Young endurance athlete recovering after several days of training, illustrating muscle fatigue, nervous system fatigue, delayed soreness, recovery, progressive overload, and overtraining.

Training Fatigue In Young Athletes Explained: Soreness, Recovery and Overtraining

One of the hardest things for athletes, and especially parents of young athletes, to understand is that how you feel today may be connected to workouts you did several days ago.

An athlete might have a hard workout Monday and feel fine Tuesday. They practice again Tuesday, lift weights Wednesday, and train Thursday. Then Friday arrives and suddenly their legs feel heavy, they're unusually sore, their normal pace feels harder, and they don't have much energy or motivation to train.

The obvious question is, "What happened?"

Sometimes nothing unusual happened yesterday. You're feeling the combined effects of several days of training.

Training puts stress on the body. Muscles use fuel. The nervous system works hard to control movement. Muscle fibers experience small amounts of damage. The body then has to repair those muscles, replace the fuel that was used, and prepare itself for the next workout.

Those things don't magically finish when you go to sleep.

Some recovery happens within minutes or hours. Other parts can take a day or two. Some changes take even longer. If another workout happens before recovery from the previous one is finished, the stress from those workouts can start to pile up.

That's why five workouts that seem fine by themselves can sometimes leave an athlete very tired by the end of the week.

Understanding the relationship between training, fatigue, recovery, and improvement is one of the most important things an athlete can learn.

TLDR: Today's Fatigue May Have Started Several Days Ago

Training fatigue isn't caused by one single thing, and it doesn't always happen right away. Sore muscles, low energy stores, the brain and nervous system, sleep, food, hydration, and the amount of training you've been doing can all affect how you feel.

Muscle soreness often feels worst about 24 to 48 hours after a hard or unfamiliar workout. That's why Monday's workout can still bother you Wednesday.
Muscles can become tired because they are using energy quickly and temporarily become less able to contract with the same amount of force.
Your brain and nervous system can become fatigued too. This can make the same workout feel harder than normal.
Soreness two days after exercise is not caused by lactic acid still sitting in your muscles.
You can start another workout before your body has completely recovered from the previous one. This allows fatigue to build over several days.
How long, how hard, and how often you train all matter.
Training needs to slowly become harder if you want to improve, but constantly adding more exercise without enough recovery eventually causes problems.
Sleep, enough food, carbohydrates, protein, fluids, easy training days, and planned easier weeks are some of the best recovery tools available.

The goal isn't to never feel tired.

The goal is to train hard enough to improve without doing so much that your body can't recover.

Why Does Soreness Sometimes Show Up Two Days Later?

Almost every athlete has experienced this. You do a hard workout Monday. Tuesday morning you're a little sore. Wednesday morning you can barely walk down the stairs.

That's called delayed onset muscle soreness, or DOMS.

Research shows that muscle soreness often becomes most noticeable about 24 to 48 hours after exercise, especially when you do a new exercise or something your muscles aren't used to doing. Research review on muscle soreness after exercise

This is important because not being sore the morning after a workout doesn't mean your body is completely recovered.

Your body may still be reacting to that workout.

No, It's Not Lactic Acid

This is a very common myth.

You may have heard that you're sore two days after exercise because lactic acid is still sitting in your muscles.

That's not what happens.

Lactate can increase during hard exercise, but your body clears it fairly quickly afterward. It's not still sitting in your legs two days later making it painful to walk down the stairs.

Instead, a hard or unfamiliar workout can create tiny amounts of damage and stress inside your muscles.

Your body responds by starting a repair process. This includes inflammation, movement of fluid into the area, and changes that make the nerves around the muscle more sensitive.

That process takes time. This helps explain why soreness can be delayed.

So the burning feeling you get during a hard workout and the soreness you feel two days later are two different things.

What Is Actually Happening When Your Muscles Become Tired?

When we say someone is fatigued, we're actually talking about several things happening at the same time.

Some of that fatigue happens inside your muscles. Some happens because of changes in your brain and nervous system. And the two systems are constantly communicating with each other.

Let's start with the muscles.

Every time a muscle contracts, it needs energy. The main form of energy your muscles actually use is called ATP.

