“I’m just tired.”

Three words, and we say them all the time. After a heavy squat session or a long run. After a week of broken sleep. After months of looking after everyone else before yourself. They feel like the same thing, but they aren’t.

Online, it gets even more confusing. Scroll for five minutes, and you’ll find someone claiming one training method creates “more fatigue” than another, as though fatigue were a single number on a dial. It isn’t. It’s a catch-all word covering very different things, with very different causes and very different fixes.

If you’re training in your 40s or 50s, especially if you combine strength and endurance work, getting this right matters more. By the end of this article, you’ll be able to tell which kind of fatigue you’re dealing with and what to do about it.

Why the word “fatigue” tells you so little

Fatigue is one of the most misunderstood ideas in training, and combining strength and endurance work makes it even harder to read. It covers multiple underlying mechanisms, each with different implications for performance and recovery. So when someone says an approach produces “more fatigue”, the useful follow-ups are:

1. What type? Is it in the muscle, the nervous system, or the whole body?
2. How big? A small dip in performance, or a serious one?
3. How long does it last? Hours, days, or weeks?

A study might find that one method causes more fatigue, but that could mean a short-lived drop in force in the trained muscle, gone by the next morning. That’s a very different thing from a drag on your energy that lasts all week. The same word describes both, but the consequences couldn’t be more different.

Your training doesn’t happen in isolation, either. Work, family, caring responsibilities and sleep all draw on the same recovery budget, and a method that looks great on paper can behave very differently once real life is added.

For hybrid athletes, there’s another layer. Lifting and endurance work stress the body in different ways: heavy lifting leaves local muscle and joint fatigue, while long aerobic sessions draw heavily on fuel and the nervous system. A hard run and a hard leg session on back-to-back days can stack up. Counting sessions isn’t enough. What matters is what each one costs and when it lands.

Acute vs chronic fatigue

Acute fatigue is what you feel during or straight after a session. It usually clears within hours to a couple of days, and it’s a normal part of the training stimulus. You don’t need to avoid it.

Chronic fatigue builds over weeks. It shows up as stalling or declining performance, poor mood, disrupted sleep and a loss of drive. This is the one that tells you the total load, training and life combined, has outrun your recovery.

In your 40s and 50s, the same session can take a little longer to bounce back from, and life stress counts towards the same total as training stress. That’s why chronic fatigue can creep up on you.

What to do:

• Plan a deload every 4 to 8 weeks, or sooner if the warning signs appear. Reduce volume rather than stopping altogether.
• Track trends, not single days. Look at performance on your key lifts and key sessions (pace or heart rate at a set effort), sleep, resting heart rate, mood and motivation over time. One bad day means very little.
• Treat two or three sessions of underperformance in a row as a signal to act, not a reason to push harder.

Peripheral fatigue: when it’s in the muscle

This is the one most of us picture when we say “fatigue”. It’s a drop in the muscle’s ability to produce force, caused by changes in the muscle itself: metabolite build-up, disrupted calcium handling and local glycogen running low.

What it feels like: burning, heaviness, local weakness, soreness the next day. Legs that feel like lead after a long run, or after a set of leg presses that leaves you unable to climb the stairs without thinking about it.

The key point is that this type is closely tied to the training stimulus. It’s part of how you get stronger, and it usually clears relatively quickly. It isn’t automatically something to avoid.

What to do:

• Match rest periods to the goal. Longer for heavy compound lifts (roughly two to three minutes or more), shorter for isolation work.
• Eat carbohydrates around higher-volume endurance sessions to support local glycogen.
• Spread volume across the week rather than cramming it into one or two sessions. It’s easier to recover from, and kinder on your joints. Try not to stack a heavy lower-body session and a hard run or ride on back-to-back days.
• Warm up properly and choose joint-friendly exercises, so local fatigue doesn’t tip into tendon or joint irritation. This matters more as we get older.

