Motivation and emotion/Book/2015/Caffeine and exercise motivation

From Wikiversity
Jump to navigation Jump to search
Caffeine and exercise motivation:
What is the effect of caffeine on exercise motivation?

Overview[edit | edit source]

Caffeine is one of the most commonly used substances in the world, and is growing in popularity among athletes due to its rapid physical and psychological effects[factual?]. Caffeine has been shown to have a positive effect on physically straining activities by increasing alertness, increasing heart rate, and reducing muscle pain (Paulus, Roth, Titus, Chen, Bridges & Woodyard, 2015). Caffeine has also been connected to several psychological effects such as improving memory, brain function and releasing serotonin and dopamine, and been linked to many psychological disorders such as anxiety and schizophrenia, as well as resulting in some specific caffeine related psychological disorders, such as caffeinism. Both the physical and psychological effects of caffeine are discussed within this book chapter, in specific relation to physical activity and motivation. Several motivation models, such as achievement motivation, self-determination theory, and exercise motivation are discussed in relation to exercise and caffeine’s effects.

What is Caffeine?[edit | edit source]

Caffeine is a natural alkaloid found in coffee beans, tea leaves, cocoa beans, and cola nuts. Due to its effective and legal nature, it has become the most commonly used behaviourally active substance in the world (Paulus, Roth, Titus, Chen, Bridges & Woodyard, 2015). It is the most widely used psychoactive drug in the world, being found in 60 known species of plants, and being included in dietary sources such as coffee, tea, cocoa beverages, chocolate, and soft drinks. Caffeine is rarely thought of as a problematic drug, due to its common dietary consumption. Coffee was first consumed in Arabia in the 13th century, and was introduced into Europe in the early 17th century. Coffee accounts for 55% of caffeine intake in the UK (Winston, Hardwick & Jaberi, 2005).

Ninety-nine percent of ingested caffeine is absorbed into the blood stream within 45 minutes of consumption, with peak plasma levels within 15 to 120 minutes and a half-life between 2.5 to 4.5 hours[factual?]. Caffeine induces changes in physiological levels of neurotransmitters such as dopamine, adrenaline, serotonin, and acetylcholine. These chemical changes have been associated with alteration in attention, mood and physiological functions. It has also been shown to enhance cognitive performance, increase alertness, reduce reaction time, increase the ability to concentrate and focus attention, and enhance short-term memory[factual?]. As well as this, it has been associated with decreasing fatigue, increasing happiness, increasing alertness, improving reaction time, and increasing vigor (Paulus, Roth, Titus, Chen, Bridges & Woodyard, 2015).

Psychological Effects of Caffeine[edit | edit source]

Caffeine can increase alertness, reduce fatigue, and elevate mood. Evidence has suggested that caffeine can have a positive effect when consumed in higher amounts[factual?]. The reinforcement of the use of caffeine is due to both pleasurable stimulatory effects and unpleasant withdrawal symptoms. Caffeine use releases dopamine in the prefrontal cortex, which is the principle area involved in both reward and addiction (Winston, Hardwick & Jaberi, 2005). Caffeine has been linked with mood-enhancing effects, and it has been found that regular consumers show more positive effects on mood than less regular consumers. Positive effects on cognitive performance have also been found to be high in caffeine consumers, and these factors contribute to withdrawal symptoms when caffeine is removed. This level of consumption has been found to be generally greater than 200 mg per day. (Attwood, Higgs & Terry, 2007).

Withdrawal from caffeine includes symptoms such as headaches, irritability, sleeplessness, confusion, nausea, anxiety, restlessness and tremor, palpitations and raised blood pressure, and are experienced by 11% of the population[factual?]. These withdrawals are at their worst within one to two days, and are rapidly relieved by caffeine intake. For mood and anxiety disorders, caffeine may have beneficial effects for depressive or low energy states, and may be detrimental for some hypersensitive patients with panic and/or performance anxiety disorder, as well as for patients with bipolar disorder. Total abstinence is not ideal, as it may be detrimental to some individuals with predominant depressive symptomatology (Lara, 2015).

Four caffeine related syndromes have been recognised in the DSM-IV (reference)[missing something?]. These include caffeine intoxication, caffeine-induced anxiety disorder, caffeine-induced sleep disorder, and caffeine-related disorder not otherwise specified. An excessive intake of caffeine can lead to a state of intoxication known as caffeinism. Caffeinism can be characterised by restlessness, agitation, excitement, rambling thought and speech, and insomnia, which are all symptoms that overlap with several other psychological disorders (Winston, Hardwick & Jaberi, 2005).