Think of ATP as the tiny packets of energy your muscle cells spend every time they do work. You don't store enough ATP inside your muscles to exercise for very long. Instead, your body has several ways to keep making more of it.

One of those systems uses phosphocreatine, which is where creatine comes into the picture. Another breaks down carbohydrates. And another uses oxygen to produce energy from carbohydrates and fats.

All of these systems work together.

When you're exercising hard, however, your muscles can need energy extremely quickly.

Eventually, changes start happening inside the muscle that make it harder to keep producing the same amount of force.

That's muscular fatigue.

Phosphocreatine Falls and Other Substances Build Up

During hard exercise, your muscles use phosphocreatine to quickly make more ATP. As you keep exercising, phosphocreatine levels fall.

At the same time, other substances inside and around the muscle change. One important example is called inorganic phosphate.

These changes can make it harder for your muscle fibers to keep contracting with the same amount of force. Research review on how muscles become fatigued

Think about doing as many pushups as you can. The first five might feel easy. Then they get harder. Eventually, you're pushing as hard as you can, but your arms simply won't lift your body anymore.

Your brain is still saying "push" but your muscles aren't able to produce the same force they could at the beginning.

That's fatigue.

Calcium Helps Your Muscles Contract

There's another important part of muscle fatigue that most people never hear about.

Calcium helps your muscles contract.

When your brain tells a muscle to move, a signal travels through your nervous system and eventually reaches the muscle.

Inside the muscle fiber, calcium is released. That calcium helps the proteins inside the muscle grab onto each other and produce force.

During hard or long exercise, the muscle can temporarily become worse at releasing and using calcium. That means your brain may still be telling the muscle to contract, but the muscle can't respond quite as well.

This is one reason muscle fatigue isn't as simple as "running out of energy." The whole system responsible for making your muscles contract can temporarily become less effective.

Researchers have also found that the amount of carbohydrate stored inside your muscles may affect this calcium system. Research review on muscle glycogen and fatigue

And that brings us to food.

Your Muscles Store Carbohydrates for Exercise

When you eat carbohydrates, your body can store some of that carbohydrate in your muscles as glycogen.

Think of muscle glycogen as stored fuel.

When you run, bike, play soccer, lift weights, or perform other demanding exercise, your muscles can use that stored carbohydrate to help produce energy.

A hard workout can use a meaningful amount of it. Afterward, eating carbohydrates helps refill those stores.

Now imagine an athlete has a hard practice Monday but doesn't eat enough afterward. They practice again Tuesday. Then again Wednesday.

If they're repeatedly using stored carbohydrate without replacing enough of it, they may begin later workouts with less fuel available.

This can contribute to that familiar feeling athletes describe as:

"My legs feel dead."

Or:

"I just don't have anything today."

Low muscle glycogen may also make it harder for muscles to release calcium normally, which could further contribute to fatigue. Research on muscle glycogen and calcium

That doesn't mean every tired athlete needs more carbohydrates. But it does mean eating enough to support your training matters.

Your Nervous System Gets Tired Too

Your muscles don't decide to move on their own. Your brain has to tell them what to do.

The brain sends signals through the spinal cord and nerves until those signals eventually reach your muscles.

During very tiring exercise, your nervous system can become less able to fully activate your muscles. Scientists call this central fatigue.

This is sometimes called "CNS fatigue" online, but people tend to make it sound much simpler than it really is. Your nervous system doesn't have a battery that drops from 100% to 10%.

And a hard workout doesn't automatically mean your nervous system needs exactly three days to recover.

Your brain and nervous system are constantly gathering information about what's happening throughout your body and deciding how hard your muscles should continue working.

Your Muscles Are Talking to Your Brain

Here's where things get interesting. The communication between your brain and muscles goes both ways. Your brain sends messages telling your muscles what to do. But your muscles also send information back.

Special nerves can detect what's happening inside and around working muscles. As exercise becomes harder, those nerves send information back toward your brain.

In very simple terms, your muscles are telling your brain:

"We're working really hard down here."

Your brain uses that information, along with many other signals, to help decide how hard you can continue working. As fatigue increases, your brain may reduce how strongly it drives the muscles.

