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The Effect of Caffeine on Heart Rate During Submaximal Exercise as Subjects Become Habituated to Caffeine Melody S. Axtell Mentor: Ted Anderson, Ph.D., Kinesiology Committee: Ryan Botts, Ph. D., Math/Computer Science, Brandon Sawyer, Ph.D., Biology/Kinesiology Abstract
Caffeine is the most commonly used stimulant in the world. Caffeine is consumed by athletes
and fitness enthusiasts to boost performance in both aerobic endurance events such as a
marathon, as well as anaerobic high intensity activities such as weight lifting. Common means
of consumption of caffeine include drinking coffee before exercise or taking a pre-workout
energy drink in which caffeine is usually combined with additional ergogenic ingredients. With
regular use, caffeine's effectiveness can diminish. Recent research suggests that tolerance, or
habituation, may be established in a very short amount of time. This study examines the rate at
which college students become habituated to the effect of caffeine as measured by their daily
performance of a sub-maximal fitness test, the Queens College Step Test. Results of this study
will be used as a first step in defining a schedule of caffeine supplementation that maximizes
caffeine effectiveness in terms of physical performance while avoiding tolerance or habituation
issues. The results of this study confirm that a tolerance to caffeine can be established with four
days of continual use, but that the rate at which habituation and tolerance to caffeine occurs are
specific to individuals. This study also confirms that low doses of caffeine (3mg/kg) are
effective at producing enhanced physical performance in submaximal exercise.
Caffeine as an ergogenic aid is supported by previous research.1 Caffeine can improve physical
performance for both endurance and power activities, as well as being able to mask feelings of
fatigue.1 These three areas of physical performance are affected by caffeine in different ways. It
is thought that caffeine improves endurance performance by increasing free fatty acid availability
thus sparing glucose during endurance events. This is accomplished by stimulating the
production of hormone sensitive lipase by increasing catecholamine levels.2,3 There are many
factors which alter caffeine's effect on heart rate in individuals during exercise. The factors are
important to understand for athletes interested in taking caffeine as an ergogenic aid, these
factors include the activity being performed, the structure of caffeine and how it metabolizes
physiologically, fitness level and gender, and timing of ingestion of caffeine.
Caffeine is popular for it's effect on energy levels. Caffeine molecular structure is similar to that
of adenosine.4 This makes caffeine a competitive inhibitor to adenosine. When adenosine
receptors bind with adenosine molecules our bodies experience relaxation, fatigue, and sleep, but
when adenosine receptors are bound to caffeine molecules, signals for the body to relax, feel
fatigued and to ultimately sleep are not sent. This is thought to be the mechanism whereby
caffeine blocks signals of fatigue.5 Caffeine does not necessarily give us energy, but it allows for
the biochemical pathway to increase energy and delay fatigue, which may improve athletic
Caffeine may also affect power for lifting and athletic performance. Caffeine may stimulate the release of additional amounts of calcium from the muscle's sarcoplasmic reticulum. Increased calcium release leads to the opening of more actin/myosin binding sites resulting in more forceful muscular contractions.4 Caffeine is also a stimulant, it is therefore logical to assume that heart rate will increase when ingesting caffeine, but this is not always the case. Current research suggests that caffeine will lower the heart rate of fit, unhabituated caffeine users during submaximal exercise.6,7,8. Previous research has suggested multiple ways in which caffeine consumption might result in a lower heart rate. Caffeine may cause a more powerful muscular contraction of the cardiac tissue which results in a greater stroke volume and a lowered heart rate but with the same cardiac output (Cardiac Output = HR X SV).9 Caffeine is a non-selective antagonist of adenosine in the A1 receptors, which when stimulated inhibits the release of norepinephrine at sympathetic smooth muscle junctions, and as A2 receptors which have a direct vasodilatory effect. 10,11 Another study suggests that caffeine increased cAMP (Cyclic adenosine monophosphate) concentration during exercise which is compatible with the caffeine antagonism of A1 receptors in skeletal muscle. 13 The fitness level of each individual affects how they will respond to caffeine. Well trained individuals have a greater response to caffeine when compared to untrained individuals at rest 7. Trained individuals have a larger increase in adrenaline, and resting metabolism.7 Researchers have also speculated that caffeine may have more impact on highly trained individuals due to trained muscle having more a sensitive response to caffeine, or that the athlete is able to exercise for a period of time long enough for the caffeine stimulus to have an effect on the slow twitch fibers. 