Woman in gym after workout

Why does exercise feel easy one day and hard the next?

Posted by for Workouts

Exercise makes us fitter, stronger and faster – but not in every single session. This is why progress isn’t always linear. 

I’ve been strength training for six years, and sometimes I walk in the gym and can only squat the weight I was lifting a year in to my programme. That isn’t a confession, it’s reality: sometimes we absolutely nail a session and can’t believe how much we just lifted. Other times, we struggle with something that should be easy by comparison. 

This isn’t just the case with resistance training either. “I’ve never been a fan of running, but during lockdown I took the plunge and tried to make it a part of my routine. But one of the most frustrating things about running has to be how unpredictable it is,” says digital writer Lauren Geall. “While on some days I’ll find it hard to complete the bare minimum, on other days, I feel like I could run forever. Last weekend, after two or three weeks of no running at all, I ran the furthest I’ve ever run. Why? It’s like my body is rewarding me for putting no work in at all.”

“It’s normal for some days to be harder than others,” says Faye Edwards, trainer and group exercise manager at Third Space. Yet unfortunately, we have been led to believe that progress is always linear: you sign up to the gym, you go three times a week and each week you get better than the last.

That is not the case. “Understanding that not every session will be us performing at our peak is really important. It’s even true for athletes, and is why competitions happen once a year or every six months, because it’s just not possible to continually improve. It’s really important to understand that and not allow those times when it’s harder to discourage us and question our training.”

Training in the gym
“Sometimes we absolutely nail a session.”

There are so many factors that can influence why we can barely run to the end of the road one day and yet fly through a 5k a few days later. Working out why you think you might be struggling in your session makes it so much easier to show yourself compassion. That’s because you’ll learn to understand that it’s not you being weak, slow or lazy, it’s one of these…


“People don’t realise how much our sleep impacts performance,” says Faye. Research shows that exercise performance is negatively affected during periods of sleep deprivation, but the reasons why are unclear. However, we do know that bad sleep effects cognitive function and mood stability, which is so important given that it is so often mind over matter when it comes to nailing our sessions. 

Given that sleep is the time our muscles rebuild and recover, a lack of sleep could be responsible for slowing muscle recovery.


Speaking of… if your body isn’t recovered from previous exercise, your next session is not going to be as good, right? Your body needs to work hard to rebuild muscles that you tore in your session, and if they don’t get a chance to do that, you’ll never get stronger.

The menstrual cycle

If you get periods, this one is huge. Our hormones hugely impact how much we can get out of a session, so it’s worth reading up on how our menstrual cycle can impact your training and adapt accordingly.


“Our performance can be down to what we have, or haven’t, eaten. We respond differently to different foods and they effect our energy levels differently,” says Faye. This is mainly trial and error, but perhaps play around with adding in a pre-workout snack and making sure you’re hitting your protein targets.

Time of day

Also related to this is the time of day you train. For some, training first thing in the morning with no food in them is when they feel the most spritely. Others need a full day of food and energy before they can even consider bouncing around the gym. There’s no right time to train, so just do what you can and pay attention to when you feel the best.


Ever felt lethargic when going through a tough time at work or in your personal relationships? “The stress of everyday life and things that are on our mind affects our energy levels,” reminds Faye, and try as we might, we can’t leave those worries on Slack while we hit the gym. 

How to measure progress in fitness

When we understand that not every week will see us adding weights onto the bar or shaving seconds off of our runs, how do we even know if we’re getting good? “Honestly, I think measuring progress every session is a bad idea,” says Faye. “Do it in long term blocks. For me, I started Olympic lifting a year ago. I’m just about to look back at my progress from year one to year two. Focusing on the journey as opposed to results is generally better, but it depends on your goal.”

This doesn’t mean go in blindly: you can still write down your session in a journal, track your heart rate or monitor your breathing. It just means stop comparing day by day or week by week. 

How to get through tough sessions

None of this means you are subject to doing nothing when you forgot to eat or you have to change the time you train or you are going through some pretty crappy personal stuff. It simply means adapting and preparing.

First of all, planning is your friend. If you know that you want to nail a long run or a heavy leg session one day, schedule it in for the day after a rest day so that your body is well rested, suggests Faye.

But we all know that sometimes a plan doesn’t come together. So it’s really important to “be in your feelings and understand how you feel on that day,” says Faye. She suggests surveying yourself once you’re out of bed and a bit more awake. Do you feel fatigued? Do your muscles feel sore? Are you ready to jump around? 

Once you have taken those things into account, change up your session. For example, don’t attempt a speedy 5K before breakfast when all you had for dinner the night before was a piece of toast and you are about to come on your period. You’ll probably just get frustrated that you can’t do it. Instead, swap it out for some gentle yoga, which you’ll no doubt nail. “Having a session, even if it’s not your strongest or what you had planed, is better than doing nothing at all. That’s still progress, you’re still digging deep and working through something,” Faye says. 

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Images: Getty

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Chloe Gray

Chloe Gray is the senior writer for stylist.co.uk's fitness brand Strong Women. When she's not writing or lifting weights, she's most likely found practicing handstands, sipping a gin and tonic or eating peanut butter straight out of the jar (not all at the same time).