Why do some workouts feel harder than others?

You begin the warm-up with the same load as usual, but your body feels unusually heavy. Your heart rate rises faster, your legs lack spring, and weights that normally move smoothly suddenly require much more effort. On other days, the opposite happens: You feel strong, coordinated, and energetic from the first working set.

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These variations do not necessarily mean that your fitness has improved or declined from one day to the next. Performance in a single session is affected by sleep, recovery, energy availability, hydration, temperature, psychological stress, and the training completed earlier in the week. Perceived effort can also change even when the external workload is identical.

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The same workload can require different levels of effort

External load describes the work you actually perform. This may be the weight on the bar, running speed, cycling resistance, number of repetitions, or duration of the session. Internal load describes how the body responds to that work.

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A five-kilometre run at the same pace represents roughly the same external load from one session to another. The internal load may still differ. Heart rate can be higher, breathing heavier, and perceived exertion greater if you have slept poorly, are dehydrated, or are still fatigued from previous training.

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The same principle applies to strength training. A 100-kilogram squat is still 100 kilograms, but the body meets the load under different conditions. How heavy the lift feels is influenced by how effectively the nervous system recruits the muscles, how much local fatigue remains, and how motivated and focused you are.

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The difference between external and internal load explains why a training programme can look identical on paper while the experience of the session is completely different.

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Perceived exertion is a combined signal

Perceived exertion describes how demanding an activity feels. It is often rated on a scale from 1 to 10, where lower numbers represent very easy work and higher numbers represent near-maximal effort.

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This experience does not arise from one signal from the muscles. The brain integrates information from several systems, including:

• breathing and ventilation

• heart rate

• muscular fatigue

• body temperature

• energy availability

• pain and discomfort

• motivation and attention

• expectations about the session

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Perceived exertion is therefore not merely a subjective feeling that should be ignored. It provides useful information about how demanding the work is for the body that day.

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At the same time, it is not a perfect measure of physical capacity. A session can feel difficult without performance being substantially worse, while motivation or competition can make hard work feel more manageable.

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Sleep affects more than how tired you feel

One poor night does not always produce a clear reduction in maximal strength, but it can make training more difficult to complete. Sleep loss can affect reaction time, concentration, decision-making, coordination, and tolerance of effort.

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When sleep is insufficient, a given workload may feel harder. This is particularly relevant during endurance exercise, long sessions, and activities requiring sustained attention or technical precision.

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A single poor night does not automatically mean the session should be cancelled. The warm-up can be used to assess how the body actually responds. If movement feels stable and the load progresses normally, the workout can often be completed with minor adjustments.

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Several nights of insufficient sleep are more important than one isolated night. As sleep debt accumulates, the body has less opportunity to recover between sessions. Motivation, tolerance of high intensity, and training quality may all be affected.

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Previous training is still present in the body

Training temporarily reduces performance before the body has had time to recover and adapt. This is a normal part of the training process.

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After a demanding session, there may be local muscular fatigue, reduced energy stores, and temporary changes in force production.

How long these effects last depends on:

• training volume and intensity

• how unfamiliar the load was

• which muscle groups were used

• the degree of muscle damage and soreness

• food intake and sleep

• training background

• the time between sessions

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A demanding leg session on Monday can still affect running or squatting on Wednesday. Even when soreness is moderate, the muscles may produce less force or fatigue more quickly.

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A large training volume can also create a more general feeling of fatigue that is not limited to one muscle group.

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This does not automatically mean the programme is poorly designed. Periods of accumulated fatigue are used deliberately in many training plans. The problem arises when load continues to increase without enough lighter days or recovery to restore performance.

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Energy availability affects work capacity

Carbohydrate is stored mainly as glycogen in the liver and muscles. During moderate- to high-intensity activity, these stores are an important source of energy.

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If glycogen stores are low after prolonged exercise, insufficient food intake, or a diet low in carbohydrate, high-intensity work may feel harder.

This is often most noticeable during:

• interval training

• prolonged endurance exercise

• circuit training with short rest periods

• high-volume strength training

• multiple sessions on the same day

• training after a long period without food

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Low energy availability does not mean that the body has no energy left. Fat stores and other energy sources remain available, but it may become more difficult to maintain a high work rate.

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The experience may be that the legs lack power, rest periods no longer feel sufficient, or the motivation is present while the body does not respond.

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A small or missing pre-workout meal does not affect every session equally. A short and easy session may be completed without difficulty, while a long or intense workout can become substantially harder.

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The response depends partly on what you ate earlier, how demanding the session is, and how accustomed the body is to training with limited available energy.

