Exercise alleviates depression by raising heart rate which increases cardiovascular fitness and improves neurochemistry promoting neurogenesis (the birth of new brain cells). Antidepressants, which I liken to chemical exercise, do the same thing in a more indirect and less effective way (but are still a better option than nothing). The research below also covers the many advantages of exercise / improved fitness, the importance of exercise intensity and frequency, as well as risks associated with overtraining.
Bottom line: The key to physical/mental health is regular intense exercise while avoiding overtraining. Current research points to HIIT (high intensity interval training) as the most effective and efficient type of exercise. An example of HIIT would be to exercise intensely until you're out of breath and then slow down or rest until you are ready to go all out again, repeating until exhaustion. This still leaves the problem of overtraining/undertraining. How do you determine if you are exercising too much or too little. Complicating things is the fact that the most conventional symptoms of overtraining are actually the same as the symptoms of undertraining - depression, sleep issues, lowered performance. My newest system to avoid overtraining/undertraining is the Altus regime.
"There were trends for the superiority of the exercise and sertraline conditions over usual care in improving SF-36 mental health scores and clinician-rated depression scores. Individuals in the exercise condition showed greater improvements in physical functioning than individuals in the usual care condition. Both sertraline and exercise show promise as treatments for late-life minor depression. However, exercise has the added benefit of improving physical functioning as well." (source)
"our study suggests that a longer course of treatment may be effective and that improvements in depression are associated with improvements in exercise tolerance." (source)
"Human and other animal studies demonstrate that exercise targets many aspects of brain function and has broad effects on overall brain health. The benefits of exercise have been best defined for learning and memory, protection from neurodegeneration and alleviation of depression, particularly in elderly populations." (source)
"at 6 months’ follow-up, those with greater aerobic fitness had much lower anxiety levels." (source)
"41 depressive women were assigned to three groups, aerobic running, anaerobic weight training, and waiting list control. depression was significantly reduced in the two training groups with no change in the control group." (source)
"anti-depressants raise heart rate" (source)
"We compared the effectiveness of an aerobic and nonaerobic exercise in the treatment of clinical depression in women. A total of 40 women, screened on the Research Diagnostic Criteria for major or minor depressive disorder, were randomly assigned to an 8-week running (aerobic), weight-lifting (nonaerobic), or wait-list control condition. Subjects were reassessed at mid- and posttreatment, and at 1-, 7-, and 12-month follow-ups. Depression was monitored by the Beck Depression Inventory, Lubin's Depression Adjective Check List, and the Hamilton Rating Scale for Depression; fitness level was assessed using submaximal treadmill testing. Results were remarkably consistent across measures, with both exercise conditions significantly reducing depression compared with the wait list control condition, and generally appearing indistinguishable from each other. No significant between-group fitness changes were noted. These findings indicate that both types of exercise conditions significantly reduce depression and that these results are not dependent on achieving an aerobic effect." (source)
"43 depressed undergraduate women were randomly assigned to either an aerobic exercise treatment condition in which they participated in strenuous exercise, a placebo treatment condition in which they practiced relaxation exercises, or a no-treatment condition. Aerobic capacity was assessed before and after a 10-wk treatment period. Self-reported depression was assessed before, during, and after the treatment period. Results show that Ss in the aerobic exercise condition had reliably greater increases in aerobic capacity and reliably greater decreases in depression than did Ss in the placebo or no-treatment condition. There was a reduction in depression that was independent of treatment; it is suggested that a no-treatment control condition is a necessity in research on depression. It is concluded that findings provide clear evidence that participation in a program of strenuous aerobic exercise is effective for reducing depression." (s0urce)
"We compared aerobic with nonaerobic forms of exercise in the treatment of clinical depression. Ninety-nine inpatients, who met the DMS-III-R criteria for major depression, dysthymic disorder, or depressive disorder not otherwise specified (NOS), took part in the study. They were randomly assigned to two different physical training conditions, aerobic and nonaerobic. In both conditions, one hour of training was performed three times a week for a period of 8 weeks. There was a significant increase in maximum oxygen uptake (Image O2 max) in the aerobic group; there was no change in the nonaerobic group regarding this variable. Depression scores in both groups were significantly reduced during the study, but there was no significant difference between the groups. The correlation between increase in physical fitness and reduction in depression scores was low. The study indicates that the antidepressive effects associated with exercises are not restricted to aerobic forms of training." (source)