13 There are detrimental effects to caffeine use as well, which should be considered when taking caffeine as an ergogenic aid. It has been reported that exposure to caffeine for as little as 3 days is sufficient to produce withdrawal symptoms.14 Withdrawal symptoms may reduce the athletes' performance, making the avoidance of withdrawal a priority. Understanding habituation effects of caffeine is essential if caffeine is to be used as an effective ergogenic aid. With regular caffeine use a tolerance to it's effects is created. Physical performance may no longer demonstrate the beneficial effects found when caffeine was initially used by naïve subjects. If subjects stop using caffeine, withdrawal effects might actually hamper performance. Currently, there is no known schedule of use and abstinence of caffeine that would maximize the positive effects of caffeine on physical performance while minimizing the potential negative effects. This study examines both timing and dosage issues associated with caffeine use. Dosages used in previous research have varied widely. Most studies have used between 3-9 mg/kg15 and Improved performance was demonstrated in all of these studies at each dosage level. The studies using large doses of 9 mg/kg showed some detrimental performance effects. Conversely, lower dosages of 3-5 mg/kg and repeated doses of 1-2 mg/kg produced superior results compared to single high doses.14 Therefore, we selected a 3 mg/kg dosage as it is considered a low and safe dose. As for the timing of caffeine ingestion, a common protocol in caffeine studies is to have subjects ingest caffeine 1 hour before exercise because plasma levels of caffeine reach their
maximum in 1 hour.13
By the daily testing of caffeinated subjects, this study examines the rate at which subjects
become habituated to the effects of caffeine during submaximal exercise, measured by heart rate.
Equipment required: 16.25 inch step (gym bleacher), stopwatch, metronome, Polar heart rate
monitor and computer software (Polar ProTrainer 5-Version 5.41.002). Personalized dosage of
caffeine (Pure anhydrous powder USP grade) capsules(double "00" Vegetable Cellulose
capsules) containing 3mg of caffeine per kg of body weight.
Subjects: Six subjects were initially tested but only three subjects provided consistent HR results
on their first two step tests. These three subjects had the same heart rate average during their
control step tests without caffeine. These 3 college students (two male, one female), 19-21 years
of age, were all caffeine free for two weeks prior to participating in the study.
Queen's College Step Test:
Submaximal exercise; this sub-maximal test provides a measure of cardiorespiratory or
endurance fitness. Subjects relative VO2max was gathered from their average control HR
without caffeine using this equation
VO2max from Step test:
Formula for men: VO2max (ml/kg/min) = 111.33 - (0.42 X HR) Formula for women: VO2max (ml/kg/min) = 65.81 - (0.1847 X HR) Recovery Avg: 118.6 111.33 - (0.42 x heart rate ( 118.6) ) 49.812 VO2 Max= 61.518(ml/kg/min
Recovery Avg: 126.5 111.33 - (0.42 x heart rate (126.5) ) 53.13 VO2 Max=58.2(ml/kg/min
Recovery Avg: 85.2 = 65.81 - (0.1847 x heart rate (bpm) ) 15.73 VO2 Max= 50.08(ml/kg/min

This chart gives an overview of the subjects and their weight, dosage, as well as vo2max result from the submaximal step test and their standings regarding fitness. Statistical Analysis:
Treatment of the data by statistical analysis is problematic due to slight variations within the
overall trend of the heart rate curve. Picking an exact point in the curve such as the highest heart
rate or the heart rate at the 3 minute mark may be an aberrational point of the overall trend. To
avoid the possibility of a single point misrepresenting the greater trend, for this study, caffeine
tolerance was indicated when 50% of the caffeinated heart rate curve matched the original non-
caffeinated heart rate curve.
Procedure: Subjects sign off on informed consent and are given the information for the study
before they begin. Subjects consume their personalized dosage of caffeine in capsule form 1 hour
before their scheduled step test. Subjects meet investigators in the gym at their assigned time
where they put on a heart rate monitor that has been set to record subject's heart rate every 5
seconds. Subjects step up and down on the 16.5" bench at a rate of 22 steps per minute for
females (88 bpm) and at 24 steps per minute (96 bpm) for males. Subjects step in unison to a
four-step cadence, 'up-up-down-down' for 3 minutes. At the 3 minute mark, the subjects stop
stepping, remain standing, breathe deeply trying to relax and recover for 1 minute. The heart rate
information is then downloaded from the subject's monitor to a computer where it is analyzed
using Polar Heart Rate software (Polar ProTrainer 5-Version 5.41.002). The graphs from the
appendices were created with this software. The caffeinated step tests continued daily until the
subject's caffeinated step test results matched their first 2 days non-caffeinated results in terms of
their heart rate results.
