Why do your legs feel heavy when you start a workout, only for that feeling to disappear later? Science has an explanation

Sales to train and the first minutes seem to be the most difficult. The legs feel heavy, it's hard to find the rhythm, and the effort feels surprisingly high. However, after a while, everything changes and you start to feel much better. But it's not just a simple impression, and the physiology of exercise helps explain a good part of this phenomenon.

The science explains why it's so hard to get going when we ride a bike

It's a situation that many cyclists and runners know perfectly well. We start a training session relatively rested, but the first pedal strokes or strides feel uncomfortable. We struggle to move our legs and have the feeling that we won't perform well that day. Sometimes, we even think about turning back.

The curious thing is that, ten or fifteen minutes later, we can be riding at a higher speed or generating more power with a significantly lower perception of effort. It's as if the body needed some time to start functioning properly.

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Although we commonly attribute it to the muscles still being cold, the explanation is much more complex. Recent research on energy metabolism, oxygen consumption, and sports warming up allows us to understand what happens in our body during those first minutes.

The body cannot go from rest to maximum performance immediately

One of the main explanations lies in a process known as oxygen consumption kinetics, or VO₂ kinetics.

When we start pedaling or running, the muscles need to rapidly increase energy production to maintain contractions. However, although the energy demand increases practically from the first moment, aerobic metabolism needs time to reach the required level of activity.

During that transition, what physiologists call oxygen deficit appears. It doesn't necessarily mean that our muscles are receiving an insufficient amount of oxygen, but rather that the aerobic system has not yet reached the rate of energy production demanded by the exercise.

To compensate for this, the body relies more on other energy production pathways while the aerobic response adjusts.

This mechanism is well studied, and a scientific review published in Sports Medicine in 2023 precisely analyzes how oxygen consumption responds to exercise and why prior effort can accelerate that adaptation.

The practical consequence is that the body does not respond exactly the same during the first minute of effort as it does after several minutes of working at a stable intensity.

The muscles also need to increase their temperature

Muscle temperature is another important factor. After spending several hours sitting, resting, or engaging in low-intensity activities, the muscles are not in the same condition as when we've been exercising for a while.

With the first contractions, muscle temperature gradually increases, which modifies various properties related to force production and contraction speed.

A systematic review with meta-analysis published in 2025 in the Journal of Sport and Health Science studied the effects of muscle warming on force production.

The researchers found that increasing muscle temperature moderately improves force production in rapid movements and the speed at which that force can be developed, although they did not observe the same benefit on the maximum capacity to produce force.

This helps to understand why, when we've been pedaling for several minutes, the movement may feel more natural and the legs start to respond better.

Not everything depends on the heart: the muscle fibers themselves need to activate

For years, one of the common explanations was that the legs felt heavy because they were still not receiving enough blood.

However, current research points to a more complex mechanism. Blood circulation adapts quickly to the onset of exercise, but the utilization of oxygen within the muscle fibers themselves also needs to adjust.

The 2023 review published in Sports Medicine concludes that the acceleration of oxygen consumption after prior effort seems to depend especially on a greater capacity for intracellular oxygen utilization and changes in motor unit recruitment.

In other words, it's not just about getting more oxygen to the muscle, but also about preparing its internal energy production mechanisms to use it.

The researchers point to the role of mitochondrial enzyme activation and changes in muscle fiber activity.

This combination of metabolic and neuromuscular adjustments helps explain why the body can respond differently after a few minutes of activity.

Warming up matters, but science has yet to find a perfect formula

One of the most recent studies on this topic was published on October 11, 2026, in the journal Sports Medicine - Open.

It is a comprehensive review of sports warming up that examined 964 studies published between 1972 and 2026, aiming to determine which strategies have the most scientific backing.

Its conclusions support the role of warming up as physiological and neuromuscular preparation for effort, but also identify significant differences between sports, protocols, and individuals.

The authors note that many studies have focused on short-term performance tests and that there is still a lack of evidence to precisely determine the best combination of duration, intensity, and exercises in each discipline.

Therefore, while we know that warming up can improve preparation for exercise, there is no universal duration that guarantees all athletes will stop feeling heavy legs.

Moreover, preparing the body for effort is one thing, and achieving a demonstrable improvement in performance is another. A meta-analysis published in 2024 in Medicine & Science in Sports & Exercise, which gathered 35 studies, did not find sufficiently solid evidence to recommend post-activation potentiation strategies as a general method for improving endurance performance.

Why do legs feel much heavier some days than others?

So far, we have talked about the physiological changes that normally occur when starting activity, but many cyclists also experience days when that initial sensation is particularly intense.

In those cases, additional factors such as accumulated muscle fatigue, lack of recovery, insufficient sleep, or reduced glycogen availability may come into play.

For example, after a long ride or intense training, the muscles may retain residual fatigue that makes the effort feel more demanding in the next session.

The intensity with which we start also influences this. If we begin training too quickly, the energy demand increases sharply before the different physiological systems have completed their adaptation.

This can be particularly evident in cycling when we start directly with a climb, ride with overly long gears, or try to reach our usual watts without a prior warm-up phase.

However, it's important to differentiate the transient sensation that disappears once we get into rhythm from persistent fatigue. If the legs remain heavy throughout the training, the problem may not necessarily be related to the normal mechanisms of starting exercise.

How to prevent the first minutes of training from becoming the hardest

For a regular bike ride, starting with a few minutes of very gentle pedaling, using a comfortable gear and gradually increasing the intensity, allows the cardiovascular, metabolic, and muscular systems to adapt to the activity.

In higher intensity sessions, such as VO₂max training, demanding intervals, or XCO competitions, it may be advisable to perform a more structured warm-up.

Research on so-called priming exercise, which involves introducing prior effort to prepare the subsequent metabolic response, shows that certain activations can accelerate oxygen consumption kinetics. However, its effectiveness depends on the intensity, subsequent recovery, and the type of effort to be performed.

Therefore, it is also not about doing such an intense warm-up that we end up accumulating fatigue before we start.

For most amateur cyclists, spending between 10 and 15 minutes gradually increasing effort can be a practical starting strategy, although it should not be interpreted as a scientifically optimal duration for everyone.

The fundamental idea is to understand that feeling heavy legs during the first minutes does not necessarily mean that we are out of shape or that we need to rest.

Our body needs to adapt to the new demands of exercise, and that transition can make the initial effort feel more uncomfortable than expected. Although a single cause for this sensation has not yet been identified, research on oxygen consumption, muscle temperature, and neuromuscular activation explains why our body does not function exactly the same when we start pedaling as it does after we have been training for a while.