Why the start of your run feels so difficult

Many runners describe the first ten to 15 minutes of a run as the hardest of all. Breathing can feel laboured, the legs can feel heavy and a pace that would normally seem manageable may suddenly feel surprisingly difficult.

It might be easy to assume the reason your run feels so difficult at the start is because you’re not fit enough. But in reality, this sensation is actually caused by a normal feature of human physiology – and one that affects almost everyone, from someone attempting their first 5K to elite endurance athletes.

One way I often explain this phenomenon to athletes and students is that when you begin running, an argument starts between your muscles and your cardiorespiratory system. While there’s nothing we can do to stop this quarrel from happening, there are many things we can do to help the body find a resolution more quickly in the future.

The moment you start running, your working muscles demand a rapid supply of energy almost instantly. Yet the system responsible for producing that energy from oxygen cannot respond quite as quickly.

To meet this increased demand, your heart rate rises, breathing becomes deeper and faster and blood flow is redirected towards the working muscles. These adjustments allow more oxygen to be transported from the lungs to the muscles, where it’s used to support aerobic energy production. While these changes occur remarkably quickly in the body when we start exercising, they’re not instantaneous.

This transition is known as oxygen uptake (VO₂) kinetics. This refers to how rapidly the body can increase oxygen consumption when we begin exercising.

The transition unfolds in three phases. During the first phase, the body focuses on increasing blood flow and oxygen, while in the second phase the muscles begin activating aerobic energy production. By the third phase, oxygen delivery and demand become largely matched – and the body reaches what’s known as a physiological “steady state”.

In simple terms, this means your body has moved from being surprised by the demand of running to being fully organised around it.

But before we can reach this steady state, the muscles still need energy. To bridge the gap during this transition, the body temporarily relies on energy stored within the muscle.

This results in those sensations most runners immediately recognise. Breathing may feel disproportionately difficult, the legs can feel heavy or sluggish and perceived effort may seem unusually high compared with the pace you’re doing.

Even elite runners find the start of a run difficult. kovop/ Shutterstock

Around ten to 15 minutes into a run, many runners report that something suddenly changes. The pace feels smoother. Breathing becomes more controlled. The run begins to settle into a rhythm. This is the point at which your body has largely resolved its internal argument.

Your body has increased its oxygen delivery, aerobic energy production is contributing more fully and the cardiovascular system is better matched to the workload being performed. Muscle temperature has also risen, which improves muscle function and movement efficiency.

Reducing the effect

Although this response is common, the speed at which the body adjusts varies between people. Studies suggest that trained endurance athletes can increase oxygen uptake around 40-60% more rapidly than amateur athletes.

This is due to their years of endurance training, which has produced a variety of adaptations within their cells, muscles and cardiovascular systems that allow them to settle into exercise more quickly.

So while the transition period cannot be eliminated entirely, there are several ways to make it less noticeable.

A gradual warm-up is perhaps the most effective strategy. Easy jogging, brisk walking or progressive increases in pace can raise muscle temperature, increase blood flow and begin activating the aerobic system before the main run starts. Research shows that warm-up exercises can accelerate the VO₂ kinetic response during subsequent exercise – a phenomenon often referred to as “priming”.

If you don’t have time to warm-up, starting slightly slower than your intended pace can also help. Easing into a run allows the body to adjust, making it a much more comfortable experience.

Regular endurance training appears to help too. This sort of training might involve performing a mixture of easy runs, tempo runs and interval sessions of varying durations and intensities. Together, these different types of training help improve the body’s ability to deliver and use oxygen, allowing it to adapt more quickly when exercise begins.

There’s also a psychological component. Experienced runners understand that the starting minutes of a run may feel uncomfortable and don’t interpret this as a sign they’re having a bad run. Instead, they trust the process and allow their body time to settle into its rhythm.

As both an ex-international runner, exercise physiologist and coach, perhaps the most important message is that the sensations you feel at the start of a run are rarely a sign something is wrong. More often, it’s evidence that the body is doing exactly what it’s designed to do: adjusting to the demands of exercise before settling into a more sustainable rhythm.