Testing schedule:
Days 1 & 2 – no caffeine, step test, used to verify step test HR consistency.
Days 3 – 6 caffeine, step test, used to determine caffeine's effect on step test HR.
The study began with all subjects performing a caffeine free step test on two consecutive days.
The results of these two step tests were compared to each other to establish that subjects will
produce consistent daily results. Three subjects produced consistent HR results on their first two
step tests and continued to the caffeinated portion of the study. Consistent meant their HR was
within 4 bpm in each control step test on these first two days.
Two subjects matched their non-caffeinated and their caffeinated results on their fourth
caffeinated day. The remaining subject matched their non-caffeinated heart rate results on
caffeinated day six.
It was expected that all subjects would have lower heart rates with the initial use of caffeine as
compared to their initial non-caffeinated step test. This was not the case. The two male subjects
showed a lower heart rate during the caffeine trial; the female subject's initial response to
caffeine was an elevated heart rate. See figures 1-3 for Heart Rate data for subjects:
The heart rate charts below show three rate curves for each subject in figures 1-3:
1. The initial step test without caffeine
2. The first step test after caffeine was ingested, which also resulted in the greatest
3. The caffeinated step test that matched the initial non-caffeinated heart rate curve, regardless of how many days it took to return to initial levels. This was considered to be a demonstration of habituation or caffeine tolerance.
Results of this study are the first steps in discovering a schedule of caffeine supplementation that
maximizes caffeine effectiveness in terms of physical performance while avoiding tolerance or
habituation issues. For example, will ingesting caffeine on an every other day, every third day or
once per week schedule eliminate habituation effects? This subject is not highly researched yet,
however one study provides the data that a moderate 3-5 mg/kg dose of caffeine before exercise
and 1-2 mg/kg repeated doses of caffeine given during prolonged exercise could be superior to a
single large (9mg/kg) dose. The researcher suggests further study to investigate whether an
athlete who regularly ingests caffeine needs to withdraw from caffeine before using it in
competition? If so, what length of time is optimal? Would the days of experiencing lethargy and
so on, during the withdrawal, affect the athlete?13 It is clear that timing of caffeine and
habituation is a subject worth investigating for athletes as they search to find their way to their
best performance.
This study suggests that daily use of caffeine is not an effective schedule of caffeine
supplementation, as subjects in this study showed habituation effects within a week of use.
Within 4 days, two subjects showed no performance effects, and within 6 days the remaining
subject also showed no performance effect.
Factors that may influence the effect of caffeine include the schedule of caffeine consumption,
the amount of the dosage, gender, physical training, and body composition (both body fat and the
amount of muscle).
The subjects in this study were all physically fit according to their VO2max calculated from their
submaximal step test . As such, it was expected that all of the subjects would initially see a
reduced heart rate during their submaximal step test.1 This was not the case for one of our
subjects who demonstrated a higher heart rate with the initial use of caffeine. Three individual
characteristics of this particular subject need to be further investigated as to their impact on the
unexpected results. These characteristics include gender, as this subject was the only female in
the study. Some studies suggest that females and males respond to caffeine differently.16 Skeletal
muscle in men may have a greater muscle sensitivity to caffeine than females.16 These results
should be investigated further in regards to subjects amount of lean muscle mass as well as their
muscle fiber type. Slow twitch muscle cells have been reported to be more responsive to the
ergogenic effect of caffeine.17 The results of this study are consistent with this study as our male
participants had a greater ergogenic response to caffeine during our step than our 1 female
Previous research shows strong support for physically fit athletes having lower heart rates with
caffeine while performing at submaximal levels. 13 While the female subject was well trained (
track athlete), her extremely small physical size (under 100 lbs.) may have mediated her resulting
raised heart rate while using caffeine. Perhaps the formula used in this study, mg/kg, needs to
be adjusted to account for body composition as well as total body weight. The two male subjects
had considerably more muscle than the female subject. There is research that suggests that
caffeine acts directly on muscle tissue, so it might make sense to give caffeine doses that take the
amount of skeletal muscle into account. The formula of 3 mg/kg used in this study only took
total body weight into account when calculating the proper dosage. A formula that utilizes
muscle mass as a factor for determining dosage, should be researched.