Caffeine use has also been linked with several psychological disorders such as anxiety, sleep disorders, eating disorders, and in severe cases, schizophrenia. The resemblance between over consumption of caffeine and anxiety is the most obvious and persistent of all the psychological disorders, and this may be due to the overactivity of the sympathetic nervous system. This is also evident in the high sensitivity that individuals with panic disorder and social phobia have to caffeine. The administration of caffeine to these prone individuals can onset panic attacks and other psychological consequences (Winston, Hardwick & Jaberi, 2005).

Caffeine is well known to produce insomnia, due to its physiological effects on the brain. It reduces slow-wave sleep in the early part of the sleep cycle and can reduce rapid eye movement (REM) sleep later in the cycle. This can be extremely detrimental to the quality of sleep and health and well-being of individuals. Caffeine is also believed to suppress appetites, and is therefore often consumed by individuals who suffer from severe eating disorders. This occurs in particular in individuals suffering with bulimia nervosa and anorexia nervosa, with 14.6% of individuals with bulimia nervosa having a high caffeine intake[factual?]. There are also important connections between adenosine receptors and the dopaminergic system, which may be relevant to the effects of caffeine in psychosis, and linked to schizophrenia. There is evidence that people with schizophrenia have higher than average intakes of caffeine, although the literature is inconsistent (Winston, Hardwick & Jaberi, 2005).

How Does Caffeine Effect Exercise?[edit | edit source]

The literature has found continuous evidence to support caffeine being a substance which improves physical performance. Due to its ability to induce legal improvements in exercise performance, professional athletes have taken to using it as an ergogenic supplement. It is widely accepted that caffeine has effects on performance endurance, for times longer than 30 minutes. The mode and intensity of exercise, caffeine consumption habits, fitness level, treatment dose and individual differences in caffeine digestion, distribution and sensitivity greatly influence the effects of caffeine on human performance. Performance enhancing effects of caffeine is more highly linked to trained athletes compared with non-trained athletes. Trained individuals have an increased resting metabolic rate, adrenaline levels, and free fatty acids compared with an untrained individual. This is also due to the level of motivation in higher trained athletes. Trained individuals will have a greater motivation to perform, and will use the caffeine as an additive to their regime rather than as a reliable substance. They will also have better nutritional preparation, and their day-to-day performance variation will be reduced (Tallis, Duncan & James, 2015).

Caffeine has been found to decrease the perception of an individual’s physical exertion, decrease muscle pain, and increase vigor. These variables all link to higher levels of performance. A meta-analysis concluded that caffeine ingestion reduces ratings of perceived exertion (RPE) by an average of 6%. They observed that greater reductions in RPE with caffeine correlated with larger performance improvements. Three other elements which were reported were the decrease of perceived muscle pain following caffeine ingestion, the decreased feelings of fatigue and an increase in feelings of vigor, and the stabilised levels of hydration and thermal status during moderate levels of consumption of caffeine (Ganio, Johnson, Lopez, Stearns, Emmanuel, Anderson & Armstrong, 2011).

Caffeine has been shown to improve performance on aerobatic activities, such as cycling, running, and rowing. Caffeine ingestion increases heart rate and oxygen uptake, and decreases the rating of perceived exertion. This is evidence that caffeine may be beneficial for high intensity exercise in highly fit individuals. The amount of caffeine found to exert these energy effects was 6 mg/kg, which could be attained in a large cup of coffee. It was therefore concluded that it would not be prudent to recommend these levels of caffeine intake for mild energy expenditure such as walking (Ahrens, Crixell, Lloyd & Walker, 2007).

Caffeine primarily affects the central nervous system (CNS), and easily crosses the blood brain barrier as well as cellular membranes of all tissues in the body. It has been shown to have effects on intramuscular pH, muscle force production, central fatigue and metabolic function. It seems that there are alterations on both central and peripheral systems, and is therefore likely to affect a combination of mechanisms that may be operating at the same time (Greer, 2010). It has also been found that caffeine reduces the level of muscle pain during and following exercise. Findings revealed significant reductions in leg-muscle pain when 10 mg/kg caffeine was ingested before 30 min of submaximal cycling. Caffeine seems to exert a blocking effect of peripheral receptors on sensory afferents or central blocking of adenosine A2B receptors that influence pain signalling. Caffeine also seems to reduce pain through its competing effects on adenosine. By inhibiting neuron excitability and synaptic transmission, there is an increase in arousal and a decrease in sleep (Astorino, Terzi, Roberson & Burnett, 2011). Caffeine is able to counteract many of the inhibitory effects of endogenous adenosine on neuro-excitability, neurotransmitter release and arousal, which is proposed to result in reduced sensations of effort and pain, increased alertness, improved neural firing rates and enhanced motor unit recruitment and frequency of activation (Buck, Guelfi, Dawson, McNaughton & Wallman, 2015).

