Why Runners Respond Differently to Training: 8 Factors Affecting Adaptation & Performance

Last Updated on November 21, 2022 by Heather Hart, ACSM EP, CSCS

Have you ever wondered why individual runners respond differently to training? Or why improvements in running performance appear to come easily to some athletes, while others seemingly have to scrape and claw their way just to achieve very minimal gains?

As a coach, more than once I’ve had athletes question their individual training based on the fact that a running friend is seemingly improving at a faster rate than they are.

As a runner myself, I’ve felt the frustration of seeing someone put in little-to-no training effort at all, and still run a faster race that I did, even though I trained my heart out.

Believe me, I get it: training can sometimes feel confusing and unfair.

But the reality is, while the training you put in is very important, it is only one piece of the overall puzzle that lends to improving your running performance.

What Does Adaptation Mean in Running?

“Adaptation” in the exercise and sports science world refers to changes in physical structure, as well as physiological and psychological function, that occur in response to repeated exposure to stress (i.e. training sessions) at increasing loads. 

Since exercise/training is a physiological stress, these adaptations prepare the body to cope better under that stimulus over time. In other words: making it easy for you to run a certain pace or distance.

But, in order to improve your performance as a runner, it takes more than just throwing yourself into the training…your body has to actually tolerate and adapt to that training, so that you can consistently continue to train.

Just ask anyone who has looked at a training program well outside of their current capabilities and thought “eh, how hard could it be?” only to crash and burn a few weeks later with an overuse injury. (It was me, in 2009, convinced I could go from a 4:30 marathon to a 3:45 in a short 4 months on 4 days of running per week.)

8 Factors Affecting Individual Adaptation to Run Training

The principle of individuality in exercise states that individuals will exhibit a range of unique responses to a given training stress, both physically and/or psychologically.

Meaning: adaptation to training is variable among runners, as are the end results.

How well you adapt to the stressors of training can be affected by a number of things. Some of these variables are out of your control, but for many runners, a lot of these areas leave room for improvement.

1. Training Structure (Specific Stress)

Let’s get the most obvious factor as to why runners respond differently to training out of the way: the training structure that you follow will have an affect on how much you can improve as a runner.

Conversely, poor training structure can be detrimental to your performance as a runner.

Training structure factors that need to be considered include:

• Specificity: the principle of specificity states that performance improves through training movement patterns and intensities of a specific task and fitness type. In other words, if you want to become a better runner, make sure the majority of your training focuses on running, and more specifically, at similar intensities and distances as your goal race.  
• Stimulus-Recovery Adaptation (SRA): in order to properly adapt to training and achieve maximal gains, there must be a balance between stimulus (training) and recovery
• Progressive Overload: the stimulus of training (running volume, intensity, etc.) must progressively increase during a training cycle in order to elicit maximal adaptations.
• Variation: in order for training gains to be consistent, strategic changes in the type of stimulus – but still within the realm of specificity – must occur.
• Fatigue Management: purposefully planned easier bouts of training, to help mitigate training decrements due to fatigue
• Phase Potentiation: making sure your phases of training are in an order that makes the most sense for performance gains.

There are endless research articles available on various types of training programming, and the influence they each have on improving athletic performance.

The programming that you follow will depend upon your end goal. But ultimately what matters when it comes to running performance is that you are following some sort of evidence-proven training structure, rather than just “winging it”.

2. Genetics

Next comes the one enjoys hearing: genetics play a large role in your fitness ceiling (how “good” you can get as a runner). You can thank your mom and dad for this one!

The genetic component to your individual athleticism manifests in a number of ways, such as:

• VO2max: research suggests that the amount in which you can improve your VO2max, or ability to utilize oxygen while exercising, is influenced up to 30-50% on your genetics (Bouchard et al.)
• Muscle Fiber Variability: studies show that 45% of the ratio of fast twitch vs. slow twitch muscle fibers an individual posses is based on genetics (Simoneau et al.).
• Body Type & Strength: a 2008 study of one million Swedish males determined that genetics are responsible for 81% of variation in height, 59% in body mass index and 50–60% in the strength measures (Silventoinen et al.)
• Exercise Tolerance & Adaptation: there is ongoing research working to identify genetic markers that determine how well an individual tolerates and responds to training. While some of these markers are present in, and believed to be linked to elite athletic status, others may actually be linked to exercise intolerance (Bray et al.)
• Psychological Response to Training: even the emotions, mood, and way you mentally approach training can be genetic. Research shows that way people respond, mentally, both during and after training can be influenced between 12-37% by genetics. The study further explains that positive affective responses were associated with higher amounts of regular exercise activity. (Schutte, et al.) 

