Why is recovery after exercise important?

Every time we exercise, we subject our body to stress. This, in turn, temporarily damages various tissues and cells.

For example, lifting a heavy weight subjects our muscles to mechanical stress, leading to the development of tiny microtears in our muscle fibres. Running a long distance puts large demands on our body’s energy systems, as we need to generate more ATP, the chemical energy currency of our cells, to support repeated, forceful muscle contractions over time. This increased generation of ATP leads to the build-up of metabolites (e.g. lactate, hydrogen ions, inorganic phosphate ions, reactive oxygen species (ROS)), depletion of muscle and liver glycogen stores, and other changes to the molecular environment of cells, all of which subject them to damage from metabolic stress.

Source: Zanuso, S., Sacchetti, M., Sundberg, C. J., Orlando, G., Benvenuti, P., & Balducci, S. (2017). Exercise in type 2 diabetes: genetic, metabolic and neuromuscular adaptations. A review of the evidence. British journal of sports medicine, 51(21), 1533-1538.

Fortunately, through a process known as allostasis, our body gradually adapts to repeated bouts of stress. The stress of exercise stimulates signaling pathways that ultimately switch on different genes (or alter patterns of ‘gene expression’), leading to the development of various training adaptations.

The mechanical stress of repeatedly lifting a heavy weight, for example, activates the mTOR signalling pathway, which switches on genes (e.g. mTOR) that repair microtears, stimulate muscle protein synthesis, and promote the remodelling and growth of muscle fibres. This culminates in increased size and strength of muscles - known as hypertrophy.

The metabolic stresses of endurance or aerobic exercise switch on genes (e.g. VEGF) that promote the growth of new blood capillaries (a process known as angiogenesis), allowing us to better supply exercising muscles with nutrients and oxygen. Endurance training also leads to the growth of new mitochondria in muscles (known as mitochondrial biogenesis), enhancing the supply of chemical energy to muscle fibres.

Source: Lavin, K. M., Coen, P. M., Baptista, L. C., Bell, M. B., Drummer, D., Harper, S. A., ... & Buford, T. W. (2022). State of Knowledge on Molecular Adaptations to Exercise in Humans: Historical Perspectives and Future Directions. Comprehensive Physiology, 12(2), 3193-3279.

As a result of these training adaptations (detailed in the diagram below), our body can better “anticipate” and more easily handle future bouts of stress. With repeated workouts, a weight that once felt heavy now feels lighter. A 10 km run that once seemed daunting can now be run with relative ease, at a faster pace and with a lower heart rate.

Crucially, the laying down of these training adaptations that improve exercise performance occur while we’re resting and recovering. If training sessions are where we subject our body to stress, post-exercise recovery periods are where our body lays down adaptations in response to this stress.

Source: Zanuso, S., Sacchetti, M., Sundberg, C. J., Orlando, G., Benvenuti, P., & Balducci, S. (2017). Exercise in type 2 diabetes: genetic, metabolic and neuromuscular adaptations. A review of the evidence. British journal of sports medicine, 51(21), 1533-1538.

In this respect, recovery allows us to:

• repair and remodel damaged tissue.
  
• replenish liver and muscle energy stores (e.g. glycogen, phosphocreatine)
• replenish enzymes used in the generation of ATP (e.g. phosphofructokinase)
• restore the internal and external environment (e.g. pH, acid/base balance) of cells.
• strengthen connections between nerves and muscle fibres
• lay down various other long-term or “chronic” adaptations that improve exercise performance.

Getting enough rest between workouts is therefore important for improving and optimising exercise performance.

(Just to be clear: your Post-exercise recovery rate trait focusses on the longer (hours to days) rest periods between individual workouts/training sessions, rather than the much shorter rest periods between sets of an exercise.)

What happens if you do not recover adequately after workouts?

Follow the schedule of any elite athlete and you’ll notice they have recovery sessions or days, whereby they either completely rest or do only light physical activity. The timetable below, for example, details the typical monthly schedule of a Premier League footballer.

Source: Ranchordas, M. K., Dawson, J. T., & Russell, M. (2017). Practical nutritional recovery strategies for elite soccer players when limited time separates repeated matches. Journal of the International Society of Sports Nutrition, 14(1), 35.

We’ve extolled the benefits of recovery in the previous section, and the rationale for designated rest and recovery sessions in elite athletes' training schedules is clear: they allow for the repair of cell damage, replenishment of energy systems, and the development of training adaptations that optimise exercise performance.