Central fatigue: when it’s your nervous system

Central fatigue is a reduction in the nervous system’s ability to drive your muscles maximally. The muscle is capable, but the signal isn’t getting through as well. Poor sleep, psychological stress, very long or demanding sessions (a long run as much as a heavy lifting day) and low energy intake all feed into it.

What it feels like: the bar feels heavy even though nothing is sore. You’re flat, unmotivated, a bit foggy and less coordinated. You can’t put your finger on what’s wrong, but the session just isn’t there.

This is where life outside the gym really shows up. For me, the last few months have been a good example. Broken sleep, a head full of decisions and some days of barely being able to stand up all feed this type directly. And no amount of “pushing through” fixes it, because the problem isn’t in the muscle.

What to do:

• Autoregulate. Use RPE or reps in reserve so the load adjusts to how you feel that day rather than what the plan says. A bad day becomes a lighter session, not a failed one.
• Save your heaviest lifts and hardest endurance efforts for days when you’re fresh, and keep sessions shorter when you’re not.
• Protect a consistent sleep and wake time. Regularity matters, not just the total hours.
• Use caffeine deliberately. A dose of 3 to 6 mg/kg of bodyweight about an hour before training can help, but start at the low end. Caffeine has a half-life of roughly 5 to 6 hours, so a late dose can still be active at bedtime. A cut-off of 8 to 10 hours before sleep is a sensible rule of thumb.
• Eat enough, especially carbohydrates. Under-fuelling shows up here quickly.

Systemic fatigue: when it’s the whole body

Systemic fatigue involves more than muscle and nerve. It can include the endocrine, immune, cardiovascular and digestive systems, all of which interact with how you recover.

This matters particularly in our 40s and 50s. Hormonal transitions such as perimenopause and menopause, age-related changes in testosterone, thyroid problems, low iron, medication side effects and ongoing illness or inflammation can all contribute. Prolonged stress and grief can also disrupt sleep, appetite and immune function, which means they can feed into this type too.

What it feels like: a deep, persistent tiredness that doesn’t lift with a rest day, often alongside other changes such as mood, sleep, appetite, getting ill more often or a feeling of running on empty.

What to do:

• Don’t assume it’s all down to training. If fatigue is persistent, unexplained or out of proportion to what you’re doing, see your GP. Blood work might look at iron and ferritin, thyroid function, vitamin D, B12 and glucose.
• Watch your energy availability. Chronic under-eating relative to your training load has effects across the whole body, not just the muscle.
• Reduce total training stress during illness, high-stress periods or poor sleep blocks rather than training through them.
• Factor in your hormonal context. Tracking symptoms or cycles, and adjusting volume during rougher phases, is good programming, not weakness.

Using one fatigue to fix another

I’ll be honest: I’ve been getting this wrong myself. Since losing my Mum, training has been one of the few things that makes sense. It gets me out of the house and out of my head. But a few weeks in, I noticed what I was doing. I was using one source of fatigue to try to fix another.

The gym felt like the answer to the stress, but my body doesn’t separate the two. Grief, broken sleep, a house to sort out, a business to launch and a hard training session all draw from the same pot. Some days the session gave me energy. Other days it just added to the pile.

What helped was asking a simple question before each session: what is this for? If the answer is “to hit a number”, I need to be fresh. If the answer is “to feel human”, it can be a walk, an easy upper-body session, or half the usual volume. Both are valid. They just aren’t the same, and they don’t carry the same cost.

A simple rule of thumb: performance sessions when you’re fresh, mood sessions when you’re stressed, and rest when both are low.

How life crowds out the basics

Here’s the pattern I see, and have lived. A demanding job, a family, caring responsibilities, early alarms and late evenings. Something has to give, and it’s almost always nutrition and recovery. Meals get skipped or thrown together. Sleep gets squeezed. Then, to make up for it, we lean on the things that feel more controllable: training harder, adding another supplement, “pushing through”.