In short, while training absolutely affects athletic performance, to what extent a runner can or will improve is greatly influenced by the genes you were given.

3. Nutrition

Nutrition is a very important piece of the training puzzle, but likely in more ways than most runners may initially assume.

First of all, food is fuel – it is what our bodies use to create ATP, or the cell’s usable form of energy.

Second, the food we consume becomes the “materials” with which we repair and rebuild damaged tissue, in order to become stronger. You can’t build a house out of thin air – you need lumber, and bricks, and manpower to get the job done. Likewise, you can’t recover from a hard workout, repair damaged tissue, and hypertrophy muscle fibers out of thin air – you need protein and amino acids to build new tissue, and energy from carbohydrates to get the work done.

Let’s look at a few ways nutrition affects running performance:

Sufficient Caloric Intake Outside of Training

Are you eating enough to support your training? The International Society of Sports Nutrition (ISSN) recommends general fitness seekers consume between 25-35 kcals/kg/day for a 110-176 lb (50-80 kg) individual, all the way up to 50-80 kcals/kg/day for athletes training at moderate and higher volumes (2-6 hours per day, 5-6 times per week.

So for example, that’s 3,410-5,454 calories a day for a 150 lb athlete. 

But research shows that female athletes, specifically female endurance athletes, are less likely to consume their recommended levels of carbohydrate or caloric intake.  The study cites that this is often due to chronic or periodic restriction of total energy intake in order to achieve or maintain low levels of body fat. (Burke et al.)

Low energy availability is described as inadequate caloric (energy) intake relative to exercise energy expenditure, and is the main factor triggering the unfavorable health and performance consequences associated with relative energy deficiency in sports (RED-S)

RED-S can result in:

• Reduced energy availability, and faster onset of fatigue
• Reduced muscle size and strength
• Impaired recovery, impaired muscle and cardiovascular adaptation from training and racing
• Increased risk of injury (Logue et al).

In order to set your body up for success, not just on race day, but in recovering and adapting to training, you must give it adequate amounts of calories to work with.

Sufficient Caloric Intake During Training

Adequate carbohydrate intake while running optimizes athletic performance and could limit exercise-induced muscle damage, reduce internal exercise load and  improve post workout or race recovery.

Further, long distance running – especially ultrarunning – relies heavily on utilizing carbohydrates as the main source of energy. Therefore it’s recommended that athlete’s consume 30-60 grams of carbohydrate per hour during workouts that last longer than 75 minutes, up to 90 g/hr for ultrarunning athletes, from mixed carbohydrate sources (glucose and fructose).

Timing of Nutrition

In addition to what you eat…when you eat matters. So much so that the the International Society of Sports Nutrition has published a 21 page position stand outlining the recommended timing of fueling and refueling for healthy, exercising adults and highly trained individuals, in regards to exercise performance and body composition.

Now, the first priority in the nutrition puzzle is making sure you are eating enough. But once that nutritional demand has been met, taking a closer look at when (and how frequently) you are eating may l end to even greater recovery from and adaptation to training.

Current general suggestions for endurance athletes include:

• Eating between 1-4 hours before training 
• Consuming 1-1.5g carbohydrate per kg bodyweight within 60 minutes of exercise, along with 20-40 grams of protein.
• Consuming protein, in 20- to 40-gram doses, throughout the day, to achieve 1.6-2.0 grams/kg/day.

For more specific recommendations, please refer to the “International Society of Sports Nutrition Position Stand: Nutrient Timing“.

Quality & Variety of Food

I promise that this isn’t where I tell you that you need to eat all organic foods and skip the post long run pizza and beer. Nope, I’m on team “everything in moderation” all the way.

Rather, this is where I point out that micronutrient deficiencies do exist, and can impact your running performance.

For example, low iron levels can result in decreased performance, and feeling fatigued. Low magnesium levels may result in achy or cramping muscles. Low calcium may compromise bone health, increasing the risk of impact injuries like stress fractures.

Therefore it’s important to make sure you eat a well balanced diet, rich in all of the necessary macro and micronutrients.

Significant and Abrupt Changes in Diet

A recent study published in the American Society for Microbiology looked at the impact of temporary high-protein or high-carbohydrate diets on endurance performance and associated gut microbial community changes. It found that short term high-protein diets resulted in less stable gut microbiota, which compromised athletic performance(Furber et al).

Point being, any shifts in your diet may temporarily affect your training and racing.

4. Hormones

Hormones play an integral role in how well your body responds to, recovers from, and makes adaptations to the stress of training.