Of course, we also need to subject our body to sufficient stress through workout sessions in order to stimulate these training adaptations in the first place. We therefore must strike the correct balance between the stress of exercise and the opportunity for adaptation during rest/ recovery periods.

With inadequate recovery, we do not afford our body enough opportunity to repair damage from the physical stress of exercise or lay down training adaptations. Consequently, cell damage from stress accumulates (in what is sometimes known as “allostatic load”) leading to prolonged inflammation, muscle soreness, fatigue, muscle weakness, and poorer exercise performance.  

Source: Learning from sport burnout and overtraining. The Open University: URL:https://www.open.edu/openlearn/mod/oucontent/view.php?id=85414&section=6

We refer to this phenomenon of exercising too much without adequate rest as overreaching, which is characterised by persistent fatigue, increased susceptibility to infection, and an increased risk of injury. Luckily, these symptoms will quickly go away and exercise performance recovers with adequate rest (usually days to weeks).

With continued inadequate recovery, however, overreaching may progress to overtraining syndrome: defined as more than 2 months of reduced exercise performance. Athletes with overtraining syndrome typically complain of prolonged muscle weakness, with studies showing their muscles cannot generate previous maximal forces. They also may suffer from low mood, loss of motivation, sleep disturbances, and changes in appetite.

The underlying mechanisms behind overtraining syndrome are unclear but may be due to:

• Depletion of glycogen in skeletal muscle - recovery allows us to replenish glycogen stores in the liver and muscle that were used up for energy during prolonged exercise. Inadequate recovery prevents this from happening.
  

• Accumulation of exercise-induced muscle damage - exercise causes damage to muscle fibres. For example, eccentric exercises in particular are shown to damage the membranes (sarcolemma) surrounding muscle fibres. Inadequate recovery means we cannot repair this damage and remodel muscle tissue.
  

• Prolonged inflammation - exercise triggers a short-term inflammatory response in muscle and other tissue, which leads to the switching on of genes that subsequently dampen inflammation, repair cell damage, remodel tissue, and lay down training adaptations. Inadequate rest can prolong the inflammatory response (chronic inflammation) causing damage to muscle tissue.
  

• Accumulation of reactive oxygen species (ROS) and oxidative stress - one of the training adaptations that occurs duing recovery is the increased production of antioxidants - substances that neutralise and protect against oxidative cell damage from highly reactive molecules known as free radicals and reactive oxygen species (ROS). Inadequate recovery can lead to the excessive generation of ROS during exercise and a poorer ability to protect against oxidative damage.

Adequate recovery after exercise can protect against these risks of overreaching and overtraining syndrome.

How do lifestyle factors affect the rate of recovery?

We all recover after exercise at different rates, with our ability to repair cell damage and form training adaptations following exercise dependent on various genetic and lifestyle (i.e. non-genetic) factors. Some of these factors, such as what we eat and drink after a workout, are within our control. Other factors, such as our age, are beyond our control.

Let’s take a look at some of the lifestyle factors that affect recovery rate.

- Training history

To state the obvious, cycling 100 km will take less of a toll on the body of a highly-trained Tour de France cyclist compared to that of a sedentary office worker. This is because, through extensive training, the Tour de France cyclist will have exposed their body to a greater cumulative load of stress from exercise and, over time, developed various chronic training adaptations to better handle this stress.

Accordingly, a highly-trained person ought to require less recovery time between workout sessions compared to a novice with little training experience. We all know this intuitively, of course, but several studies back up this intuition. For example, one study compared the muscle strength of resistance-trained and untrained men following a workout session consisting of 10 sets of 6 maximal voluntary eccentric actions of the elbow flexors.

As explained earlier, eccentric muscle contraction (which involves contraction while the muscle is lengthening, such as during the lowering phase of a bicep curl) in particular causes exercise-induced muscle damage, and a temporary loss of muscle strength before recovery.

Source: Newton, M. J., Morgan, G. T., Sacco, P., Chapman, D. W., & Nosaka, K. (2008). Comparison of responses to strenuous eccentric exercise of the elbow flexors between resistance-trained and untrained men. The Journal of Strength & Conditioning Research, 22(2), 597-607.

As can be shown in the graph above, trained individuals suffered significantly less decrement in muscle strength (as assessed by changes in isometric torque) after the workout session and more quickly recovered muscle strength. Trained individuals returned to baseline strength after just three days (as indicated by the lack of # sign in the graph above), whereas untrained individuals had not yet returned to baseline strength on day 5 (as indicated by the # sign).

