Muscles 101

Key Takeaways:

• Muscles are made of muscle fibers containing myofibrils. Myofibrils contain actin and myosin which work to contract the muscles.

• ATP powers the muscles and is only stored in small amounts, which is why rest between sets, plays, or sprints is so important.

• Muscle strength and growth is seen after 4-6 weeks, which is when increasing intensity is indicated.

• A muscle strain happens when muscle fibers are stretched or torn too much.Tendinitis happens when a repetitive motion causes tiny tears to the tendon. Strains and tendinitis get better with rest, ice, compression, and elevation (R.I.C.E.).

---

Whether you play soccer, basketball, dance, run track, or lift weights, your muscles are working every time you move. Understanding how muscles work can help you train smarter, recover faster, and avoid injuries.

What Is a Muscle?

A muscle is a body part that helps you move. Muscles are made of long cells called muscle fibers. Inside these fibers are tiny parts called myofibrils, which contain proteins named actin and myosin. These proteins are the real workers that help muscles move.

Muscles are attached to bones by strong tissues called tendons. When a muscle moves, it pulls on a bone and makes a joint move.

How Do Muscles Contract?

A muscle contracts when it shortens to create movement. This happens when your brain sends a message through nerves to the muscle.

Here’s what happens:

• Actin and myosin slide past each other

• This sliding pulls the muscle shorter

• The muscle creates force

This is called the sliding filament theory. You use muscle contraction to jump, throw, sprint, and lift.

Where Does the Muscle Get Energy?

When you move, jump, sprint, or lift, your muscles use ATP (adenosine triphosphate) as their main energy source. ATP powers the myosin heads so they can grab onto actin and pull, which makes the muscle contract. Each time a muscle contracts, ATP is broken down into ADP and phosphate, releasing energy.

The problem is that muscles only store a small amount of ATP—enough for just a few seconds of hard effort. That’s why rest is so important between plays, sets, or sprints. During rest, your body uses oxygen and nutrients to rebuild ATP inside the muscle cells, mainly in the mitochondria. Without enough rest, ATP levels stay low, muscles feel weak or heavy, and performance drops. Rest helps reload your muscles so they’re ready to fire again at full power.

How Do Muscles Stretch?

Muscles stretch when they lengthen instead of shorten. This happens when:

• You move a joint in the opposite direction

• Another muscle pulls against it

• You hold a stretch on purpose

Stretching helps muscles move through a bigger range of motion, which is how far a joint can move safely. It can help maintain muscle balance and keep your joints moving in the correct alignment, reducing injury risk.

How Do Muscles Get Bigger?

Muscles grow bigger through a process called muscle hypertrophy.

When you:

• Lift weights

• Sprint

• Do resistance exercises

Tiny micro-tears form in the muscle fibers. Your body repairs them using protein, making the fibers thicker and stronger. This repair happens during rest and sleep, not during the workout.

That’s why recovery days are so important.

How Long Does it Take for Muscles to Grow?

Muscle growth doesn’t happen overnight—it takes consistent training, good nutrition, and enough rest for your body to rebuild stronger muscle fibers after workouts.

• Your body repairs the small muscle tears after workouts over the following 24–72 hours

• Most teen athletes start to notice small strength gains within 2–3 weeks

• Visible muscle growth usually takes about 6–8 weeks of regular training

• After 4-6 weeks, your muscle strength improves and you can slowly increase intensity

Getting enough protein, eating balanced meals, drinking plenty of water, and sleeping 8–10 hours a night are just as important as the workouts themselves. To keep making progress, train consistently, use proper form, increase weights or difficulty gradually, and avoid overtraining—rest days help your muscles grow.

How Do Muscles Become More Flexible?

Muscle flexibility improves when you stretch regularly. Stretching helps:

• Muscle fibers relax

• Fascia (the thin tissue around muscles) loosen

• Joints move more freely

Types of stretching include:

• Static stretching (holding a stretch- best done immediately after your workout)

• Dynamic stretching (moving while stretching- best done as part of a warm-up)

Flexible muscles are less likely to get injured.

What Is a Muscle Strain?

A muscle strain happens when muscle fibers are stretched or torn too much. This can occur from:

• Sprinting suddenly

• Lifting too heavy

• Sudden increase in workout intensity

• Not warming up

• Being very tired

Signs of a muscle strain:

• Pain

• Tightness

• Swelling

• Weakness

How Does a Muscle Strain Heal?

To help a strained muscle heal, athletes often use R.I.C.E.:

• Rest – stop the painful activity

• Ice – reduce swelling and pain

• Compression – gentle pressure with a wrap

• Elevation – raise the injured area

As healing continues, gentle stretching and strengthening help the muscle recover fully.

Mild strains (Grade I) take days to a few weeks, moderate ones (Grade II) take several weeks to a couple of months, while severe tears (Grade III) can need several months or longer.

Why Do Massage, Stretching, and Foam Rolling Help?

After exercise, muscles can feel sore because of:

• Small muscle damage

• Inflammation

• Buildup of waste products like lactic acid

Massage helps by:

• Increasing blood flow

• Bringing oxygen and nutrients to the muscle

• Helping muscles relax

Stretching helps reset muscle length and reduce tightness.

Foam rolling is a type of self-myofascial release. It helps:

• Loosen tight fascia

• Improve circulation

• Reduce muscle stiffness

All of these can make muscles feel looser and less sore.

Why Do Some Athletes Use Ice Baths?

Ice baths use cold therapy to help muscles recover. The cold:

• Decreases inflammation

• Slows nerve signals (less pain)

• Reduces swelling

After the body warms back up, blood flow increases again, which may help healing. Some athletes love ice baths, while others prefer gentler recovery methods.

What is Tendinitis?

Tendinitis is an injury that happens when a tendon (the tough tissue that connects muscle to bone) becomes irritated or inflamed, usually from:

• Overuse

• Repetitive motions

• Sudden increases in training

• Poor technique

Teen athletes often get it in places like the knee, shoulder, elbow, or ankle after doing the same movement again and again without enough rest.

How Does Tendinitis Heal?

Your body heals tendinitis by reducing inflammation and repairing tiny tears in the tendon, but this takes time and patience. To heal faster:

• Rest the injured area

• Ice it for 15–20 minutes a few times a day

• Gently stretch and strengthen as pain decreases

• Follow your coach’s or doctor’s advice—sometimes a brace or physical therapy helps.

  
  

Mild tendinitis may improve in 1–3 weeks, while more serious cases can take 6–8 weeks or longer if you keep stressing it. To prevent tendinitis:

• Warm up before practice

• Cool down and stretch afterward

• Build strength gradually

• Use proper form and equipment

• Cross-train to avoid overworking one muscle group

• Stay hydrated

• Listen to your body—pain is a warning sign, not something to push through

Take Care of Your Muscles

Strong, healthy muscles help you perform better and stay injury-free. Remember to:

• Warm up before activity

• Stretch after exercise

• Eat enough protein

• Sleep well

• Listen to your body

Your muscles work hard for you—take care of them, and they’ll take care of you 💪