This study provided a pilot for future research regarding caffeine and it's effect on heart rate during submaximal exercise. Further research is needed to analyze the habituation effect of caffeine and a schedule that would allow for optimum ergogenic effects for athletes. Further research is also encouraged to look into the dosage amount of caffeine, as well as the calculation of skeletal muscle rather than overall body weight or just lean body weight. Caffeine can be an effective ergogenic aid, discovering more about how to most effectively use caffeine will benefit athletes and fitness enthusiasts. Limitations and Strength section:
Limitations of this study include small population size of 3 subjects and limited time. This does
not provide a statistically significant population. Ideally we would have a larger population, and
test them for a longer period of time to further research habituation timing. Originally the study
was supposed to be a crossover study, where we compared caffeine to a placebo of vegetable
cellulose. Another aspect that would be interesting with this is comparing fit vs. unfit with our
own population where VO2max testing would be conducted to determine subjects fitness levels
before. This study provides those interested in this topic to view some trends in habituation we
found in our 3 subject population, and in turn provide a pilot study for further research.
1.Palmer TM, Stiles GL. Review: neurotransmitter receptors VII: Adenosine receptors. Neuropharmacology 1995; 34: 683-94 2.Essig D, Costill DL, Van Handel PJ. Effects of caffeine ingestion on utilization of muscle glycogen and lipid during leg ergometer cycling. Int J Sports Med 1980; 1: 86-90 3. Kovacs EMR, Stegen JHCH, Brouns F. Effect of caffeinated drinks on substrate metabolism, caffeine excretion, and performance. J Appl Physiol 1998; 85: 709-15 4. Shryock JC, Belardinelli L. Adenosine and adenosine receptors in the cardiovascular system: biochemistry, physiology, and pharmacology. Am J Cardiol 1997; 79: 2-10 5. Fredholm BB. Adenosine, adenosine receptors and the actions of caffeine. Pharmacol Toxicol 1995; 76: 93-101 6. McClaran SR, Wetter TJ. Low doses of caffeine reduce heart rate during submaximal cycle ergometry. JISSN. 2007; 4:11 7. LeBlanc J, Jobin M, Cote J, et al. Enhanced metabolic response to caffeine in exercise-trained human subjects. J Appl Physiol 1985; 59: 832-7 8. Pasman WJ, van Baak MA, Jeukendrup AE, et al. The effect of different dosages of caffeine on endurance performance time. Int J Sports Med 1995; 16: 225-30 9. Daniels JW, Mole PA, Shaffrath JD, et al. Effects of caffeine on blood pressure, heart rate, and forearm blood flow during dynamic leg exercise. J Appl Physiol 1998; 85:154-159 10. Smits P, Boekema P, De Abreu R, Thien T, Van't Laar A. Evidence for the antagonism between caffeine and adenosine in the human cardiovascular system. J Cardiovasc Pharmacol. 1987;10:136–143. 11. Pincomb GA, Lovallo WR, Passey RB, Whitsett TL, Silverstein SM, Wilson MF. Effects of caffeine on vascular resistance, cardiac output and myocardial contractility in young men. Am J Cardiol. 1985;56:119–112. 12.Smith, Abbie E., Christopher M. Lockwood, Jordan R. Moon, Kristina L. Kendall, David H. Fukuda, Sarah E. Tobkin, Joel T. Cramer, and Jeffrey R. Stout. "Physiological Effects of Caffeine, Epigallocatechin-3-gallate, and Exercise In overweight and Obese Women." Applied Physiology, Nutrition, and Metabolism 35.5 2010; 607-16. 13. Graham TE, Spriet LL. Performance and metabolic responses to a high caffeine dose during prolonged exercise. J Appl Physiol 1991; 71: 2292-8 14. Palmer TM, Stiles GL. Review: neurotransmitter receptors VII: Adenosine receptors. Neuropharmacology 1995; 34: 683-94 16. Kalow W. Pharmacogenetic variability in brain and muscle. J Pharm Pharmacol 1994; 46: 17. Mitsumoto H, DeBoer GE, Bunge G, et al. Fiber-type specific caffeine sensitivities in normal human skinned muscle fibers. Anesthesiology 1990; 72: 50-4 Appendices
Caffeine Study Questionnaire
Are you currently on medication, if so what is it?