"Although this article focuses on the augmentation of dentate-gyrus neurogenesis by serotonin, other means of increasing neurogenesis might also have clinical relevance. For example, it is well known that exercise, especially running, has an antidepressant action, and we recently found that 4-10 days of running on a wheel induces a significant increase in cell proliferation in a mouse’s dentate gyrus. After several weeks of running, neurogenesis increased as well. Also, norepinephrine appears to increase cell division in the dentate gyrus. These factors might also play roles in depression." (source)
"Investigated the psychophysiological effects of exercise on measures of cardiovascular fitness, depression, and anxiety in male psychiatric outpatients. In addition to self-report measures, electromyogram (EMG) assay and digital skin temperature were performed to measure physiological concomitants of anxiety. The 9 Ss in the running treatment group ran 3 days/week while the 9 Ss in the corrective therapy group were involved in noncardiovascular exercise for 1 hr, 3 times/week. Nine Ss were in a waiting list control group. Results demonstrate significant improvements in cardiovascular conditioning for the running treatment group and significant decrements in depression for the running treatment group compared with the waiting list controls. Results of EMG activity demonstrated that the running treatment group was significantly less tense; however, running was not effective in reducing cognitive anxiety." (source)
"This study identifies the relative importance of psychological dimensions that discriminate between habitually physically active and sedentary men. The data support the notion that physical activity has psychological benefits. Subjects (N = 22) were normal, medically healthy middle-aged men (40–60 years of age). Data were collected on selected physiological (treadmill) and psychological (MMPI) variables and replicated within four months. The Student's t-test and discriminant function analysis were used in the statistical analysis. Physically active men (n = 11) have lower depression (scale 2) and lower social introversion (scale 10) than the sedentary men (n = 11). The physically active men may exhibit MMPI scale differences in ‘neurotic’ tendencies from the sedentary men, but there is no apparent difference in scales suggestive of ‘psychotic’ tendencies. Depression (scale 2) is the most powerful discriminator between physically active and sedentary men, followed by hysteria (scale 3) and social introversion (scale 10). In conclusion, our data identify depression as the most important MMPI scale that discriminates between physically active and sedentary men." (source)
"A common trait of antidepressant drugs, electroconvulsive treatment and physical exercise is that they relieve depression and up-regulate neurotrophic factors as well as cell proliferation and neurogenesis in the hippocampus. In order to identify possible biological underpinnings of depression and the antidepressant effect of running, we analysed cell proliferation, the level of the neurotrophic factor BDNF in hippocampus and dynorphin in striatum/accumbens in ‘depressed’ Flinders Sensitive Line rats (FSL) and Flinders Resistant Line (FRL) rats with and without access to running-wheels. The FRL strain exhibited a higher daily running activity than the FSL strain. Wheel-running had an antidepressant effect in the ‘depressed’ FSL rats, as indicated by the forced swim test. In the hippocampus, cell proliferation was lower in the ‘depressed’ rats compared to the control FRL rats but there was no difference in BDNF or dynorphin levels in striatum/accumbens. After 5 wk of running, cell proliferation increased in FSL but not in FRL rats. BDNF and dynorphin mRNA levels were increased in FRL but not to the same extent in the in FSL rats; thus, increased BDNF and dynorphin levels were correlated to the running activity but not to the antidepressant effect of running. The only parameter that was associated to basal level of ‘depression’ and to the antidepressant effect was cell proliferation in the hippocampus. Thus, suppression of cell proliferation in the hippocampus could constitute one of the mechanisms that underlie depression, and physical activity might be an efficient antidepressant." (source)