The exercise also begins to feel harder. This is one reason fatigue isn't just something happening inside the muscles. It's a conversation between your muscles, brain, and nervous system.

This Is Why Nervous System Fatigue Can Feel Different From Sore Muscles

Muscle soreness is easy to notice.

"My legs hurt."

Nervous system fatigue can feel different.

An athlete might say:

"I feel slow."
"My legs feel heavy."
"I can't get going today."
"I just feel tired."
"Everything feels harder than normal."

The pace may be exactly the same. The weight on the bar may be exactly the same. But suddenly it feels like much more work. That doesn't automatically mean the athlete is being lazy.

The same workout really can feel harder when someone hasn't fully recovered. That's an important thing for coaches and parents to understand.

Brain Chemicals May Play a Role Too

Your brain uses chemicals called neurotransmitters to help nerve cells communicate. You may have heard of some of them, including dopamine, serotonin, and norepinephrine.

Scientists have studied whether changes in these chemicals during long or difficult exercise may affect things like motivation, mood, alertness, effort, and fatigue.They probably play some role.

But it's much more complicated than saying:

"Your dopamine ran out."

Your brain is paying attention to lots of things at once.

How hot are you?
How much fuel do you have?
How tired are your muscles?
How hard is your heart working?
Did you sleep enough?
Are you stressed?
How hard does this exercise feel?
How motivated are you?

All of those things can help determine how tired you feel.

Fatigue isn't one thing caused by one system.

Why Several Days of Training Can Suddenly Catch Up With You

Now we can put everything together. Imagine a young runner has

A hard interval workout Monday.
Tuesday they complete an easy run.
Wednesday they lift weights.
Thursday they have another hard running workout.
By Friday they feel terrible.

It's easy to blame Thursday's workout. But Monday's workout may still matter. Their muscles may still have been repairing themselves Tuesday and Wednesday.

They had to replace the carbohydrate they used. Their muscles and nervous system needed time to recover.

Then Tuesday's workout added another small amount of stress. Wednesday added more. Thursday added more.

Now add normal life.

Maybe the athlete had a test at school.
Maybe they only slept six hours Wednesday night.
Maybe they didn't eat enough Thursday.
Maybe Thursday's practice was hot and they lost a lot of fluid through sweat.

By Friday, there may not be one single reason they're tired.

The stress has simply piled up faster than their body has been able to recover from it. That's accumulated fatigue.

Your Body Doesn't Reset at Midnight

This may be the easiest way to explain the entire idea to a young athlete. Your body doesn't reset every morning. Every workout creates some amount of fatigue. Then your body begins recovering.

If you're completely recovered before the next hard workout, great. But sometimes you're not.

Imagine Monday's workout creates $100 of "recovery debt."

By Tuesday, you've paid back $80.

You start Tuesday still owing $20.

Tuesday's workout adds another $60.

You recover $50 before Wednesday.

Now you owe $30.

Keep doing that for several days and you can end up very tired even though none of the individual workouts was too difficult by itself.

The body obviously doesn't really use dollars to measure recovery.

But the example makes the point. Workouts add up.

That's why coaches need to look at an entire week of training instead of judging each workout by itself.

Training Makes You Fitter and More Tired at the Same Time

This sounds strange, but it's extremely important. Training can make you fitter while also making you tired.

Suppose a runner completes several weeks of hard training. Their heart and muscles are adapting. They're becoming fitter. But they're also carrying a lot of fatigue.

Because of that fatigue, they may actually feel slower even though their fitness has improved.

Then their coach gives them an easier week. Their body catches up on recovery. Suddenly they feel fast again.

They didn't magically gain all that fitness during the easy week.

The hard training built the fitness. The easier week allowed the fatigue to disappear enough for that fitness to show.

This is one reason athletes use recovery weeks and tapers.

How Much Training Stress You Create Depends on More Than One Thing

Not all workouts are equal.

Several things determine how stressful a workout is:

Volume means how much work you do.
Intensity means how hard you work.
Duration means how long the workout lasts.
Frequency means how often you train.

All four matter.

A 30 minute easy run isn't the same as 30 minutes of hard intervals. Running five miles on flat pavement isn't necessarily the same as running five miles up and down a mountain.