Do you have any allergies if so what are they?
Are there any health concerns that may not allow you to participate in exercise?
Have you ever completed a submaximal exercise step test?
Activity Level: (Briefly describe your workout routine, in hours a week and what kind of exercise.)
(example - wts. 3 x week, running 3 x week, high intensity, 1.5 hrs per day)
Do you ever have shortness of breath, or feel faint while exerting minimal effort during exercise?
Do you have a history of cardiovascular disease in your family?
Caffeine Intake:
0 intake 1 cup a week 1-2 cups a week 3-4 cups a week 4-5 cups a week every day (if so write in how many cups a day) other _
0 intake 1 per week 2 times per week 3 times per week 4 times per week 5 times per week 6 times per week 7 times per week Brand of Pre-Workout Other consumption of Caffeine or other stimulant how often source of caffeine Would you say you "need" caffeine to get going in the morning or to workout?(yes/no) _
Informed Consent
Introduction: As a subject of this study I understand that I am voluntarily a part of this study. I
understand that I can withdraw at any time without penalty. I understand that I must honestly
answer the questions on the Caffeine questionnaire as well as about my health history before I
begin the study. I understand that as a subject of this study I have the right to understand all
procedures before being a part of the study. The purpose of this study is to understand the effect
and habituation of caffeine on heart rate during submaximal exercise.
Procedure: I understand that the proposed length of my participation in this study consists of 1
week 1 session per day. I understand that I will receive a complete schedule of the sessions,
location and meeting time for each day that week.
Caffeine Intake: I understand that participating in this study I will receive caffeine and
sucralose capsules of a dosage of 3 milligrams per kilogram of body weight.
I understand that I should not consume caffeine outside of this study while participating. I
understand I must inform the investigators of any caffeine use outside of the study, for my own
health. I understand that there will be no consequences of this, except that I will not be able to
participate in the study.
Exercise Test: I understand that I will be a part of a 3 minute exercise test called the Queens
College step test. (Step up and down on a 16.5 inch step, men 96 bpm, women 88 bpm)
Freedom of Consent: I understand that my participation is strictly voluntary, that it involves
physical activity. I understand that for most people it is at a submaximal level of intensity,
meaning I will not be exerting myself to full ability. I understand that I am free to stop the test at
any point, and that I should not continue the activity if I feel faint or otherwise uncomfortable
with the cellulose, caffeine, or exercise.
Risk Concern: I understand that as a subject of this study I may be at risk for side effects of
caffeine such as; sleeplessness, nausea, dehydration, head aches, and increased heart rate. I
understand that upon feeling any side effects I am fully in control of my participation in this
study and may withdraw at any point.
Benefits: I understand that as volunteering to be a subject in this study, the results found may
help broaden the understanding of caffeine and habituation during submaximal exercise.
I understand that my personal information may be useful to me, and the group information may
be useful in the health, nutrition and exercise community.
Confidentiality: I understand that any personal information will be locked away, for only
investigators to see. I understand that the results will have information but there will be no
reference to identity to any of the subjects. I understand that I may ask questions at any point,
and that I can contact the investigators, if I have any concerns regarding the study.
Debriefing: I understand that there will be a debriefing session once the study is over, which
will be held by the investigators if I choose to participate. I understand that I have a right to have
all my questions answered regarding this study and my participation.
Receipt of Informed Consent: I have received two copies of the informed Consent forms, I will
return one to the investigators of this study signed, and I will keep one for my own reference. I
understand that I may call the investigators with the contact information below If I have any questions or concerns with the study. I understand that If I have concerns with the nature of this study I may contact PLNU's IRB Dr. Ross Oakes Mueller (619-849-2905 or [email protected]). I have read and understand the above information and I consent to volunteer to participate in the study of my own free will. Name: Class: Fr So Jr Sr Age: Gender: Male / Female Signature: I am 18 yrs of age. Date: Contact Information of Investigators: if you have any concerns or questions regarding your participation, research, or physical necessity about the study Melody Axtell: EmailCell: (805)290-8765 Ted Anderson: Emal:Office: (619)849-2244



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