- Training intensity

It takes us longer to recover from a more intense workout session compared to a light one. Again this is because more intense workouts subject our body to greater mechanical and metabolic stress, which takes our body longer to recover from.

- Age

It is an inescapable fact that the older we get, the longer it takes us to recover from a workout. As we age our bodies become less adept at repairing exercise-induced cell damage, meaning an older person will take longer to recover from the same bout of exercise compared to a younger person.

- Diet

Several nutritional factors affect how quickly we recover post-exercise.

Source: Trommelen, J., Betz, M. W., & van Loon, L. J. (2019). The muscle protein synthetic response to meal ingestion following resistance-type exercise. Sports Medicine, 49(2), 185-197.

During recovery, our muscles replenish their stores of glycogen and phosphocreatine. Studies suggest that consuming carbohydrate after a workout can assist this refuelling process, with more intense endurance exercise requiring higher carbohydrate intakes. For example, a high carbohydrate meal eaten within two hours of exercise and containing at least 1.2 g carbohydrate per kg bodyweight per hour for the first four hours of recovery is one way to expedite the replenishment of glycogen stores.

As well as refueling, we also increase protein synthesis during recovery to repair muscle damage and develop training adaptations that increase the size and strength of muscle fibres. Intake of high quality, easily-digested protein, containing essential amino acids, can enhance this process. There is also evidence that creatine monohydrate benefits muscle repair following exercise.

For interested readers, the International Society of Sports Nutrition (ISSN) has a wealth of information on the best, evidence-based nutritional strategies to improve recovery.

It is worth pointing out that while some nutritional factors enhance recovery, other factors can impair and prolong recovery. Not consuming enough calories can delay repair of damaged muscle tissue, for example. Unhealthy diets followed over longer term can also promote low-grade inflammation that prolongs the inflammatory response following exercise and impairs recovery.

- Sleep

Sleep is a crucial, yet often overlooked, aspect of recovery from exercise.

During sleep, our body secretes growth hormone, which plays a key role in repairing damaged muscle, stimulating muscle growth, and developing metabolic training adaptations.

Slow-wave sleep or “deep sleep” is particularly important for some of the motor adaptations to exercise, with sleep disruption shown to impair the learning of new motor skills that are important in various sports and exercises.

Several studies show that sleep restriction or total sleep deprivation can significantly impair subsequent exercise performance. Similarly, insufficient and poor quality sleep may promote inflammation and cause imbalances in the autonomic nervous sytem  that delay recovery from exercise.

- Alcohol

Consuming alcohol is demonstrated to impair recovery from exercise by interfering with the signaling pathways (e.g. the mTOR pathway) that stimulate training adaptations in response to stress.

Source: Parr, E. B., Camera, D. M., Areta, J. L., Burke, L. M., Phillips, S. M., Hawley, J. A., & Coffey, V. G. (2014). Alcohol ingestion impairs maximal post-exercise rates of myofibrillar protein synthesis following a single bout of concurrent training. PLoS One, 9(2), e88384.

Alcohol can also attenuate the increase in muscle protein synthesis (MPS) that comes from ingesting protein after a workout. In one small study, individuals consuming alcohol alongside protein or carbohydrate were shown to lower rates of muscle protein synthesis (this is illustrated in the graph above).

Furthermore, alcohol can also disrupt sleep, leading to further impairment of recovery.

How do genetic factors affect the rate of recovery?

Our genetics also govern how quickly we can recover from exercise. Several gene variants likely affect the processes of exercise-induced muscle damage, inflammation, and subsequent tissue repair and laying down of training adaptations.

In terms of single gene variants that affect post-exercise recovery, some of the candidates include: IL-6, ACTN3, and CKM. Let’s take a look at these individually.

- IL-6

Interleukin-6 (IL-6) is a cytokine - a signaling molecule that helps coordinate an inflammatory response. During prolonged exercise, our muscles secrete IL-6, causing IL-6 levels in the blood circulation to rise. It is thought that this muscle-derived IL-6 helps to increase the supply of glucose and fatty acids as an energy source for exercising muscles.

During recovery from exercise, immune cells infiltrate into damaged muscle and also secrete IL-6. This immune-derived IL-6 released after exercise promotes muscle repair and muscle growth. However, excessively high levels of IL-6 after exercise may prolong inflammation and worsen muscle damage.

Source: Hennigar, S. R., McClung, J. P., & Pasiakos, S. M. (2017). Nutritional interventions and the IL‐6 response to exercise. The FASEB Journal, 31(9), 3719-3728.

