Acute Response to Endurance Exercise Stress

Written by Ben Bunting: BA, PGCert. (Sport & Exercise Nutrition) // British Army Physical Training Instructor // S&C Coach.

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Physical activities require the coordinated interactions of several acute physiological responses. Exercise can take different forms, from concentric, isometric or eccentric exercise - each exerting pressure on different areas of the neuromuscular system.

Recent studies have demonstrated that endurance-trained individuals exhibit lower physiological reactivity to psychosocial stress. However, research comparing concurrent endurance and resistance training yields inconsistent results.

Muscle Damage

Exercise-induced muscle damage (EIMD) occurs due to structural muscle fiber damage and an accompanying inflammatory response.

This consists of leukocyte infiltration into damaged areas and an elevation in creatine kinase (CK) levels in muscle enzymes such as creatine kinase (CK).

Signs and symptoms associated with EIMD include pain and soreness, an impairment in pressure pain threshold thresholds, localized swelling, elevated lactate dehydrogenase activity levels and myoglobin concentrations among others.

As endurance athletes return to training after engaging in one bout of damaging exercise, acute physiological reactions often diminish over time due to something known as the interference effect.

While its exact cause remains elusive, evidence points towards muscle damage via AMPK/mTOR signaling pathways as a likely culprit. It's therefore vital that athletes understand how an acute response to damaged muscle affects their performance during concurrent exercise stimulus.

Studies have explored whether intense exercise intensity negatively impedes on subsequent adaptations in strength training muscles. Investigations using work and duration-matched concurrent endurance and resistance training programs to probe this question.

Results from these investigations generally failed to demonstrate such interference effects.

However, results of other investigations of this topic vary significantly and have even shown positive interference effects. These discrepancies may be attributable to differences in experimental methods and design.

Many studies have explored the effect of nutrition on acute muscle injury responses. One such investigation discovered that supplementing with HMB-C before engaging in potentially damaging exercise helped decrease CK flux and soreness levels by 25%; however, this finding has yet to be replicated in other investigations.

Studies on supplementation with antioxidants to aid recovery after damage-inducing exercise have also been conducted, with participants either receiving 3 grams of HMB-C or placebo before engaging in damaging exercise and then being tested 24 hours postexercise for markers of EIMD.

Consumption of HMB-C reduced CK flux and soreness while not altering an exercise-induced decrease in muscle function.

Cortisol Release

Cortisol is a hormone released when you experience physical or emotional stress. It increases sugars in your bloodstream, increases brain glucose usage, and increases substances that repair damaged tissues.

Too much cortisol over an extended period can have damaging consequences - cortisol levels tend to peak when we wake up in the morning before decreasing as time progresses.

As soon as you start working out at an intensity that requires endurance exercise, your adrenal glands release glucocorticoids in response to HPA axis stimulus from hypothalamus and pituitary gland.

An increase in glucocorticoids increases glucose entering the bloodstream for use during endurance exercise. Furthermore they prevent excess glucose being stored as fat within your body as insulin production decreases due to their effect on insulin release and production.

One bout of endurance exercise should be sufficient to trigger glucocorticoid secretory response from adrenal glands in healthy individuals. However, for individuals with chronic disease or family histories of autoimmune disorders this threshold intensity of endurance exercise increases significantly.

As you exercise, your adrenal glands produce catecholamines epinephrine and norepinephrine that work together with cortisol to create your body's response to physical stressors like an imminent threat or unexpected challenge.

They increase heart rate and blood pressure as well as providing large muscles with energy to perform unfamiliar movements more easily. Additionally, they inhibit liver and skeletal muscle breakdown so more glucose enters your bloodstream for instant energy to power those muscles.

The adrenal glands produce cortisol in small doses on a normal 24-hour cycle and in larger amounts during times of physiological stress. Acute exercise causes elevated cortisol levels shortly afterward, though its effects often wear off several hours later.

With continued training however, cortisol responses become less significant; suggesting that regular training better equips your body to cope with physical stressors while attenuating its HPA axis response.

Endurance Exercise Related Low Testosterone

Exercise research has predominantly examined resistance and strength-based activities and their effects on testosterone, yet less attention has been paid to how long-term endurance training affects this vital hormone.

Indeed, some high profile athletes have reported retiring due to clinically low testosterone caused by prolonged endurance-based training (a condition known as Exercise hypogonadal male condition or EHMC).

Endurance exercises refer to any physical activity you can maintain for an extended period at a lower intensity, like cycling, jogging or swimming.

While endurance exercises should form part of your regular fitness regime it should never go beyond what your body can handle otherwise overtraining could occur leading to loss of muscle mass, reduced energy and an inability to tolerate other forms of physical exercise.

Testosterone is the main male sex hormone responsible for stimulating facial and body hair growth, increasing muscle mass, protein production/synthesis and aiding sexual function/sperm production.

Studies have demonstrated that men who train intensely at an endurance level often display reduced resting levels of testosterone, hypogonadism and changes to the HPG axis.