Three easy strength training sets aren't the same as ten hard sets taken close to failure.

That's why saying someone "worked out five times this week" doesn't tell us very much. We need to know what those workouts actually looked like.

Progressive Overload Doesn't Mean Every Workout Has to Be Harder

To improve, your body eventually needs a bigger challenge. This is called progressive overload. But people often misunderstand what that means.

It doesn't mean every workout needs to be harder than the workout before it.

Progress can mean:

Slowly running farther.
Running a little faster.
Lifting slightly more weight.
Doing another set.
Practicing more often.
Improving your technique.

Or gradually doing harder workouts as your body becomes ready for them. The important word is gradually.

Research and sports medicine recommendations emphasize slowly increasing training stress instead of suddenly making huge jumps in workload. IOC research on training load and injury risk

Being able to finish a workout doesn't always mean you're ready to recover from doing that workout repeatedly.

That's an important difference.

Soreness Doesn't Mean You Had a Better Workout

A good workout doesn't have to make you sore. Soreness is often worse when you do something new.

Try an exercise you've never done before and you may be extremely sore afterward. Keep doing that same exercise for several weeks and you'll usually become less sore.

Your body has adapted. Scientists call this the repeated bout effect.

But becoming less sore doesn't mean the exercise stopped working. That's why athletes shouldn't judge a workout by how difficult it is to walk the next morning.

Soreness tells you something happened. It doesn't tell you how good the workout was.

Some Fatigue Is Actually a Good Thing

We also shouldn't make fatigue sound bad. Exercise is supposed to make you tired.

Sometimes coaches even plan periods of harder training that make an athlete temporarily perform worse. Then the athlete gets extra recovery. After recovering, they may perform better than before.

Scientists call this functional overreaching. Research consensus on overreaching and overtraining

Think about it like digging a small hole. A hard training block puts you in the hole. Recovery allows you to climb back out.

If the training was appropriate, you may climb out a little stronger than before. The problem starts when you keep digging and never give yourself enough time to climb out.

When Helpful Fatigue Becomes Too Much

Suppose the athlete continues training hard:

They're getting more tired.
Their performance keeps getting worse.
They aren't sleeping well.
Their legs constantly feel heavy.
They're losing motivation.
They keep getting small aches and pains.

At some point, the extra training may stop helping. This can become nonfunctional overreaching. Instead of needing a few days to recover, the athlete may need much longer.

That's why coaches and parents should look for patterns. One bad workout isn't a big deal. One tired day isn't a big deal. One sore morning isn't a big deal.

Several problems continuing together are much more important.

Overtraining Syndrome Is Much More Serious

People say they're "overtrained" far too easily. You had one terrible workout? Probably not overtraining syndrome. Your legs hurt after squats? Probably not overtraining syndrome.

True overtraining syndrome is a much more serious condition where performance stays worse for a long time and the athlete may also experience physical and mental changes.

Recovery can take a long time.

Researchers also don't have one simple blood test that can tell us whether someone has overtraining syndrome. Overtraining syndrome consensus statement

And sometimes something else is causing the problem.

A tired athlete could be sick.
They could have low iron.
They might not be eating enough.
They may not be eating enough carbohydrates.
They could be sleeping poorly.
They might be dealing with a lot of stress.

That's why long lasting or unusual fatigue shouldn't automatically be called overtraining.

Young Athletes Aren't Small Professional Athletes

This is especially important for parents. A teenager might have school practice Monday through Friday:

Then club practice.
Then strength training.
Then a private coach.
Then games all weekend.

Each coach may only see their own practice.

But the athlete's body experiences all of it.

The American Academy of Pediatrics has warned about excessive training, poor recovery, overuse injuries, overtraining, and burnout in young athletes. American Academy of Pediatrics report on young athletes

And sports aren't the only stress in a kid's life.

School matters.
Homework matters.
Tests matter.
Travel matters.
Growing matters.
Sleep matters.
Emotional stress matters.

Your body doesn't keep one bucket for school stress and another bucket for sports stress.

It all adds up.

Sleep May Be the Best Recovery Tool We Have

If I could choose one recovery tool for a young athlete, it wouldn't be a supplement. It would be sleep.