"The relation between level of physical activity and risk of subsequent depression was examined using three waves of data from the Alameda County Study. Among subjects who were not depressed at baseline, those who reported a low activity level were at significantly greater risk for depression at the 1974 follow-up than were those who reported high levels of activity at baseline. Adjustments for physical health, socioeco-nomic status, life events, social supports, and other health habits did not affect the association appreciably. Associations between 1965–1974 changes in activity level and depression in the 1983 follow-up suggest that the risk of depression can be altered by changes in exercise habits, although these associations were not statistically significant after adjustment for covariates. These results provide somewhat stronger evidence for an activity-depression link than do previous studies, and they argue for the inclusion of exercise programs as part of community mental health programs, as well as for further studies that focus on the relation between life-style and mental health." (source)
"Presents psychological and physiological arguments for the case of aerobic/oxygen-consuming exercise as a remedy for dysphoria. Psychological rationales include the beliefs that aerobic exercise enhances self-esteem and self-confidence, fosters a feeling of accomplishment that counteracts learned helplessness, and promotes an exercise distress that desensitizes Ss to somatic anxiety. Physiological arguments suggest that exercise decreases the biological responses to physical stressors with increased endurance, is a natural muscle relaxant, promotes sound sleep, and reduces the amount of lactate (an acid product linked to anxiety). Other physiological benefits of exercise, which are notably lacking in depressed and anxiety-ridden patients, involve an increase in brain norepinephrine production, relief of chronic fatigue, and an increase of slow-wave sleep." (source)
"Aerobic exercise at a dose consistent with public health recommendations is an effective treatment for MDD (moderate major depressive disorder) of mild to moderate severity. A lower dose is comparable to placebo effect." (source)
"The results of this cross-sectional study suggest that individuals who exercised at least two to three times a week experienced significantly less depression, anger, cynical distrust, and stress than those exercising less frequently or not at all. Furthermore, regular exercisers perceived their health and fitness to be better than less frequent exercisers did. Finally, those who exercised at least twice a week reported higher levels of sense of coherence and a stronger feeling of social integration than their less frequently exercising counterparts." (source)
"The results indicate a consistent association between enhanced psychological well-being, as measured using a variety of psychological inventories, and regular physical exercise." (source)
"This study of relationships between personality and exercise frequency was based on 22,448 residents (40 to 64 years old) in Miyagi, Japan. They completed the self-administered questionnaires on their exercise frequency and the Japanese version of the short form Eysenck Personality Questionnaire—Revised. Higher scores on Extraversion were positively related to exercising and higher scores on Neuroticism and Psychoticism were positively related to "not exercising" in a general population for both sexes. These observed relationships were consistent even when an analysis of covariance was conducted with controlling for possible confounding factors of age, marital status, and education." (source)
"Frequent daytime sleepiness predicted impaired physical function (OR = 2.76, 95%CI = 0.237–0.553, P = 0.001) after controlling for age, BMI, income and number of co-morbid conditions. The conclusion was that daytime sleepiness in older adults is associated with physical functional impairments and decreased exercise frequency." (source)
"Women who engaged in vigorous exercise at least once per week had an age-adjusted relative risk (RR) of NIDDM (non insulin dependent diabetes melitus) of 0·67 (p<0·0001) compared with women who did not exercise weekly. After adjustment for body-mass index, the reduction in risk was attenuated but remained statistically significant (RR=0·84, P=0·005)" (source)
"Exercise vigorous enough to work up a sweat is associated with decreased stroke risk in men." (source)
"After adjusting for age, smoking, alcohol use, history of hypertension, history of high cholesterol level, and other covariates, the relative risks (RRs) of developing type 2 diabetes across quintiles of physical activity (least to most) were 1.0, 0.77, 0.75, 0.62, and 0.54 (P for trend <.001); after adjusting for body mass index (BMI), RRs were 1.0, 0.84, 0.87, 0.77, and 0.74 (P for trend = .002)." (source)
"It was found that regular exercisers scored higher as a group on the mental decrement test than did the nonexercise group. In general, better fitness measures were found to accompany higher fluid intelligence scores." (source)
"After 3 months of exercise, results indicated that cognitive function and walk distance improved significantly. Results also indicated that the gain in cognitive function was reliably predicted by the decrease in VE at VO2peak." (source)
Aerobic exercise strengthens your cardiovascular system. Anaerobic exercise strengthens your muscles. Both are important and beneficial. Based on the research, higher intensity exercise has more benefits (lower mortality risk, more time efficient, etc.) than moderate intensity exercise.