As aforementioned, the Benefits associated with caffeine ingestion in this [which?] population include delayed feelings of fatigue, reduced sensations of pain and exertion, increased time to exhaustion, increased fatty acid oxidation, increased mean power output, decreased times to complete a set amount of work, stimulation of motor activity, as well as an increase in alertness, feelings of subjective energy and ability to concentrate. It has been established that the benefits associated with caffeine ingestion i.e. the ability to do more work could be useful in motivating initial exercise in individuals who are feeling a strong sense of effort (Wallman, Goh & Guelfi, 2010).

Motivation[edit | edit source]

Exercise Motivation[edit | edit source]

Regular engagement in physical activity is important for a healthy lifestyle, and has [missing something?] shown to create improvements in both physical and psychological well-being. Research has shown that regular exercise is linked to the prevention of cardio-vascular disease, type 2 diabetes, cancer, hypertension, obesity, osteoporosis, and depression (Duncan, Hall, Wilson & Jenny, 2010). Although this is common knowledge, the majority of adults remain at a very low frequency duration and intensity of exercise. It has been found that the effective promotion of physical activity has the potential to prevent as many as two million premature deaths and nearly 20 million disability-adjusted life years worldwide. Many interventions of physical activity are consistently ineffective (Weman-Josefsson, Lindwall & Ivarsson, 2015).

In 2009, data indicated that, on a typical week, 60% of adults in Europe engaged in no physical exercise or sports. In the US, less than 50% of adults are considered regularly physically active, while in Canada, new accelerometer data shows that only 15% of adults meet national physical activity recommendations[factual?]. These findings suggest that there is a heavy lack of motivation in individuals to be physically active. This can be explained by people not feeling sufficiently interested in exercise or its value outcomes to make it a priority in their lives. Individuals who experience stress and timely demands from educational, career, and family obligations may feel as though it would not be worth investing time in physical activities. People may also feel incompetent at physically activities, feeling either not physically fit enough or skilled enough to exercise or they may have health limitations that present a barrier to activity. The lack of motivation in individuals is a concern; however, the reason behind the motivation in individuals who exercise may be more controlled. Controlled motivations mean that participation in activities like going to the gym or running regularly is based on a feeling of “having to” rather than truly “wanting to” participate.

Motivation has been found to be an important and central factor in explaining human behaviour, and this applies directly to the practice of exercise. There is evidence that almost half of those who start an exercise program quit within the first six months. It has been found that eight weeks are needed for someone begining to exercise to become a regular exerciser, but even after 6 months, the motivation of initiating exercisers remains significantly lower than long-term regular exercisers. Low levels of motivation and self-efficacy, time-shortage, low familiarity with exercise, and poor social and cultural support are considered the primary reasons why individuals fail to adhere to physical exercise programs after they begin. A study found many individuals to believe that exercise will not bring any benefit or that they are not able to perform exercise satisfactorily (Rosa, de Souza, de Lima, Rodrigues, de Aquino Lemos, da Silva Alves, de Mello, 2015).

The theory of planned behaviour suggests that the determinant of behaviour is one’s intention to engage in that specific behaviour[factual?]. This intention is influenced by three factors: attitudes, subjective norms, and perceived behavioural control. Attitudes can be positive or negative, representing overall evaluations regarding a specific behaviour. Subjective norms reflect the individual perception of social pressures from significant people to assume or not assume a specific behaviour. Perceived behavioural control assesses the individual perception of control in order to assume or not assume a specific behaviour (Martinent, Guillet-Descas & Moiret, 2015).

