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Journal of Exercise Physiologyonline
December 2013
Volume 16 Number 6
Official Research Journal
of The American Society of
Exercise Physiologists
(ASEP)
JEPonline
Segmental Trend Lines Define Heart Rate Response to
Increased Work
ISSN 1097-9751
Frank B. Wyatt, Alissa Donaldson
Department of Athletic Training and Exercise Physiology, Midwestern
State University, Wichita Falls, TX
ABSTRACT
Wyatt FB, Donaldson A. Segmental Trend-Lines Define Heart Rate
Response to Increased Work. JEPonline 2013;16(6):59-68. The purpose
of this study was to establish mathematical regression trend lines for
segmental changes in heart rate (HR) response to increased work in 10
male subjects with a mean (±SD) age of 29.6 ± 8.1 yrs. Pre-exercise
testing included height (cm), weight (kg), age (yrs), percent body fat (%),
and seated resting HR. The subjects were classified as fit given their
mean (±SD) VO2 max of 70.3 ± 6.03 mL·kg-1·min-1. Each subject began
the bicycle ergometer test by pedalling at 150 watts (w) of work at a
pedal rate of 80 to 90 rev·min-1 for the first 5 min followed by an increase
in work of 25 w·min-1 until volitional fatigue. The following measurements
were taken during the test: expired ventilation (VE), oxygen consumption
(VO2), carbon dioxide (VCO2), and HR. Statistical analysis included the
descriptive mean (±SD) of subjects and group test results, and the mean
(±SD) values across subjects for HR at each measure. Statistical
significance was set at P≤0.05. Segments of change for each phase
were determined using logarithmic and linear regression analysis of
group mean HR. Trend lines of best fit were used for each of the phases.
Phase I indicates the withdrawal of the parasympathetic nervous system.
Phase II indicates sympathetic influence, and Phase III indicates
peripheral afferent signalling. Three phases accompany the subjects’ HR
response during the incremental bicycle ergometer test. Each phase
indicates an established physiological response of the subjects’ HR in a
sequence of an initial rapid rise, a slower steady rise, and a final rise
with a plateau from the onset of exercise to volitional fatigue.
Key Words: Parasympathetic, Sympathetic, Peripheral Afferents
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INTRODUCTION
Vagal neural influence on the myocardium is enhanced with fitness level (1,2,4,9). This increase in
parasympathetic stimulus results in a reduced resting heart rate (HR) in trained individuals. At the
onset of exercise, HR rises dramatically (burst morphology) as a result of parasympathetic withdrawal
(5,9,19). This also occurs when blood pH is reduced through hyperventilation or carbon dioxide (CO2)
blow-off. Following parasympathetic withdrawal, as workload increases the sympathetic influence on
the heart allows for additional increase in HR (1,4,5,8,10,16).
With increasing workloads, a small but discernible breakpoint of HR increase known as the heart rate
threshold (HRT) occurs (3,6,7,11,13,17). This point of additional increase in HR has been linked to
direct afferent signals emanating from the muscle tissue as a result of metabolite (CO 2, H+)
accumulation (21,22,36). While the interplay between signals to the heart (i.e., parasympathetic,
sympathetic, and metabolite afferent) has been recognized, the characteristic of rate change has not
been established. Therefore, the purpose of this study was to establish mathematical regression
trend lines for segmental changes in HR response to increased workload.
METHODS
Subjects
Ten males acted as subjects. Each subject was well-trained, given his responses to a health
questionnaire and the Par-Q Fitness Readiness QuestionaireTM. All subjects signed an Informed
Consent prior to testing. The research protocol was approved by the Midwestern State University.
The following pre-exercise testing measures were taken: height, weight, age, body fat, and seated
resting HR (Table 1).
Table 1. Mean (±SD) of Subject Demographics.
Variable
Age (yrs)
Height (cm)
Weight (kg)
VO2 peak (mL·kg-1·min-1)
Body Fat (%)
Maximal Power (w)
Maximal Heart Rate (beats·min-1)
Mean ± SD
29.6 ± 8.1
178.3 ± 5.1
81.4 ± 6.8
70.3 ± 6.03
10.5 ± 3.8
377.5 ± 23.6
187 ± 9
Procedure
After the resting measures were taken, each subject was fitted to the VelotronTM bicycle ergometer. A
seated resting HR was taken at this time. The Australian Institute for Sport (AIS) Protocol for the cycle
ergometer was utilized for maximal testing purposes. With this protocol, each subject then began the
cycle ergometer test by pedaling at 150 watts (w) at a pedal rate between 80 to 90 rev·min-1 for 5 min.