"The epidemiologic studies consistently found a greater reduction in risk of cardiovascular disease with vigorous (typically ≥6 METs) than with moderate intensity physical activity and reported more favorable risk profiles for individuals engaged in vigorous, as opposed to moderate, intensity physical activity. Clinical trials generally reported greater improvements after vigorous (typically ≥60% aerobic capacity) compared with moderate intensity exercise for diastolic blood pressure, glucose control, and aerobic capacity, but reported no intensity effect on improvements in systolic blood pressure, lipid profile, or body fat loss. In conclusion, if the total energy expenditure of exercise is held constant, exercise performed at a vigorous intensity appears to convey greater cardioprotective benefits than exercise of a moderate intensity." (source)
"Twenty-one stable CAD (coronary artery disease) patients were randomized to supervised treadmill walking at either high intensity (80–90% of VO2peak) or moderate intensity (50–60% of VO2peak) three times a week for 10 weeks.
Results After training VO2peak increased by 17.9% (P = 0.012) in the high intensity group and 7.9% (P = 0.038) in the moderate intensity group. The training-induced adaptation was significantly higher in the high intensity group (P = 0.011).
Conclusions High intensity aerobic interval exercise is superior to moderate exercise for increasing VO2peak in stable CAD-patients. As VO2peak seems to reflect a continuum between health and cardiovascular disease and death." (source)
"Men who ran for an hour or more per week had a 42% (mortality) risk reduction (RR, 0.58; 95% CI, 0.44-0.77) compared with men who did not run (P<.001 for trend). Men who trained with weights for 30 minutes or more per week had a 23% risk reduction (RR, 0.77; 95% CI, 0.61-0.98) compared with men who did not train with weights (P = .03 for trend). Rowing for 1 hour or more per week was associated with an 18% risk reduction (RR, 0.82; 05% CI, 0.68-0.99). Average exercise intensity was associated with reduced CHD risk independent of the total volume of physical activity. The RRs (95% CIs) corresponding to moderate (4-6 METs) and high (6-12 METs) activity intensities were 0.94 (0.83-1.04) and 0.83 (0.72-0.97) compared with low activity intensity (<4 METs) (P = .02 for trend). A half-hour per day or more of brisk walking was associated with an 18% risk reduction (RR, 0.82; 95% CI, 0.67-1.00). Walking pace was associated with reduced CHD risk independent of the number of walking hours." (source)
"Total energy expenditure and energy expenditure from vigorous activities, but not energy expenditure from nonvigorous activities, related inversely to mortality. After adjustment for potential confounders, the relative risks of dying associated with increasing quintiles of total energy expenditure were 1.00 (referent), 0.94, 0.95, 0.91 and 0.91, respectively (P [trend] < .05). The relative risks of dying associated with less than 630, 630 to less than 1680, 1680 to less than 3150, 3150 to less than 6300, and 6300 or more kJ/wk expended on vigorous activities were 1.00 (referent), 0.88, 0.92, 0.87, and 0.87, respectively (P [trend] = .007). Corresponding relative risks for energy expended on nonvigorous activities were 1.00 (referent), 0.89, 1.00, 0.98, and 0.92, respectively (P [trend] = .36). Analyses of vigorous and nonvigorous activities were mutually adjusted. Among men who reported only vigorous activities (259 deaths), we observed decreasing age-standardized mortality rates with increasing activity (P = .05); among men who reported only nonvigorous activities (380 deaths), no trend was apparent (P = .99).