Another motivating factor of physical activity involves the human evolutionary perspective, where physical activity was the salient feature of human lifestyle historically. Our ancestors were physically active for other reasons than what we know today, as their way of life required them to be. The lifestyle demanded many daily physically straining activities. This includes activities such as the style of hunt-gathering life, which was composed of numerous physical activities, for example, running to capture wounded prey, transporting children, building shelters, and social activities, such as visits to neighbouring camps, dances as part of religious ceremonies or for recreation. The increase in physical activity created an increase in aerobic and physical capacity allowed access to new food resources, influencing the development of brain components and consequently cognitive improvement in human. It has also been noted that physical exercise associated with substance has been associated with brain size and cognitive function during human evolution, increasing neurotrophins and growth factors that led to brain development. This leads to selective traits, for example, a strong selection to running long distances in humans (Rosa, de Souza, de Lima, Rodrigues, de Aquino Lemos, da Silva Alves, de Mello, 2015).

Achievement Motivation[edit | edit source]

Regular exercisers have perceived themselves to have more autonomy, personal growth, and purpose in life, positive relations with others and conditioning than non-exercisers. Although motivation has been found to have long-term stability, it has also been found to be dependent on situation and context, as well as personality. The Big Five traits are positively associated with exercise motivations including health, appearance, stress management, enjoyment and weight control, with Extraversion and Openness identified as the traits that most positively related to exercise enjoyment and frequency. Although personality traits independently effect exercise frequency separate to motivation, the influence of traits will, to a certain extent, be expressed through the motivational system (Lewis & Sutton, 2011).

The social cognitive model of achievement motivation characterises challenge seeking, high persistence, exerted effort, enjoyment and task mastery as achievement striving behaviours. Maladaptive behaviour is described as avoidance of challenging situations, low persistence when faced with obstacles, low enjoyment, and a performance orientation. This model relies on three main constructs: implicit theories of ability, achievement goals and perceived competence. Exercise performance would be motivationally adapted, and would be the focal goal point. When low perceptions are theorised, learned helplessness is exhibited (Stevenson & Lochbaum, 2008).

Variations in motivation describe why some people choose optimally challenging tasks, try harder, and persist longer than others. Achievement goals are a large factor in supporting this, and have been suggested to differentially influence the behaviours of physical activity participants. The goal perspective theory states that individuals strive to display high ability and avoid demonstrating low ability. An individual’s definition of success and failure is identified through two goal orientations; a task goal orientation and an ego goal orientation. A task goal orientation is defined as an individual’s personal and inner beliefs as to their own personal competence, with an emphasis on mastery of skills and performance improvement. An ego goal orientation is perceived as the norm-referenced perceptions of competence and holds an emphasis on favourable and positive comparisons between oneself and others (Hayashi, 1996).

In relation to sports and physical exercise, it has been found that task-oriented individuals believed that the purpose of athletic involvement was to foster an ability to cooperate and strive for personal mastery. Task-orientated athletes were also found to focus on adaptive achievement strategies, for example, persistence. Contrariwise, the purpose of athletic involvement for ego-oriented individuals was to feel important and be competitive. As well as this, ego-orientated individuals were found to focus on maladaptive strategies, for example, chance and social approval. These differences in motivation orientation have also been established cross culturally, with Anglo-American marathon runners having higher levels of competitiveness, and Japanese runners having greater levels of win-orientation (Hayashi, 1996).

Ames proposed that achievement motivation differs in response to various goal/reward structures. These structures have been defined as a setting that influences an individual's achievement goals, basis for evaluation, and rules for interacting with others. Three goal/reward structures have been established: competitive (individuals work against each other in the pursuit of a reward), individualistic (an emphasis on personal effort and skill mastery), and cooperative (the pursuit of shared goals) (Hayashi, 1996).

Self Determination Theory[edit | edit source]

The self-determination theory (SDT) posits that individuals have a predisposition to be active and to move their lives in desired and specific directions, rather than being passive and completely subjected to environmental forces. SDT considers the individuals interactions within the environment, and this framework is based on the concept of “need”. When SDT is applied to is applied to physical activity, there is an element of attempting to understand it, explore it, and deepen the practice. This behavior is commonly associated with feelings and sensations of interest, psychological well-being and joy (Rosa, de Souza, de Lima, Rodrigues, de Aquino Lemos, da Silva Alves, de Mello, 2015).

