Struggling to Maintain Speed? A Weak Core Might Be the Real Problem

​Most runners assume that when they slow down, their legs are to blame.

Quads burn. Hamstrings tighten. Calves feel unstable. It makes sense to think those muscles are the limiting factor.

But research suggests something less obvious.

When fatigue sets in, the breakdown often starts higher up in the body. Your core and upper body may be the reason your pace drops.

​What Happens to Your Form When You Get Tired

A study published in Royal Society Open Science tracked experienced runners during intense workouts designed to create fatigue.

As expected, their performance declined. They became less efficient and slowed down.

What was unexpected was where the changes happened.

Most of the breakdown occurred in the torso and pelvis, not the legs.

As fatigue increased, runners showed:

• More forward tilt of the pelvis, which arches the lower back
• Side to side hip drop
• Excessive rotation of the torso and hips
• More vertical movement, or “bounce”

These changes are small, but they reduce efficiency. Instead of moving forward smoothly, the body starts wasting energy in extra motion.

​Why Your Core Matters More Than You Think

​Your legs generate power, but your core controls how that power is used.

If your core is weak or fatigued, your body becomes unstable. Energy is lost instead of being transferred forward.

A weak core can also affect your breathing. If your posture collapses and your shoulders round forward, your ribcage cannot expand fully. That makes it harder to get enough oxygen.

In simple terms, your system becomes less efficient.

​How to Improve Stability and Maintain Speed

​To run well under fatigue, you need strength and control in your core, hips, and upper body.

1. Strengthen Your Core and Control Pelvic Position

When your pelvis tilts too far forward, your glutes cannot work effectively. That reduces your ability to generate power.

To fix this, focus on:

• Glute exercises: hip thrusts, glute bridges, Romanian deadlifts, step ups
• Core stability: planks and dead bugs

These help keep your pelvis in a more neutral position, allowing your muscles to work efficiently.

​2. Train Your Hip Stabilizers

If one hip drops while you run, it is often due to weak stabilizing muscles in your hips.

This creates side to side movement that wastes energy.

To improve stability, include:

• Side planks
• Banded side steps
• Clamshells
• Side lying leg raises

These exercises build endurance in the smaller muscles that keep your pelvis level.

​3. Strengthen Your Upper Body

Your upper body helps maintain posture and supports breathing.

When you get tired and start to slump forward, your breathing becomes less efficient.

To prevent this, add:

• Rows
• Pull ups or assisted pull ups
• Face pulls
• Resistance band pull aparts

You do not need heavy training. You just need enough strength to hold good posture during a run.

​4. Reduce Excessive Bounce

As you fatigue, your movement often shifts upward instead of forward.

This wastes energy.

A slight forward lean from your ankles helps direct your movement forward. Increasing your cadence slightly can also reduce bounce.

Simple drills like high knees can help reinforce this pattern.

​5. Practice Good Form When You Are Fresh

It is difficult to focus on form when you are tired.

Instead, build good habits during easy runs and training sessions.

Keep it simple:

• Run tall
• Lean slightly from your ankles
• Keep your arms controlled and close to your body

Over time, these patterns become automatic.

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​The Key Takeaway

Losing speed late in a run is rarely just tired legs. It’s usually a breakdown in overall efficiency.

When the core fatigues, pelvic stability drops, glute activation weakens, and stride power becomes less effective. At the same time, posture collapses and breathing becomes more restricted, reducing endurance.

This creates wasted movement and higher energy cost, so you slow down even if effort increases.

You can do these exercises to build a stronger core. A strong core helps maintain stability, support breathing, and keep your stride efficient, allowing you to hold your pace longer when fatigue sets in.