These data demonstrate a graded inverse relationship between total physical activity and mortality. Furthermore, vigorous activities but not nonvigorous activities were associated with longevity. These findings pertain only to all-cause mortality; nonvigorous exercise has been shown to benefit other aspects of health." (source)
"The psychological benefits of walking and jogging were compared in 52 symptomatic neurotics over an 8-week training period and subsequent 6-month follow-up. Both groups showed marked reduction of anxiety, depression and global symptoms. Joggers had greater aerobic gain, but no greater psychologic benefit. Significantly larger numbers of joggers dropped out of the study. There was no relationship between aerobic gain and reduction of symptoms at the end of the program. However, at 6 months’ follow-up, those with greater aerobic fitness had much lower anxiety levels. Changes in exercise frequency and aerobic capacity were also maintained at follow-up. Depression levels were not associated with aerobic fitness at follow-up." (source)
"Subjects included 42 healthy male and female volunteers (M age = 39.12; SD = 11.53, range 17-64 years) enrolled in four aerobics classes. The classes were randomly assigned to either low intensity (< or = 60% maximum heart rate) or high intensity (> or = 75% maximum heart rate) conditions. Subjects participated in a 50-minute bench-stepping routine at their assigned intensity level. Pre-exercise and post-exercise assessment of transient mood was assessed with the Profile of Mood States (POMS) inventory.
Results from a 2 x 2 (pre-exercise/post-exercise x high intensity/low intensity) ANOVA suggested that tension, depression, fatigue and anger decreased while vigor increased in both conditions. Additionally, subjects in the high intensity group reported less fatigue and anger than those participants in the low intensity group." (source)
Center for Disease Control (CDC) exercise intensity recommendations - http://www.cdc.gov/physicalactivity/everyone/guidelines/adults.html
The CDC recommends 150-300 minutes of moderately intense or 75-150 minutes of vigorously intense aerobic exercise per week in addition to anaerobic strength training. A heart rate monitor can be useful in getting a better sense of exercise intensity. Up to the point of overtraining (which will vary from individual to individual), the more frequently you exercise, the better.
"Patients (114 males and 16 females; age range, 32-70 years) were randomized into either a high-frequency or a low-frequency exercise training program (10 versus 2 sessions per week, respectively) as part of a 6-week multidisciplinary cardiac rehabilitation program. The General Health Questionnaire and the RAND-36 were used to assess changes in psychological distress and subjective health status.
Results. After 6 weeks, high-frequency patients reported significantly more positive, change in "psychological distress" (P < 0.05), "mental health" (P = 0.05), and "health change" (P < 0.01), than low-frequency patients. Apart from changes in mean scores, individual effect sizes indicated that a significantly greater percentage of high-frequency patients experienced substantial improvements in "psychological distress" (P < 0.01), "physical functioning" (P < 0.05), and "health change" (P < 0.05), compared with low-frequency patients. In addition, deterioration of quality of life was observed in a considerable number of high-frequency patients (ranging from 1.7% to 25.8% on the various measures)." (source)
"This study evaluated the effects of exercise frequency on functional fitness in older women participating in a 12-week exercise program. Participants (67.8 ± 4.6 years) were divided into three different exercise groups (I, II, and III; n = 34) and a control group (Group C; n = 11). Group I participated in a 90-min exercise program once a week, for 12 weeks, while Group II attended it twice a week, and Group III attended three times a week. The exercise program consisted of a 10-min warm-up, 20 min of walking, 30 min of recreational activities, 20 min of resistance training, and a 10-min cool-down. The following items were measured before and after the program: muscular strength, muscular endurance, dynamic balance, coordination, and cardiorespiratory fitness (6-min walking distance). Comparisons of baseline and post-intervention measures showed significantly greater improvements in body weight, coordination, and cardiorespiratory fitness for Group III compared to the other groups (p < 0.05). In addition, the greatest improvements in body fat, muscular endurance, and dynamic balance were also observed in Group III (p < 0.05). However, no significant differences were found in muscular strength. Older women who participate in an exercise program three times a week gain greater functional fitness benefits than those who exercise less frequently. In order to improve functional fitness in older women, an exercise frequency of at least three times each week should be recommended." (source)