A quick science refresher: hormones are chemical messengers regulated and released by the endocrine system (hypothalamus, thyroid, pituitary gland, etc.). They travel through your blood to the targeted area of the body to regulate various bodily functions and help maintain homeostasis.

Some of these hormones include (but are not limited to)

• Testosterone: the major promoter of muscle growth and subsequent increase in muscle strength in response to resistance training in men (though also present in smaller amounts in women)
• Estrogen: female sex hormone responsible for fat storage, increased rate of bone growth, improves muscle mass and strength, and increases the collagen content of connective tissues. Newer research suggests that estrogen (Krause et al.)
• Human Growth Hormone: stimulates growth, cell reproduction, and cell regeneration in humans. HGH also plays a role in the breakdown and utilization of fatty acids, protein synthesis, retention of electrolytes, and immune cell function.
• Insulin-Like Growth Factors (IGF’s): play a role in formation, maintenance, and regeneration of skeletal muscles
• Insulin: regulates the amount of glucose in the blood, promotes glycogen storage, and is involved in protein synthesis
• Cortisol: a hormone produced in response to stress that plays several roles, including regulating metabolism, blood pressure, blood sugar, inflammation response, and circadian rhythm. .
• Catecholamines: hormones produced in response to stress , including dopamine, norepinephrine, and epinephrine, that act as central motor stimulators and peripheral vascular dilators to enhance enzyme systems and calcium release in muscle, therefore responsible for acute expression of strength and power.
• Leptin: responsible for the regulation of appetite and fat storage.

The amount of hormones present in any given individual can vary greatly, and can be affected by things such as (but not limited to): nutritional status, chronic or acute stress, menstrual cycle, pharmaceuticals (from prescription medicines, to birth control, to even caffeine), medical conditions (such as PCOS, thyroid issues, or cancers), endocrine disruptors (such as BPA’s in plastics or naturally occurring phytoestrogens in soy), and aging.

5. Sleep

As a mom who discovered my love for running around the same time my then 3 month old son refused to sleep more than two hour bouts at a time, I am no stranger to training and racing while sleep deprived.

But like many things, just because it’s possible, doesn’t mean it’s ideal. Here’s just a few ways not getting enough sleep can affect running performance:

• Tissue Repair: Your body releases Growth Hormone throughout the day, however, the majority of it (up to 70%) is released during sleep, and in the overnight hours. Further, the majority of tissue repair (and thus, adaptation to training) occurs while you are sleeping.
• Increased Cortisol:  Sleep deprivation results in increased cortisol levels, which affects the secretion pattern of testosterone and GH.  This causes a shift in anabolic-catabolic patterns, meaning you are more likely to suffer from muscle mass loss rather than growth.
• Faster Time to Exhaustion:  Multiple studies on sleep deprivation in runners show that lack of sleep may decrease running performance, and increase physical discomfort as well as time to exhaustion, due to a decrease in respiratory response. (Azboy et al.) (Souissi et al.)
• Cognitive Effects: Both short term and long term sleep deprivation have shown to cause cognitive slowing, increased rate of perceived exertion, memory impairment, decreased sustained attention, shifts in optimum response capabilities, decreased reaction time, and dysfunctional emotional regulation. (Chennaoui et al)
• Immune System: Adequate sleep promotes optimal function of the immune system, whereas poor sleep habits are associated with lower resistance to illness (Prather et al)

So how much sleep do you actually need? Well…science isn’t quite sure.

A 2021 study in the International Journal of Sports Physiology and Performance Elite male and female athletes need 8.3 hours of sleep to feel rested. However, a majority of athletes (71%) fail to meet this need on most nights. (Sargent et al)

But, in the consensus statement published in the British Journal of Sports Medicine, experts state that a one-size-fits-all approach to athlete sleep recommendations (eg, 7–9 hours/night) is unlikely ideal for health and performance, and therefore, an individualized approach that considers the athlete’s perceived sleep needs is recommended.

6. Non-Specific Stress

You’ve probably heard it before: “stress is stress”, as far as our bodies are concerned. In order to become a better runner, you need to present a certain level of specific stress in order for your body to adapt to it, and become stronger.

However, adapting to the stress of that stimulus takes a lot of effort from your body. Therefore, minimizing non-specific stress can help optimize your adaptation to training. Here’s a few examples of common non-specific stresses that runners face:

Environmental Stress

Environmental stress, like extreme heat, extreme cold, higher altitudes, smoke, pollen, smog, etc. all put excessive stress on your body as your body needs to work harder to maintain homeostasis while running in these environments.

Greater stress loads require greater recovery times.