Figure 1. psychological components of need according to the self determination theory

SDT is the most common and universal method of looking at the motivation of exercise in individuals. The framework of SDT suggests that human motivation lies along a continuum which represents varying degrees of autonomy. Autonomy refers to behaviours being self-determined, or freely initiated by the individual. Within this continuum, motivation is determined by either intrinsic or extrinsic components (See figure 1). Intrinsic motivation, also known as autonomous motivation, stays at the most self-determined end of the continuum, and involves motivation derived from the pleasure and satisfaction of engaging in the behaviour itself. The individual will formulate goals that are mainly focused on developing their personal interests, values, and potential. For example, an exerciser who is intrinsically motivated might run, as they thoroughly enjoy the feeling of the wind moving through their body (Duncan, Hall, Wilson & Jenny, 2010). Extrinsic motivation, also known as controlled motivation, suggests that the individual engages in activities due to consequences. The individual formulates and bases their goals on outward orientation, and less on self-determination. This means that motivation lies in external indicators of worth, such as wealth, fame, and appealing image (Martinent, Guillet-Descas & Moiret, 2015).

A review of SDT in relation to physical activity has shown that greater participation results from higher motivation techniques such as affiliation and social engagement, challenge, and skill development. It highlights amotivation, which is a motivational condition observed in individuals who are not adequately able to identify a good reason to perform any physical activity. In order to combat this, it has been hypothesised that a higher degree of satisfaction of needs is associated with increased exercise through a more self-determined motivation. In saying this, it has been established that variable factors could contribute to the differences across motivation. When looking specifically into physical activity and exercise, factors such as age, culture and gender do play an important role. Considering social context support, as well as how these behavioural regulations emerge is also significant (Weman-Josefsson, Lindwall & Ivarsson, 2015). Integrated regulation is the belief that an individual holds in relation to a behaviours significance to their identity and consistency with personal values. It refers to being motivated to perform a behaviour because it is personally significant and results in outcomes which are valued by the individual. For example, an individual might go running because they believe they are a ‘runner’, and may carry out this behaviour as they know it is good for their health (Duncan, Hall, Wilson & Jenny, 2010).

SDT also proposes the concept of dispositional tendencies, named causality orientations. This describes the way people preferentially orient towards their environments, resulting in characteristic motivational and behavioral patterns. Some individuals may feel more motivated and inclined to seek out and follow their internal indicators of preference in choosing their course of action, others may more naturally tend to align with external directives and norms, while still others may reveal to be generally amotivated, more passive, and unresponsive to either internal or external events that could energize their actions (Teixeira, Carraça, Markland, Silva & Ryan, 2012).

Quiz[edit | edit source]

1 How many species of plants can caffeine be found in?


2 Caffeine can be most positively linked to which of the following psychological disorders?

Narcissistic personality disorder

3 In the self determination theory, Intrinsic motivation is also known as which of the following?

Perceived motivation
Extrinsic motivation
Autonomous motivation
Controlled motivation

Conclusion[edit | edit source]

Caffeine has the capacity to affect motivation to undertake physical activity in mild forms[factual?]. Caffeine can improve an individual’s perceived ability to perform at a higher level, in particular, in trained individuals who are preparing for performance. Individuals who have a high intake of caffeine have also shown both physical and psychological reactions, such as increase alertness, reduce fatigue, and elevate mood (Winston, Hardwick & Jaberi, 2005). It[what?] has also been shown to enhance cognitive performance, reduce reaction time, increase the ability to concentrate and focus attention, and enhance short-term memory (Paulus, Roth, Titus, Chen, Bridges & Woodyard, 2015). These are all factors that enhance the motivation to undertake a higher level of physical performance[factual?]. Several models of motivation that explain physical activity, such as the self-determination theory, suggest that there is an internal need, and these behaviours satisfy those needs. Caffeine is a substance that enhances the motivation for behaviours such as physical activity (Rosa, de Souza, de Lima, Rodrigues, de Aquino Lemos, da Silva Alves, de Mello, 2015)[Provide more detail].

See also[edit | edit source]

Self-determination theory

Motivation and exercise

Sport and emotion

References[edit | edit source]

Ahrens, J. N., Crixell, S. H., Lloyd, L. K., & Walker, J. L. (2007). THE PHYSIOLOGICAL EFFECTS OF CAFFEINE IN WOMEN DURING TREADMILL WALKING. Journal Of Strength & Conditioning Research (Allen Press Publishing Services Inc.),21(1), 164-168.

Astorino, T. A., Terzi, M. N., Roberson, D. W., & Burnett, T. R. (2011). Effect of Caffeine Intake on Pain Perception During High-Intensity Exercise. International Journal Of Sport Nutrition & Exercise Metabolism, 21(1), 27-32.

