I have become an avid triathlete the past two years, and i have started my base to prepare myself for my 70.3 training I have in May. I am a fitness instructor (spin and core stuff), and then I train 6 days a week on top of that. My typical schedule on a Wed and Friday is teach a 45 minute spin class and then I lift weights right after for 30-45 minutes. When you say "avoid strength training right after heavy endurance exercise", would a pretty laid back 45 min spin class (i do push myself and sweat, but it is nothing compared to my typical bike training) be considered "heavy endurance"? I understand that I shouldnt go on a 10 mile run and then lift weights. But what are your thoughts on lifting after a 45 min spin class?
Thank you for your time.
My response (which I sort of edited since I sent the email haha)
Thanks for the email. There are several issues with concurrent endurance and strength training, so I'll try to tell the whole story of why this even matters and give you some recommendations on how you should structure your training...
Why it's NOT good to strength train after endurance exercise:
First off, there are issues of fatigue with concurrent training. You're likely to be more fatigued if you strength train after endurance and therefore your technique might suffer. If you're lifting relatively heavy weights, that could be a big problem. The fatigue may also prevent you from lifting to your potential so your strength workout won't be as quality as it would be were you "fresh." Some reasons for that may be: 1) lower glycogen levels post-endurance (even just 45 min of exercise), which impairs the anabolic (muscle growth) response of strength training; 2) endurance --> neuromuscular fatigue --> slowed muscle recruitment --> impaired rate of force development --> impaired power (no bueno). Now, all that said, if a 45-min bike isn't that big a deal for you, I'm sure it's not terribly detrimental to ride then lift (more on this below).
Secondly, with concurrent exercise there's hormonal/chemical issues. Endurance exercise prior to strength will suppress protein synthesis, which negatively interferes with gains in strength, power and hypertrophy, meaning you're not getting the max potential benefits from your strength workout. An enzyme called AMPK is released with endurance exercise, and AMPK inhibits markers of muscle growth, specifically a protein called mTOR (result: less protein synthesis is going on = your strength workout isn't reaching its potential). This effect can last up to 2-3 hours; the longer/harder the endurance exercise, the longer the effect.
However, strength training will not negatively affect your endurance capabilities; i.e. aerobic capacity and muscular endurance are not negatively affected with concurrent training. So lift away and you'll still be an aerobic machine. Plus, strength training is beneficial for triathletes/endurance athletes for a million reasons, and don't worry you won't bulk up as long as your endurance workouts are legit. But that's a whole other topic...
The bottom line:
It's best to separate strength and endurance training into two separate sessions hours apart when possible. But if you can't, there are two ways to go1) do the key workout of the day first so your most fresh for that, i.e. you get the most out of it.
2) do strength then endurance (for the reasons above).
Now, practically speaking:
Personally, I am forced to combine the two all the time due to time constraints. Most times, I like to do a short warmup (i.e. 10 min functional movement, 5-10 min treadmill), then a strength circuit, short rest, then endurance -- with quality nutrition throughout. But I switch it up if I need to be fresh/focused for the endurance workout. For example, I'm working hard on running technique these days so I'll sometimes do a 30-50 min easy/moderate run then strength train. (Described that workout in a recent post.) That said, I never do strength training after a long and/or hard run (hour+) or long bike (1.5+ hours).
I say, if the endurance workout is less that 60 minutes and not a total ball-buster, I think it's OK to do strength afterward without too many detrimental effects, though it's not ideal. Just rest about 10-20 minutes between and get in good nutrition during everything & after -- primarily glycogen, but protein too.
However, you might want to consider doing your strength first because it sounds like the spin class isn't that big of deal for you, and the strength is the focus. Thus you'll want the most gains from the strength and even 45 minutes spinning might elicit some fatigue, lowered glycogen, inhibition of protein synthesis, etc. So maybe try switching it up and see if you notice a difference!
Another option is combining lower-body endurance exercise with upper body strength, and vice versa. The inhibition of protein synthesis is localized to the muscles you use in the endurance workout, so if it's a bike ride, your legs release the AMPK, but your upper body will be relatively unaffected.
Hope that helps!