me to recover. I have rotated in and out of sereral workouts in the last year. I have not combined programs. Since January, I have been doing the 88 program from crossfit football (a program that John Wellborn “says” is designed for retired former players with dementia symptoms). I think it is a mixture of Wendler and Westside with a few mostly short met-com’s thrown in that are not terribly strenuous. The program allows for lots of recovery. I also do a lot of dog walking. I would like to do a hilly bike ride in Tennessee in early May. The distances are 100k to 100 miles. Since reading Mark Sisson and looking at Crossfitendurance I have markedly decreased my cycling mileage. My best ride was a year ago after a few months of the endurance programing. I was stronger on 30 miles per week training that I had been in the past on 200 or so miles per week. The weather here in Indiana will probably break in early March and I will regularily be able to get outside. I suspect, based on this post, you are not crazy about crossfit endurance. Do you have suggestions about how I can become fit enough to ride a hilly 100k or a century by May. Thanks

    1. thatpaleoguy Post author

      I’m not much a fan of any cookie cutter approach. I like aspects, but I’d pick out bits I think applicable to an individual and leave those that weren’t.

  7. Luke Terry

    I just found your blog on a recommendation from a friend of mine. I have to say, this is a fantastic article. I’m a health professional that has (in the past) recommended CF to a variety of clients.

    What I’ve seen recently is that the athletes just get toasted adrenals fairly quick, doing mostly metcon’s 4-5 days a week. They get sloppy, weak, and tired, especially coupled with the everyday stresses of life.

    I’m working at finding a balance on the bike as well as in the gym, and have come to find my body can tolerate sprinst 1-2x weekly, 1 heavy lifting session, a la Rippetoe, and several light days of mobility/balance work.

    Any suggestions for working in some longer rides in getting ready for mountain biking season?



    1. thatpaleoguy Post author

      Thanks Luke.

      Look at periodising everything – you might be able to peel back the MOB/BAL work. And don’t get trapped into a 7 day cycle. There is no reason why you can’t spread your load over 14 days and not really sacrifice a whole lot.


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