Attwood, A. S., Higgs, S., & Terry, P. (2007). Differential responsiveness to caffeine and perceived effects of caffeine in moderate and high regular caffeine consumers. Psychopharmacology, 190(4), 469-477. doi:10.1007/s00213-006-0643-5

Buck, C., Guelfi, K., Dawson, B., McNaughton, L., & Wallman, K. (2015). Effects of sodium phosphate and caffeine loading on repeated-sprint ability. Journal Of Sports Sciences,33(19), 1971-1979. doi:10.1080/02640414.2015.1025235

DOUGLAS G., B., & TOM M., M. (2003). Effect of Repeated Caffeine Ingestion on Repeated Exhaustive Exercise Endurance. Medicine & Science In Sports & Exercise,35(8), 1348-1354.

Duncan, L.R., Hall, C.R., Wilson, P.M., & Jenny, O. (2010). Exercise motivation: a cross-sectional analysis examining its relationships with frequency, intensity, and duration of exercise. International Journal of Behavioral Nutrition and Physical Activity, 7(7), doi:10.1186/1479-5868-7-7

Ganio, M. S., Johnson, E. C., Lopez, R. M., Stearns, R. L., Emmanuel, H., Anderson, J. M., & ... Armstrong, L. E. (2011). Caffeine lowers muscle pain during exercise in hot but not cool environments. Physiology & Behavior,102(3/4), 429-435. doi:10.1016/j.physbeh.2010.12.005

Greer, F. (2010). Myth buster: Caffeine does not exhibit a diuretic effect during exercise performance. Indian Journal Of Medical Research, 132(1), 11-13.

Hayashi, C. T. (1996). Achievement motivation among Anglo-American and Hawaiian male physical activity participants. Journal Of Sport & Exercise Psychology, 18(2), 194-215.

Lara, D.R. (2015). Caffeine, mental health, and psychiatric disorders. Journal of Alzheimer’s disease, 20(2010), 239–248. doi:10.3233/JAD-2010-1378

Lewis, M., & Sutton, A. (2011). Understanding Exercise Behaviour: Examining the Interaction of Exercise Motivation and Personality in Predicting Exercise Frequency. Journal Of Sport Behavior, 34(1), 82-97.

Martinent, G., Guillet-Descas, E., & Moiret, S. (2015). A Reciprocal Effects Model of the Temporal Ordering of Basic Psychological Needs and Motivation. Journal Of Sport & Exercise Psychology, 37(2), 117-126. doi:10.1123/jsep.2014-0093

PAULUS, R., ROTH, A., TITUS, L., CHEN, R., BRIDGES, M. C., & WOODYARD, S. (2015). Impact of Various Caffeine Vehicles on Mood and Cognitive, Neurpological and Physiological Functions Over Five Hours. Ohio Journal Of Science, 115(2), 51-62.

Rosa, J. P., de Souza, A. L., de Lima, G. O., Rodrigues, D. F., de Aquino Lemos, V., da Silva Alves, E., & ... de Mello, M. T. (2015). Motivational and evolutionary aspects of a physical exercise training program: a longitudinal study. Frontiers In Psychology, 61-9. doi:10.3389/fpsyg.2015.00648

Stevenson, S. J., & Lochbaum, M. R. (2008). Understanding Exercise Motivation: Examining the Revised Social-Cognitive Model of Achievement Motivation. Journal Of Sport Behavior, 31(4), 389-412.

Tallis, J., Duncan, M.J., James, R.S. (2015). What can isolated skeletal muscle experiments tell us about the effects of caffeine on exercise performance? British Journal of Pharmacology, 172 3703-3713. doi: 10.1111/bph.13187

Teixeira, P. J., Carraça, E. V., Markland, D., Silva, M. N., & Ryan, R. M. (2012). Exercise, physical activity, and self-determination theory: A systematic review. International Journal Of Behavioral Nutrition & Physical Activity, 9(1), 78-107. doi:10.1186/1479-5868-9-78

Wallman, K. E., Goh, J. W., & Guelfi, K. J. (2010). Effects of caffeine on exercise performance in sedentary females. Journal Of Sports Science & Medicine, 9(2), 183-189.

Weman-Josefsson, K., Lindwall, M., & Ivarsson, A. (2015). Need satisfaction, motivational regulations and exercise: moderation and mediation effects. International Journal Of Behavioral Nutrition & Physical Activity, 12(1), 1-11. doi:10.1186/s12966-015-0226-0

Winston, A.P., Hardwick, E., & Jaberi, N. (2005). Neuropsychiatric effects of caffeine. Advances in Psychiatric Treatment, 11(6)432-439. doi:10.1192/apt.11.6.43211

External links[edit | edit source]