"Exercise frequency may be more important than intensity in improving HDL cholesterol and LDL:HDL and total:HDL ratios in men with CHD (coronary heart disease)." (source)
"Examined the relationship between exercise frequency and measures of shyness and loneliness in 882 college students. Ss completed a self-administered survey during their regularly scheduled physical education classes which included the UCLA Loneliness Scale, the Cheek and Buss Shyness Scale, and self-report indices of exercise frequency, perceived attractiveness, current height and weight, and satisfaction with weight. Results provide evidence that among college students, not exercising or exercising infrequently is associated with higher shyness and loneliness scores. Ss who reported not exercising during an average week scored substantially higher on shyness and loneliness than did all of the other exercise frequency groups. Also, Ss who exercise 7 times a week scored significantly lower than the other exercise-frequency groups." (source)
"Experiment 1 (M age = 32.6 years, SD = 7.9) demonstrated significant (p < .05) reductions on both depression and tension scores for the medium (16-22 sessions, n = 26) and high (24-30 sessions, n = 24) frequency groups, while the low frequency group (10-15 sessions, n = 22) had a significant reduction only on tension. No significant changes were found for the wait list control group (n = 26). ANOVAs with Scheffeacute follow-up comparisons indicated significantly greater reduction on depression and tension scores for the high and medium frequency groups. Experiment 2 (M age - 32.1 years, SD = 7.7) demonstrated significant (p < .05) reductions in tension scores for groups completing 15-min (n = 20) and 30-min (n = 20) exercise sessions, but not for the no exercise control group (n = 18). Only the group completing 30 min of exercise demonstrated significant change on depression. ANOVAs with follow-ups indicated no significant differences in reduction on either psychological measure, between either exercise condition. The study provided data that may lead to determining lower end exercise frequency and duration “thresholds” for significant depression and tension reductions." (source)
"A study found the stereotypic behaviors of four children with developmental disabilities reduced by 51.6 percent in a single frequency treatment consisting of one daily ten minute walk/jog session, however, the behaviors reduced 58.9 percent following a multiple frequency conditions that involved three ten minute walk/jog session per day." (source)
"We examined the interaction of two different frequencies of aerobic exercise training (30 min at 50-60% of maximal heart rate reserve per session) and a self-administered caloric restriction program on the changes in subcutaneous (SFM) and visceral (VFM) fat mass over a period of 13 wk. Twenty-six sedentary young women (27.9% body fat) were randomized into three groups: nonexercising control (C, N = 8); 1-2 sessions/wk plus a 240 kcal caloric restriction (1-2SW, N = 9); and 3-4 sessions/wk without caloric restriction (3-4SW, N = 9). There was a equivalent decrease in the percentage of body fat and total fat mass in both exercise groups compared with that in C. Reduction in SFM was significant in 3-4SW, but not in 1-2SW or C. A negative correlation was observed between training frequency and changes in SFM (r = -0.65). In contrast, VFM decreased significantly and equivalently in both 1-2SW and 3-4SW, but there was no correlation between training frequency and changes in VFM (r = 0.20). It is suggested that the decrease in SFM, but not VFM, is proportional to the amount of aerobic exercise training." (source)
"an exercise prescription that incorporated 170 min of exercise/wk improved insulin sensitivity more substantially than a program utilizing 115 min of exercise/wk, regardless of exercise intensity and volume." (source)
"Data were analyzed for linear, quadratic, and cubic trends relative to exercise frequency. Mean values for wet weight RCCI were 98.82, 99.16, 91.04, and 90.21 mg per planimeter unit for groups exercising 0, 1, 5, and 10 times weekly (P < 0.01 for linear trend). Regressed mean ventricular dry weights exhibited expected significant linear increases of 4.89%, 5.66%, and 6.12% over sedentary control rats. The RCCI was clearly shown to undergo a significant linearly graded decrease with increasing frequencies of exercise." (source)
Center for Disease Control exercise frequency recommendations - http://www.cdc.gov/physicalactivity/everyone/guidelines/adults.html
Overtraining can be an issue if you exercise a lot but it's not an area that researchers have a clear sense of (or have any ability to objectively measure currently). If you are getting sick more often, your performance is going down, or you are suddenly more prone to injuries / joint weakness and/or instability, you may be overtraining.