Further, training in extremes can often limit the ability to train at an optimal intensity, thus inhibiting adaptation. In the case of altitude, this is where the “live high, train low” approach comes into play: athlete’s cannot push variables like speed, lactate, or aerobic thresholds as well at higher altitudes as they can at sea level. So they train at lower altitudes in order to push hard, yet reap the benefits of adaptation to the higher altitudes by living/recovering there.

Concurrent Training

Concurrent training refers to combining both resistance and endurance training. To be well rounded, strong, injury free runners (and simply healthy adults) we need to have both incorporated in our training.

However…when it comes to optimal strength gains, we have to be cognizant of how we combine these two modes of training.

Without getting TOO sciencey, here’s what you need to know: there are two key signaling pathways in our bodies that are considered “master regulators” of cell metabolism and growth.  Those pathways are mTOR (Mammalian Target of Rapamycin) and AMPK  (Adenosine Monophosphate-activated Protein Kinase).  

The mTOR pathway is activated during anaerobic based, nutrient rich conditions (think: heavy strength training, plyometrics, etc.) and plays a big role in muscle hypertrophy.   

 On the other hand, AMPK pathway is activated during aerobic based, energy depleted states (like, you guessed it, running for extended periods of time). It stimulates energy production, increasing glucose uptake, and oxidation of fatty acids, while inhibiting processes that consume energy, such as protein and lipid synthesis.

In short: AMPK inhibits mTOR, therefore inhibiting protein synthesis. So, combining running and resistance training too closely together (or in the same training session) can eventually result in a diminished returns, as the mTOR and AMPK pathways will compete with one another.

That’s not to say that you can’t ever combine strength training and endurance training, you absolutely can. However, by constantly combining the two, you may sell yourself short on the gains you are hoping to achieve from resistance training.

Emotional/Mental Stress

The number of things in our day to day lives that can cause perceived or actual stress are endless:  life events, job strain, role strain (parent, caretaker, etc.), work life balance, relationships, etc.  The list goes on and on.  

While running can absolutely be beneficial for reducing stress, chronic emotional stress may actual hinder running performance, and inhibit recovery from training.

Studies show that individuals reporting high levels of perceived or life stress showed significantly reduced muscle force production and feelings of energy during testing.  Further, these participants had slower recovery from exercise or training than individuals who are not under chronic stress.  (Stults-Kolehmainen, et al)

7. Age

This is another one most athletes don’t want to hear: research shows that runners can expect a 0.5 to 1 percent decline in performance per year from age 35 to 60 with consistent training. After age 60, performance decrement tends to increase at a faster rate. 

VO2max (maximal oxygen consumption) appears to be the primary mechanism associated with declines in endurance performance with age. Reduction in lactate threshold also contributes to the reduction in endurance performance with aging. 

Scientists aren’t entirely sure if these decrements in performance and cardiovascular fitness are caused by declines in training volume or vice versa.

Here are a few things we do know about the aging process, and how it affects training:

• It is believed that aging skeletal muscle experiences greater exercise-induced fatigue or damage and has a slower rate of repair and recovery from this fatigue or damage (Fell, et al)
• There is a preferential loss of type II (fast twitch) muscle fiber motor units with age, which can affect both strength and endurance.
• Sarcopenia (loss of skeletal muscle mass and strength) is age related, and involuntary
• Muscle fibers decrease in their maximum shortening velocity
• Hormones are altered with age, including decreases in testosterone and IGF-1 (both responsible for muscle tissue growth)

The good news? There is plenty of positive research demonstrating that regular resistance training can reduce these declines in both endurance and strength/power.

8. Psychological Factors

The human mind is powerful and complex, and it should be no surprise that it can have a great influence on whether or not a runner will be successful in their training and racing goals.

A study published in 2021 set out to analyze the psychological variables of 356 trail/mountain ultrarunners and their association with athletic performance and success.   Researchers found that mental toughness, resilience, addiction, and passion were all factors present that lent to the success of the runners.  

Further, the study found positive correlations between mental toughness and the athlete’s level, meaning more elite runners scored higher on the mental toughness scale.

This, of course, isn’t the first study to determine that successful long distance runners have higher levels of motivation and mental toughness (such as this one, this one, and this one, just to list a few).

Point being? There’s something to be said for putting aside self doubt, believing you can improve as a runner, then putting in the hard work in order to succeed.

Final Thoughts:

These eight factors are not the ONLY factors that contribute to how well you’ll improve as a runner, and whether or not your training will actually work. But, they are eight important ones.

So if you feel like you are struggling to make improvements, I highly recommend diving deep into the things you can change: appropriate training structure, nutrition, sleep, managing stress, and believing that you can do this.