Sleep helps your body repair itself. It supports the brain, immune system, muscles, learning, memory, mood, reaction time, and athletic performance.

The problem is that athletes sometimes train more while sleeping less. Morning practice can mean waking up earlier. Evening practice can mean going to bed later.

Now the athlete has increased how much recovery they need while decreasing the amount of time available to recover. That's backwards.

Sports sleep experts emphasize the importance of protecting sleep as part of an athlete's overall training plan. Expert recommendations on sleep and athletes

If training increases, sleep becomes more important, not less.

Carbohydrates Help Replace the Fuel You Used

Remember the muscle glycogen we talked about earlier? Your body needs to replace it. That's one reason carbohydrates matter for athletes.

Foods such as rice, potatoes, pasta, oats, cereal, fruit, bread, and other carbohydrate sources can help replace the carbohydrate used during training. The harder and more often someone trains, the more important this becomes.

An athlete who trains once Saturday has a lot of time to recover. An athlete who finishes practice at 7 p.m. and has another hard session the next afternoon doesn't.

If you're asking the body to do more work, you also need to give it enough fuel to do that work and recover afterward.

Protein Helps Your Body Repair and Rebuild

Exercise creates a reason for muscles to adapt. Protein provides the amino acids the body uses during muscle repair and remodeling.

Eating some protein after training can be useful.

But there's no need to panic if you don't drink a protein shake within five minutes of finishing practice. What's more important is eating enough protein throughout the entire day.

And protein can't fix a diet that doesn't contain enough food overall.

Eating Enough Food Is Especially Important for Young Athletes

Kids and teenagers have another challenge. They're still growing. Their bodies need energy for normal growth and development.

Then we add school.
Daily activity.
Practice.
Strength training.
Games.
And recovery.

That's a lot.

If a young athlete keeps increasing training without increasing how much they're eating, they may simply not have enough energy available to support everything their body is being asked to do.

That can contribute to fatigue and poor recovery.

Over time, consistently eating too little can also affect health and performance. So before spending money on another recovery supplement, ask a very simple question:

Is this athlete eating enough food?

Hydration Matters Too

Athletes lose water when they sweat. How much they lose depends on the athlete and the workout.

A 45 minute indoor weight training session isn't the same as a two hour soccer practice outside in hot weather.

The longer and hotter the workout, the more fluid someone may lose. Athletes who sweat heavily can also lose meaningful amounts of sodium.

The goal isn't to drink as much water as possible. It's to replace enough of what you lose. That's why hydration needs can be different for every athlete.

What About Ice Baths, Massage Guns, Compression Boots, and Supplements?

Some of these things can help with soreness or make an athlete feel better. There's nothing wrong with that. But they're not where I would start.

Imagine an athlete sleeps six hours, barely eats breakfast, doesn't eat enough carbohydrates, is dehydrated, and trains hard every day. Buying a massage gun isn't going to fix the real problem.

Start with the basics:

Sleep.
Enough food.
Carbohydrates.
Protein.
Fluids.
Easy days.
Rest.
A well designed training program.

Then worry about the fancy stuff.

This Is Where Deload Weeks Can Help

A deload is simply a planned period of easier training.

You might train fewer miles.
Lift fewer sets.
Use lighter weights.
Remove a hard interval session.
Or simply have several easier days.

The purpose is to reduce training stress so your body has a chance to catch up on recovery. There isn't one rule saying everyone needs a deload every fourth week. Different athletes need different amounts of recovery.

But the basic idea is important:

Training can't keep getting harder forever.

Sometimes you need to reduce the workload so your body can actually benefit from the work you've already done.

Recovery Days Don't Mean You Have to Sit on the Couch

Sometimes complete rest is exactly what an athlete needs. Other times, very easy activity is fine.

Walking.
An easy bike ride.
Light swimming.
Mobility work.
An easy run.
Easy skill practice.

The important word is easy. If your recovery run turns into a race, it's not a recovery run anymore.

Easy days may feel like you're not accomplishing much. But easy days help make hard days possible.

They're part of training too.

How Do You Know When an Athlete May Need More Recovery?

Don't judge an athlete from one bad day.