Variants of the IL6 gene, which encodes the IL-6 cytokine, are shown to affect circulating levels of IL-6 and may influence recovery from exercise. More specifically, the ‘G’ allele (rs1800795) of the IL6 gene is associated with higher circulating IL-6 levels.  

It is thought that, by elevating IL-6 levels after exercise and promoting muscle repair, the ‘G’ allele may be beneficial for recovery. On this note, one study found that ‘G’ allele carriers suffered less exercise-induced muscle damage following a bout of eccentric exercise, as evidenced by lower levels of creatinine kinase - an enzyme in muscle cells that is released into the bloodstream following muscle damage. Those with the CC genotype, by contrast, experienced higher levels of muscle damage after eccentric exercise, and may require longer recovery periods following exercise.

Source: Yamin, C., Duarte, J. A. R., Oliveira, J. M. F., Amir, O., Sagiv, M., Eynon, N., ... & Amir, R. E. (2008). IL6 (-174) and TNFA (-308) promoter polymorphisms are associated with systemic creatine kinase response to eccentric exercise. European journal of applied physiology, 104(3), 579-586.

Conversely, it is also possible that higher IL-6 levels in 'G' allele carriers has a detrimental effect on post-exercise recovery, particularly if an individual has low-grade inflammation. Under this hypothesis, 'G' allele carriers may be more likely to have excessively high immune-derived IL-6 levels, which prolongs inflammation and exacerbates muscle damage following exercise.

- ACTN3

The ACTN3 gene is widely dubbed the ‘gene for speed’ as it encodes a protein involved in high-velocity, forceful contraction of fast-twitch (Type II) muscle fibres.

One variant of the ACTN3 gene, the R allele (rs1815739), results in production of the alpha-actinin-3 protein and is significantly more common in strength, power and sprint athletes – hence the “gene for speed” moniker.

By contrast, people carrying two copies of the X allele / variant do not produce any alpha-actinin 3 protein at all. Evidence also suggests these people with the XX genotype suffer greater exercise-induced muscle damage, particularly following eccentric exercise.

For example, one study of football (soccer) players found that those with the XX genotype had higher levels of creatinine kinase and alpha-actin (both markers of muscle damage) in the hours following eccentric exercise. This is shown in the graphs below.

Source: Pimenta, E. M., Coelho, D. B., Cruz, I. R., Morandi, R. F., Veneroso, C. E., de Azambuja Pussieldi, G., ... & De Paz Fernández, J. A. (2012). The ACTN3 genotype in soccer players in response to acute eccentric training. European journal of applied physiology, 112(4), 1495-1503.

Eccentric-contraction based workouts are particularly taxing on our muscles and are likely to induce damage to parts of muscle fibers called Z-discs. Z-discs are the borders between the individual contractile units of muscle fibers (called sarcomeres). They’re made out of various structural proteins, including alpha-actinin-3 and the closely related protein, alpha-actinin-2.

Individuals with the XX genotype, however, do not produce alpha-actinin-3. As such, their Z-discs are mainly composed of alpha-actinin-2 instead. This, however, makes them less stable and more susceptible to damage from eccentric contraction. Accordingly, people with the XX genotype may require more recovery time following eccentric exercise.

- CKM

The CKM gene encodes an enzyme known as creatinine kinase ( skeletal muscle type). This enzyme is usually confined to muscle cells and is involved in the generation of energy.

Levels of creatinine kinase in the bloodstream rise, however, following strenuous exercise. This is thought to be due to muscle damage allowing creatine kinase to “leak” into the bloodstream. With recovery, creatinine kinase levels then return to baseline (as shown in the graphs below. The graph shows creatinine kinase levels rising after 90 minutes of cycling exercises (Ex.1-3) on days 0, 1, and 2, before subsiding during recovery on days 2- 9).

Source: Baird, M. F., Graham, S. M., Baker, J. S., & Bickerstaff, G. F. (2012). Creatine-kinase-and exercise-related muscle damage implications for muscle performance and recovery. Journal of nutrition and metabolism, 2012.

Creatinine kinase levels may therefore be a proxy of exercise-induced muscle damage. Variants of the CKM gene may affect creatinine kinase activity and levels following exercise, which, in turn, can impact upon recovery rates.

For example, one study found that people with the GG genotype (rs1803285) of the CKM gene were more likely to experience exertional rhabodmyolysis - a condition whereby there is pathological breakdown of muscle tissue in response to exercise. It is possible that those with the GG genotype may therefore require greater recovery periods following strenuous exercise, although this remains to be shown conclusively in studies.

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