"Overtraining is an imbalance between training and recovery. Short term overtraining or 'over-reaching' is reversible within days to weeks. Fatigue accompanied by a number of physical and psychological symptoms in the athlete is an indication of 'staleness' or 'overtraining syndrome'. Staleness is a dysfunction of the neuroendocrine system, localised at hypothalamic level. Staleness may occur when physical and emotional stress exceeds the individual coping capacity. However, the precise mechanism has yet to be established. Clinically the syndrome can be divided into the sympathetic and parasympathetic types, based upon the predominance of sympathetic or parasympathetic activity, respectively. The syndrome and its clinical manifestation can be explained as a stress response. At present, no sensitive and specific tests are available to prevent or diagnose overtraining. The diagnosis is based on the medical history and the clinical presentation. Complete recovery may take weeks to months." (source)
"The aim of this study was to determine whether quality of self-determined motivation at the start of the competitive season in elite athletes and symptoms of overtraining can predict athlete burnout propensity at the end of the season. The participants were 141 elite winter sport athletes. In September, at the beginning of the season, the athletes responded to a self-determined motivation questionnaire, while they answered questions assessing overtraining symptoms and burnout in March, at the end of the season. Findings indicated that self-determined motivation and symptoms of overtraining were negatively and positively linked respectively to dimensions of athlete burnout. The results suggest that self-determined motivation and symptoms of overtraining are both independently linked to signs of burnout in elite athletes and that although no moderating effect was found, pairing self-determined motivation with symptoms of overtraining increased the prediction of burnout in athletes at the end of the season. Our findings are in line with those of recent research (Cresswell & Eklund, 2005; Lemyre, Treasure, & Roberts, 2006) and support a motivational approach to study burnout in elite athletes." (source)
"Athletes experience minor fatigue and acute reductions in performance as a consequence of the normal training process. When the balance between training stress and recovery is disproportionate, it is thought that overreaching and possibly overtraining may develop. However, the majority of research that has been conducted in this area has investigated overreached and not overtrained athletes. Overreaching occurs as a result of intensified training and is often considered a normal outcome for elite athletes due to the relatively short time needed for recovery (approximately 2 weeks) and the possibility of a supercompensatory effect. As the time needed to recover from the overtraining syndrome is considered to be much longer (months to years), it may not be appropriate to compare the two states. It is presently not possible to discern acute fatigue and decreased performance experienced from isolated training sessions, from the states of overreaching and overtraining. This is partially the result of a lack of diagnostic tools, variability of results of research studies, a lack of well controlled studies and individual responses to training.
The general lack of research in the area in combination with very few well controlled investigations means that it is very difficult to gain insight into the incidence, markers and possible causes of overtraining. There is currently no evidence aside from anecdotal information to suggest that overreaching precedes overtraining and that symptoms of overtraining are more severe than overreaching. It is indeed possible that the two states show different defining characteristics and the overtraining continuum may be an oversimplification. Critical analysis of relevant research suggests that overreaching and overtraining investigations should be interpreted with caution before recommendations for markers of overreaching and overtraining can be proposed. Systematically controlled and monitored studies are needed to determine if overtraining is distinguishable from overreaching, what the best indicators of these states are and the underlying mechanisms that cause fatigue and performance decrements. The available scientific and anecdotal evidence supports the existence of the overtraining syndrome; however, more research is required to state with certainty that the syndrome exists." (source)
"Since the publication of the serotonin hypothesis, numerous theories involving the accumulation or depletion of different substances in the brain have been proposed to explain central fatigue. Although the theoretical rationale for the “serotonin-fatigue hypothesis” is clear, several seemingly well-conducted studies have failed to support a significant role for 5-hydroxytryptamine in the development of fatigue. As brain function appears to be dependent upon the interaction of a number of systems, it is unlikely that a single neurotransmitter system is responsible for central fatigue. Several other mechanisms are involved, with evidence supporting a role for the brain catecholamines. Fatigue is therefore probably an integrated phenomenon, with complex interaction among central and peripheral factors. When prolonged and excessive training happens, concurrent with other stressors and insufficient recovery, performance decrements can result in chronic maladaptations that can lead to the overtraining syndrome (OTS). The mechanism of the OTS could be difficult to examine in detail, perhaps because the stress caused by excessive training load, in combination with other stressors, might trigger different “defence mechanisms” such as the immunological, neuroendocrine, and other physiological systems that all interact and probably therefore cannot be pinpointed as the “sole” cause of the OTS. It might be that, as in other syndromes, the psychoneuroimmunology (study of brain-behavior-immune interrelationships) might shed a light on the possible mechanisms of the OTS, but until there is a definite diagnostic tool, it is of utmost importance to standardize measures that are now thought to provide a good inventory of the training status of the athlete. It is very important to emphasize the need to distinguish the OTS from overreaching and other potential causes of temporary underperformance such as anemia, acute infection, muscle damage, and insufficient carbohydrate intake." (source)
The Importance of Adequate Sleep
Slow-wave sleep (SWS) is your deepest level of sleep. During this stage of sleep growth hormone (GH) is released which is essential to the repair of your body. As you age both SWS and GH levels diminish substantially so they are highly tied to the aging process. Children spend as much as 30% of their sleep time in SWS. SWS progressively declines after you stop growing. By the age of sixty, most adults experience close to zero SWS. Maximizing SWS/GH is the best way to minimize physical deterioration and it appears improving physical fitness is essential to that. So, provided you are very physically active, you should prioritize getting as much sleep as you need.