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Hydration and temperature change the physiological load

When you sweat, the body loses both fluid and electrolytes. If the loss becomes substantial without adequate fluid intake, blood volume decreases.

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The heart must then often beat faster to maintain circulation, while temperature regulation becomes more demanding.

Dehydration may particularly increase perceived exertion during endurance exercise, especially when training takes place in warm conditions or lasts for a long time.

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There is considerable individual variation, and smaller fluid losses do not necessarily affect all forms of performance in the same way.

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Heat also makes exercise harder even when you begin well hydrated. More blood must be directed to the skin to release heat, and sweat production increases. This can produce a higher heart rate at the same running speed or cycling resistance.

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Cold conditions may make the muscles feel stiffer and reduce fine motor control if the warm-up is insufficient. The workload may therefore feel heavy at the beginning but become easier as body temperature rises.

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Practical factors such as room temperature, clothing, humidity, and time of day can therefore explain some of the variation between sessions.

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Mental fatigue can make physical work more demanding

A long working day, demanding studies, repeated decision-making, or sustained concentration can create mental fatigue.

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The body is not necessarily physically exhausted, but the ability and willingness to maintain effort may be reduced.

Mental fatigue appears to affect activities that last over time and require continuous regulation of pace and effort.

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Physiological measurements such as heart rate and oxygen consumption may not change substantially, even though the work feels harder. One central explanation is that perceived exertion increases.

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This can create a characteristic experience: The body functions during the warm-up, but it feels unusually difficult to push further. Rest periods seem short, motivation falls quickly, and maintaining technical focus becomes harder.

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Music, a training partner, or a clear plan may sometimes reduce the experience of mental load. On other days, it may be more appropriate to choose a simpler session with lower demands on concentration and self-regulation.

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Psychological stress affects training through several pathways

Psychological stress activates systems that are also involved in physical strain. Heart rate, muscle tension, breathing, and attention may all change.

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Acute activation can improve performance in some situations, while prolonged stress often reduces available energy and recovery.

Stress does not only affect the body during the workout. It can also influence:

• sleep quality

• appetite and meal timing

• muscle tension

• motivation

• concentration

• tolerance of discomfort

• choice of training intensity

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A busy period can therefore make sessions feel harder through several indirect mechanisms at the same time.

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It is difficult to separate stress from sleep, nutrition, and mental fatigue because these factors often occur together.

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Exercise can still be useful during stressful periods, but it is not always appropriate to complete every planned session at full intensity. A lighter workout may provide movement and mental relief without adding unnecessary strain.

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Motivation and expectations influence the experience

The brain does not only assess what the body is doing. It also estimates how long the activity is expected to last and how important the goal is.

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A load may feel easier when you know only one set remains than when you believe five sets are left.

Motivation can increase tolerance of discomfort and make it easier to maintain intensity. Competition, music, a training partner, or a clear goal can therefore improve performance without changing the underlying physiological capacity.

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Expectations can also work in the opposite direction. If you have already decided that your body is poorly recovered, you may become more attentive to heavy breathing, stiffness, and small deviations.

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This does not mean the sensation is imaginary. Attention influences how bodily signals are interpreted.

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A low recovery score on a wearable device can, for example, change your expectations before the session. Such measurements may be useful, but individual values should not automatically overrule how the body actually responds during the warm-up.

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Time of day can matter

Body temperature, hormonal activity, alertness, and coordination vary throughout the day. Many people perform best physically in the afternoon or early evening, when body temperature is generally higher.

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Others function best in the morning because it suits their sleep rhythm, habits, and daily routine.

There is also a clear adaptation effect. A person who always trains early may perform well in the morning even if average circadian patterns would suggest otherwise.

Time of day also influences the conditions surrounding the session:

• how long you have been awake

• how many meals you have eaten

• how much fluid you have consumed

• how mentally fatigued you are

• how much time is available

• whether the body has had time to become warm and mobile

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A morning session and an evening session are therefore not always directly comparable, even when the programme is identical.

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The menstrual cycle may influence some sessions

Hormonal variation across the menstrual cycle can affect temperature regulation, fluid balance, symptoms, and perceived well-being.

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Some people notice clear changes in energy, pain, or performance, while others experience little or no difference.

Average research findings often show small and inconsistent effects on performance. It is therefore difficult to provide general rules stating that certain phases are always better or worse for training.

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The practical approach should be individual. If the same pattern appears repeatedly, it may be useful to record:

• energy level

• menstrual pain

• sleep

• perceived exertion

• performance in selected exercises

• bleeding volume

• need for load adjustment

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Heavy menstrual bleeding can contribute to iron deficiency in some people, which may cause unusual fatigue and reduced endurance.