This is similar to what can be observed among female overtrainers - after engaging in endurance training at an intense endurance level. These changes have been coined "Exercise Related Relative Hypogonadism - S," or RED-S for short.

Blood Pressure Changes

Your blood pressure tends to temporarily increase during physical exercise as your heart pumps oxygen-rich blood to your muscles.

This rise should dissipate within several hours after exercise if your health permits; otherwise it should return to its resting level within several days after each bout of activity.

Your diastolic blood pressure should gradually decline over time; this is normal and helps protect you against potential stroke or heart attack risks.

But, any specific changes should be monitored closely; for those with preexisting conditions like high or low blood pressure, high cholesterol, or coronary artery disease it is wise to consult their physician prior to beginning any new exercise program.

Blood pressure can also be affected by how much sweat you produce and for how long. As more fluid is lost through exercise, your systolic blood pressure will drop post-exercise. This phenomenon is called post-exercise hypotension and typically lasts 24-48 hours post-workout.

Studies have revealed that regular endurance training can lower systolic blood pressure by 5 to 7 mmHg, which can drastically lower overall blood-pressure readings and significantly improve readings overall.

A reduction in systolic blood pressure also reduces your risk of cardiovascular disease when coupled with weight loss and healthy diet choices.

Studies have also demonstrated how exercise type can alter systolic blood pressure responses to head-up tilt (HUT) testing, with endurance-trained individuals responding differently than resistance-trained ones to HUT testing.

One such study demonstrated how HUT caused divergent SBP responses between endurance- and resistance-trained participants and found LV wall thickness and aortic pulse wave velocity to explain 41% of variability in this measure.

These findings indicate that chronic resistance and endurance exercise training differentially impacts systolic blood pressure responses to HUT, likely through training-specific changes to morphological adaptations of the left ventricle (LV) and large conduit arteries.

This potentially provides a better prognosis for future hypertension development, especially among individuals who have experienced EIH before; likely through increased sympathetic tone.

Mood Changes

Experienced runners know the feeling of "runner's high," the result of physical exercise's power to lift mood. Partly due to endorphin production - natural painkillers which help lower stress levels and enhance sense of well-being. Their effects may last several hours after you work out!

Endurance exercises can also provide relief from depression and anxiety, stimulating the release of hormones which promote relaxation and better sleep - thus further decreasing stress levels.

Studies have also demonstrated how physical activity such as endurance sports can decrease overall anxiety, elevate mood and relieve depression in both men and women.

As a trainer, one of the key considerations should be understanding how your clients' bodies react to different exercises. Knowing this will enable you to prescribe optimal durations and intensities for their training sessions for maximum results.

Participants were required to perform a 5-min warm up jog before engaging in two endurance exercise conditions matching intensity and duration: (MIC - constant load cycling at W equaling 65% VO2peak; HIIC - three min intervals at W between 85-45% of VO2peak); plus one control condition consisting of seated rest.

Every five minutes throughout this test period, heart rate (HR), blood lactate concentration (BLa), and rating of perceived exertion (RPE) measurements were collected and recorded - recording was collected every five minutes during this exercise test period for analysis.

Data presented showed that both MIC and HIIC caused positive mood changes compared to sedentary control conditions.

These differences were statistically significant. PANAS positive affect scale scores significantly correlated with both BLa, RPE, but not HR levels of discomfort; in addition PANAS positively correlating with antinociceptive effects but not discomfort levels.

This study provides novel evidence that the antinociceptive effect induced by various exercise intensity levels does not solely depend on mood changes but also fitness status.

These results support previous research which indicates that exercise intensity and fitness level play an influential role in terms of its antinociceptive response.

Conclusion

"Stress" refers to any physical activity which disrupts homeostasis and produces an identifiable physiological response

For instance, rising from a chair to make tea could qualify as exercise stress; although it would not result in muscle-tendon strain leading to delayed onset muscle soreness (DOMS).

You can measure relative intensity by monitoring how long it takes you reach steady state and changes in muscle metabolites (such as phosphoryl creatine kinase) or oxygen uptake.

High-intensity endurance exercise is an established stressor that induces profound metabolic changes within cells of skeletal muscles and other tissues, including increased mitochondrial mass, aerobic enzyme activity, angiogenesis (capillary formation) resulting in greater capillary density, among many others.

Exhaustive endurance exercise elicits a systemic inflammatory response, culminating in an accumulation of the cellular damage markers CK and myoglobin (Mb).

Myoglobin seems to respond more promptly following single bouts of exercise due to leakage from damaged muscle fibers more rapidly. Renal excretion clearing more rapidly from blood, and less susceptibility to degradation by phosphorylating hexokinase degradation.

Studies have demonstrated the beneficial effects of regular endurance and resistance training on cardiovascular and adrenal reactivity to psychosocial stressors.

In a research study, twelve endurance-trained men, ten resistance-trained men, and twelve healthy but untrained men were exposed to a standard psychosocial stressor; free salivary cortisol responses, heart rate responses, and mood responses were then measured during exposure.

Both endurance- and resistance trained groups demonstrated lower cortisol responses than untrained groups as well as significantly lower heart rate responses than untrained group.