Look for patterns.

A few warning signs become more important when several happen at the same time:

Normal workouts keep feeling much harder than usual.
Performance keeps getting worse.
Soreness lasts much longer than normal.
Easy workouts don't feel easy anymore.
Sleep gets worse.
Motivation drops.
The athlete becomes unusually moody or irritable.
They feel tired even when they're not exercising.
Small aches and pains keep appearing.
They seem to get sick more often.
Their appetite or body weight changes unexpectedly.

None of those things alone proves someone is overtraining.

But several happening together are worth paying attention to.

If a young athlete has long lasting pain, unusual fatigue, major changes in performance, repeated illness, unexpected weight loss, or other concerning symptoms, don't just tell them to push through it. That's when it's worth talking with an appropriate healthcare professional.

A Better Way to Think About Training

Training should challenge your body. Then you recover. Your body adapts. Then you challenge it again. Repeat that process and you gradually become fitter, faster, and stronger.

The mistake is thinking:

"If one hard workout is good, two must be twice as good." Eventually, more training creates more fatigue without creating more improvement.

The best training program isn't always the one with the most miles, the most practices, or the hardest workouts. It's the program with the most useful training the athlete can recover from consistently.

The Bottom Line

Feeling sore or tired one or two days after exercise can be completely normal. Monday's workout really can affect how you feel Wednesday. Then Tuesday's and Wednesday's workouts get added on top of it.

Inside your muscles, your body is dealing with changes in energy supply, phosphocreatine, stored carbohydrate, calcium, muscle damage, and the repair process.

At the same time, your brain and nervous system are paying attention to what's happening throughout the body and adjusting how difficult exercise feels and how strongly your muscles can continue working.

That's why fatigue can feel different from day to day.

Sometimes your muscles hurt.
Sometimes your legs feel heavy.
Sometimes you're mentally tired.
Sometimes your normal workout just feels much harder than it should.

None of this means athletes shouldn't train hard.

Hard training is part of getting better. So is progressive overload. But hard training only works when the body gets enough time and resources to recover from it.

That means:

Getting enough sleep.
Eating enough food.
Eating enough carbohydrates and protein.
Replacing lost fluids.
Having easy days.
Using recovery or deload weeks when needed.
And paying attention when fatigue keeps getting worse instead of better.

For athletes, coaches, and parents, the most important lesson may be this:

Don't judge a training program by how tired it makes an athlete. Judge it by whether the athlete is recovering, adapting, and getting better.

The goal isn't to find out how much training someone can survive.

The goal is to find the amount of training they can recover from, adapt to, and repeat consistently.

That's how athletes improve.

About the Author

Matt Mosman, MS, CISSN, CSCS, is the founder and Chief Science Officer of Simply Good Supplements. With a background in exercise science, sports nutrition, and strength and conditioning, Matt focuses on turning research into practical advice that athletes, parents, and everyday active people can actually use.

References:

Meeusen R, Duclos M, Foster C, et al. Prevention, Diagnosis, and Treatment of the Overtraining Syndrome: Joint Consensus Statement of the European College of Sport Science and the American College of Sports Medicine. Medicine & Science in Sports & Exercise. 2013;45(1):186–205. PubMed
Soligard T, Schwellnus M, Alonso JM, et al. How Much Is Too Much? (Part 1) International Olympic Committee Consensus Statement on Load in Sport and Risk of Injury. British Journal of Sports Medicine. 2016;50:1030–1041. British Journal of Sports Medicine
Schwellnus M, Soligard T, Alonso JM, et al. How Much Is Too Much? (Part 2) International Olympic Committee Consensus Statement on Load in Sport and Risk of Illness. British Journal of Sports Medicine. 2016;50:1043–1052. British Journal of Sports Medicine
Brenner JS, et al. Overuse Injuries, Overtraining, and Burnout in Young Athletes. Pediatrics. 2024;153(2):e2023065129. American Academy of Pediatrics
Walsh NP, Halson SL, Sargent C, et al. Sleep and the Athlete: Narrative Review and 2021 Expert Consensus Recommendations. British Journal of Sports Medicine. 2021;55:356–368. British Journal of Sports Medicine