Caffeine is commonly used as an ergogenic aid by many athletes due to the almost unequivocal support in the literature on the beneficial impacts of ingesting caffeine on submaximal performance. Its effects have been found to improve upon many paradigms including aerobic, anaerobic and muscular strength through the enhancement of both mental and physical performance (Killen et al, 2013).
Studies have shown that, as compared with a placebo, caffeine improved performance by approximately 11%, however, this finding does not extend itself to exercise termination, as this typically coincides with physiological limits such as maximal oxygen consumption or maximal heart rate, which have not been found to be interfered with through the consumption of caffeine (Doherty Smith, 2005). The purported mechanisms behind this have spanned from the augmented mobilisation of free fatty acids and associated glycogen sparring, sympathetic nervous system enhancement, reduction in central fatigue, depletion of neuromuscular conduction block and a potentiation of muscular force output for a given input. However, the results behind these proposed mechanisms remain equivocal, with each lacking consistent support (Doherty & Smith, 2005).
Although the exact reasons behind augmented performance remain elusive, there is one result which continues to appear, and that is that moderate caffeine ingestion during submaximal exercise leads to an attenuated rating of perceived exertion (RPE). Borg (1962) developed this rating system (from 1 – 20) as a means of acute estimation of effort during exercise, with higher reading signifying that the individual is closer to reaching volitional exhaustion. A greater RPE scope (RPE end of exercise – RPE during exercise) thus allowing athletes a greater capacity to sustain exercise when experiencing feelings of discomfort, ultimately masking feelings of fatigue when performed under conditions of voluntary exhaustion (Killen et al, 2013).
Consuming between 2 – 5 mg·kg-1 of caffeine has been found to be adequate to augment performance, with levels any greater than 6 mg·kg-1 being found to be deleterious to performance due to increase heart rate and over arousal. This could consequently effect concentration and hinder technique (Ganio et al, 2009).
Caffeine’s ergogenicity has also been found to extend to short-term high-intensity exercise, where moderate caffeine ingestion has been found to increase power output as compared to a placebo trial (Doherty et al, 2004). This finding was synonymous with augmented blood lactate concentrations suggesting that a higher workrate was achieved in the caffeine trial, whilst subjects simultaneously achieved decreased RPE readings (Doherty et al, 2004). This finding has been stipulated as having important ramifications for novel nutritional strategies, where athletes complete training in a fasted or glycogen depleted state in order to promote cellular and molecular adaptations. However, as athletes may find it difficult to perform with low glycogen availability, the addition of caffeine has been found to partially rescue the reduction in power output associated with training under such conditions (Lane et al, 2013).
On a cellular level, it is stipulated that caffeine works at various sites across the body through the antagonism of adenosine A1 and/or A2a receptors on the skeletal muscle membrane (Lynge & Hellston, 2000) consequently improving excitation-contraction coupling. This is proposed to occur via an increased release of Ca2+ from the sarcoplasmic reticulum and/or improved Na+/K+ ATPase pump activity (Hodgson et al, 2013). This can then act upon the central nervous system, whereby a decrease in the neuronal activation threshold of motorneurons and/or alterations in muscle contraction force result in an attenuation of muscle sensory processing that could reduce RPE due to a larger amount of motor units being recruited for a given task, therefore force for a given task would be greater (Doherty et al, 2004). An alternative explanation which offers an interpretation of this relationship, is that modified perceptual response during exercise could allow athletes to centrally recruit and engage more motor units and hence allowing for increased power output, as hypothesised during studies whereby participants selected higher exercise intensity with caffeine than without (Cole et al, 1996).
Additionally to this, but by no means independently, caffeine has been associated with analgesic properties with reference to both ischemic muscle contractions (Myers et al, 1997) and whole-body ones (O’Conner et al, 2004). This proposition ties in with research conducted in 1982 by Laska and colleagues, who stipulated that the analgesic effect expressed by patients in the study was primarily a result of changes in mood and overall sense of wellbeing. This theory can thus be applied to reductions in RPE observed during exercise. Backhouse et al (2004) identified the accompanying mood states associated with self-rated happiness, calmness and alertness during exercise can offer a better understanding of why perceptual response is affected by caffeine ingestion. However, it would appear most likely that the relationship between caffeine and perceptual response is multifactoral, and this has consistently manifested itself as an increase in work output at a given RPE or effort sense (Doherty & Smith, 2005).
In conclusion, due to the lack of understanding surrounding perceptual response to exercise and the multifarious effects of caffeine on the body, the exact causes of the reduction in RPE remains unknown. As a result, more studies are needed to study the interactions between performance, caffeine, and RPE whilst accounting for possible mediators such as mood, pain and cardiorespiratory dynamics (Doherty & Smith, 2005). One of the proposed implications of a greater understanding of caffeine responses with regards to RPE, could result in an effect tool of improving physical activity adoption and maintenance through the analysis of pre and post exercise ratings. Session RPE which can be obtained 20-30 minutes following the termination of the exercise bout, and acts as a subjective estimation of the difficulty of the entire exercise session, thus helping to detect overtraining in athletes (Killen et al, 2013). The addition of caffeine has been found to decrease levels of session RPE, thus potentially allowing coaches to examine more ways in which the sessions could be altered and help to monitor athlete more effectively (Killen et al, 2013).
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