It’s an understandable instinct, but it’s backwards. Grinding harder on top of a poorly fuelled, poorly recovered base adds to central and systemic fatigue, and it rarely produces better results. Supplements can’t replace the foundations either. They sit on top of them.

So instead of “get more sleep” and “eat better”, here’s what I’d actually do.

Protein

• Aim for roughly 1.6 to 2.2 g per kg of bodyweight per day.
• Spread it across three to four meals, at around 0.3 to 0.4 g/kg per meal.
• Make it easy: a default breakfast, a go-to lunch and a couple of back-up options for chaotic days.

Carbohydrate

• Match carbohydrate to the intensity and duration of the planned session, not just to whether you’re training.
• A resistance session on its own doesn’t need much more than a basic intake. A moderately intensive aerobic session lasting over 60 minutes does.

Day type	Typical training	Carbohydrate (g/kg bodyweight/day)	Example: 80 kg athlete
Light or rest day	Rest, easy movement or a resistance session	1 to 4	80 to 320 g
Moderate intensity day	Moderately intensive aerobic session, 60+ minutes	5 to 7	400 to 560 g
High intensity or double session day	Hard intervals or long hard efforts, or two sessions in a day	8 to 10	640 to 800 g

These are daily totals. On a double-session day, refuel between sessions, not just after the second. And if your sessions feel flat and your sleep and stress are poor, check carbohydrate and total intake before changing the programme.

Total energy

• Avoid a chronic deficit unless it’s a deliberate, time-limited phase. Chronic under-eating shows up as central and systemic fatigue.

Hydration and micronutrients

• Food first. If you suspect a deficiency, get bloods done rather than guessing with a cupboard full of tablets.

Supplements, in order of evidence

1. Creatine monohydrate. After loading, a maintenance dose is 0.03 to 0.05 g/kg per day, which is roughly 2.4 to 4 g for an 80 kg person. Loading is optional, as the same saturation is reached at maintenance doses in around three to four weeks.
2. Caffeine, used strategically. 3 to 6 mg/kg about an hour before training, starting at the low end and respecting the cut-off for sleep.
3. Everything else is a distant third.

Build systems, not willpower

• Batch-prep protein once or twice a week.
• Decide your default meals in advance, so you’re not making food decisions when you’re depleted.
• Have a minimum viable session for chaotic weeks: 30 minutes, a few key lifts or an easy aerobic session, done. It keeps the habit alive without adding to the load.

Which fatigue is this?

Here’s a simple checklist you can screenshot and use.

What you notice	Likely type	First response
Local soreness, burn or heavy legs; clears in a day or two	Peripheral	Usually fine. Adjust volume or rest periods if needed.
Flat, heavy, unmotivated, no real soreness	Central	Check sleep, stress and fuelling. Then autoregulate the session.
Weeks of declining performance, poor mood and sleep	Chronic	Deload and review your total load, including life stress.
Persistent, unexplained and affecting daily life	Systemic	Get it checked by your GP. Don’t just train around it.

Real life isn’t this tidy, and you can have more than one type at once. But asking “which one is this?” is a far better starting point than “how do I push through?”

This article is general information, not medical advice. If your fatigue is persistent or worrying, speak to your doctor.

Closing thought

Go back to those three words: “I’m just tired.”

It can mean sore legs after squats or a long run. It can mean a nervous system running on poor sleep and too much stress. It can mean weeks of accumulated load, or something in the body that needs medical attention. Each one needs a different response, and none of them is fixed by simply pushing harder.

For me, the hardest lesson has been that training can be both the thing that helps and one of the things adding to the load. I’m still working that out. But asking the right question has made a difference: not “how tired am I?” but ”what kind of tired is this?”

Context matters. Mechanisms matter. And when it comes to training in our 40s and 50s, nuance matters most of all.

What kind of fatigue are you dealing with right now? Let me know in the comments.