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Persistent low energy, shortness of breath, or a clear drop in performance should be assessed by a healthcare professional.

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A difficult warm-up does not always mean a poor session

The first few minutes can be misleading. After prolonged sitting, a demanding day, or training in the cold, the body may need more time to increase temperature and activate movement patterns.

A gradual warm-up can:

• increase muscle temperature

• improve nerve conduction

• increase blood flow

• make joint movement feel easier

• prepare technique

• provide information about the day’s capacity

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During strength training, several gradual warm-up sets may cause a load that initially felt heavy to move normally later.

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During running, the first ten minutes may feel stiff before breathing and rhythm stabilise.

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If the heavy feeling decreases during the warm-up, it is often possible to complete the session as planned. If it increases, or technique and coordination are clearly impaired, adjustment may be sensible.

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How to adjust the session without cancelling it

A difficult day does not necessarily require complete rest. The workload can be modified so the session still provides value without creating unnecessary fatigue.

Possible adjustments include:

• reduce the weight slightly

• remove one or more working sets

• leave more repetitions in reserve

• reduce interval speed

• extend rest periods

• choose simpler exercise variations

• perform technique work

• replace a hard session with easy activity

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Autoregulation can be used in strength training. Instead of completing a fixed load regardless of readiness, the weight is adjusted according to how the warm-up and first working sets feel.

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A planned set of eight repetitions can, for example, be performed with a load that leaves approximately two repetitions in reserve. On a good day, the weight may be slightly higher. On a difficult day, it is reduced.

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The training goal remains the same, but the load is matched to the day’s capacity.

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During endurance training, intensity can be guided by heart rate, breathing, or perceived exertion. If the goal is an easy session, reducing speed is often more appropriate than forcing the usual pace at a much higher internal intensity.

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Do not judge your fitness from one day

Performance naturally fluctuates. One weak set or one heavy run says little about progress over time.

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The trend across several weeks is more useful for determining whether the training is working.

It may be helpful to record a few factors after each session:

Factor	Example of recording
Sleep	Duration and perceived quality
Training load	Exercises, sets, repetitions, or duration
Perceived exertion	Scale from 1 to 10
Performance	Load, speed, or completed work
Energy and stress	Low, normal, or high
Symptoms	Pain, illness, or unusual breathlessness

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The log does not need to be detailed. The goal is to identify patterns, not to analyse every small fluctuation.

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If difficult sessions repeatedly follow insufficient sleep, high training volume, or long periods without food, the cause becomes easier to adjust.

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Wearable measurements may be a useful supplement, but should be interpreted together with perceived readiness and actual performance. Resting heart rate and heart rate variability are affected by many factors and cannot alone explain why the body feels tired.

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When persistently difficult sessions should be taken seriously

Occasional difficult training days are normal. It is more important to respond when performance remains reduced over time, particularly when lighter training and several rest days do not lead to improvement.

Relevant signs include:

• persistent decline in strength or endurance

• unusually high heart rate during easy activity

• prolonged sleep disturbance

• marked loss of motivation

• repeated infections

• unexplained weight loss

• dizziness or fainting

• chest pain

• significant breathlessness at low intensity

• persistent muscle or joint pain

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In these situations, the cause may be more than ordinary training fatigue.

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Infection, iron deficiency, insufficient energy intake, medication side effects, hormonal conditions, and other medical problems can all affect capacity.

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Acute chest pain, fainting, severe breathing difficulty, or new neurological symptoms during exercise should be assessed promptly by a healthcare professional.

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Summary

Some workouts feel harder because the same external workload can create a different internal load from day to day. Sleep, previous training, glycogen stores, hydration, temperature, mental fatigue, stress, and motivation all influence how the body responds. One difficult session rarely means that fitness has declined. The warm-up can be used to assess readiness, and the session can be adjusted through lighter weights, fewer sets, a slower pace, or longer rest periods. The most important approach is to follow the trend over time and respond if the decline in performance becomes persistent or is accompanied by unusual symptoms.

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Sources

• Kong, Y., et al. (2025). Effects of sleep deprivation on sports performance and perceived exertion in athletes and non-athletes: A systematic review and meta-analysis. Frontiers in Physiology, 16, 1544286.

• Deshayes, T. A., et al. (2022). Impact of dehydration on perceived exertion during endurance exercise: A systematic review with meta-analysis. Journal of Exercise Science & Fitness, 20(3), 224–235.

• Van Cutsem, J., et al. (2017). The effects of mental fatigue on physical performance: A systematic review. Sports Medicine, 47(8), 1569–1588.

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Why do some workouts feel harder than others? Learn how sleep, recovery, hydration, energy, stress, and daily readiness affect performance.