"fit subjects had higher levels of SWS than unfit subjects. Second, following exercise, the level of SWS increased in fit, but remained unchanged in unfit subjects." (source)
"Sleep recordings were carried out on athletes on four successive nights after completing a 92-kilometer road race. Significant increases in total sleep time and slow-wave sleep were found after this metabolic stress. The results show a definite exercise effect on sleep and support sleep-restoration hypotheses." (source)
"A progressive increase in SWS over the whole-night sleep record was found with progressively increasing physical fatigue. A fall in rapid-eye-movement (REM) sleep and at higher exercise levels, of stage 2 sleep, was found. The results support the hypothesis that SWS is involved in the recovery process from fatigue." (source)
"Slow wave sleep (SWS) formed a very high percentage of total sleep in the onset (26%) and peak (31%) training periods, but was significantly reduced following precompetition taper (16%), supporting the theory that the need for restorative SWS is reduced with reduced physical demand." (source)
Overtraining is a risk which I think is increased when you are compulsive in your approach to exercise as opposed to observant and reflective. If you are not paying attention to how you feel, whether you are actually making gains, all your efforts can be wasted or even worse, harmful to your health.
"The exercise-deprived group reported significant withdrawal-like symptoms of depressed mood, reduced vigour and increased tension, anger, fatigue and confusion (measured by POMS), as well as significantly elevated RHR, within 24 hours after the missed training session. The control group showed no changes in mood or RHR." (source)
"This study examined associations among excessive exercise, eating disorders, and selected psychological characteristics in college women ( N = 586). Participants were recruited from university classes and administered the Obligatory Exercise Questionnaire, Eating Disorders Inventory--2, Bulimia Test-Revised, and other psychosocial measures. Results indicated that obligatory exercise is best viewed as multidimensional. These dimensions were used, through cluster analysis, to generate a typology of exercisers. One identified group clearly manifested eating disorder traits and behaviors, as well as signs of psychological disturbance. Another group who exercised with equal intensity but less emotional fixation showed the fewest signs of eating disorders and psychological distress." (source)
Find/determine an exercise regime you can stick to for the rest of your life. Experiment. Push yourself to improve. Find and maintain the sweet spot between undertraining and overtraining. From what I've read, most people recover from an intense workout in 24-48 hours. Experiment with 24, 36, 48 hour recovery periods and see which works best for you. In any case, your recovery period may change as your fitness improves.
Recent research points to intensive short duration training as being far more effective and efficient than longer less intense training (source). One study found that several 30 second sprints every other day resulted in significantly more endurance gains than one would expect from traditional longer endurance training (source). Another study found interval training had the same benefits as endurance training as far as aerobic endurance with the added benefit of improved anaerobic performance/endurance (source). Consequently, focusing on interval training seems to be more effective than traditional endurance training (even in improving endurance). VO2max is considered the best measure of cardiovascular health and is best improved by intense exercise (source) which is another reason to focus more on interval training.