After the initial 5 min of pedaling, the workload was increased work at 25 w·min-1 until volitional
fatigue.
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Measurements
The following measurements were taken with the ParVo MedicsTM metabolic system during the cycle
ergometer test: beat-by-beat HR (PolarTM RS800CX); expired ventilation (VE), oxygen consumption,
(VO2), and carbon dioxide production (VCO2). Upon termination of the test, subjects were allowed a
cool-down period before being de-briefed on the results.
Statistics
Statistical analysis included the following: descriptive mean (±SD) of the subjects and group test
results. Mean (±SD) values across subjects for HR at each measure (i.e., beat-by-beat). Segments of
change for each phase were determined using logarithmic and linear regression analysis of group
mean HR. Trend lines of best fit were used for each of the established phases.
RESULTS
Mean HR (beats·min-1) with logarithmic and linear regression lines are presented in Figure 1. The
cross-over of these regression lines established the points of phase distinction (2). Phases I, II, and III
are presented in Figure 1. Phase I is the parasympathetic withdrawal (via the vagus nerve) with an
immediate increase in HR (1,4,5,16,25-27,32 33). Phase II is the sympathetic influence that allows for
the increase in HR that matches the increase in workload (1,2,4,5,12,14-16,18,19,22,24,25,27,30,31,
34). Phase III represents the increase in peripheral metabolites, chemoreceptors (i.e., carotid body),
and the arterial baroreflex that stimulate the afferent signal increase in HR (3,6-8,10-13,15,2123,28,29,31,35,36).
Mean HR
I
200
180
160
140
120
100
80
60
40
20
0
II
III
Mean HR
Log. (Mean HR)
Linear (Mean HR)
0
500
1000
1500
2000
2500
Figure 1. Group Mean Heart Rate Response to Work with Associated Phases.
Phase I indicates the withdrawal of the parasympathetic nervous system at the onset of exercise,
which was identified with a logarithmic line of best fit (r2 = 0.91). Phase II followed, indicating an
increase in sympathetic influence on the subjects’ HR response. It was identified with a polynomial
line of best fit (r2 = 0.99). Lastly, the Phase III HR response through peripheral afferents to volitional
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fatigue was identified with a 4th order polynomial (r2 = 0.99). All trend lines established for each phase
was significantly correlated at P = 0.001.
DISCUSSION
From the analysis, it was determined that the three phases accompany the HR response to an
incremental workload increase. These phases were determined through a logarithmic/linear
regression cross-over technique (35). It has been well-established that the physiological responses of
HR follow a sequence of: (a) an initial, rapid rise; (b) a slower, steady rise; and (c) a final rise with a
possible plateau from the onset of exercise to volitional fatigue (5,25,27). With the onset of exercise,
the primary goal is to rapidly increase HR and VO2 to meet the oxygen needs at the muscle tissue
level. The most efficient manner for this goal to be met is with parasympathetic withdrawal and
hyperventilation. Therefore, the logarithmic pattern response of HR established the best fit to describe
a response characterized by a powerful, rapid rise followed by a leveling off as steady state is
reached to signal the transition into Phase II.
Previous studies have categorized the HR response in Phase I as non-linear, curvilinear, and monoexponential (4,5,9,27). All of these classifications hint at the rapid increase in HR required by the
onset of exercise to realize an increase in cardiac output. This study identified the pattern response of
group mean HR in Phase I as logarithmic (r2 = 0.91), which was considerably a better fit than a linear
trend line (r2 = 0.60). This can be seen in Figure 2. The onset of exercise required an initial rapid rise
in HR driven primarily by parasympathetic withdrawal, which was well fit by a logarithmic trend line
with its initial rapid rise followed by a slower increase.
In Phase I, trained cyclists have typically demonstrated a rapid response to the onset of exercise that
is steeper than an untrained response. Past research findings have noted that endurance trained
subjects have faster HR responses at the onset of exercise compared to sedentary subjects
(2,20,34). This would lead to a steeper pattern in Phase I due to a more efficient cardiovascular
response to the increased energy demand imposed by the onset of exercise.
Phase I
140
130
R² = 0.9074
120
Phase I
110
Log. (Phase I)
100
90
80
0
20
40
60
80
Figure 2. Heart Rate Trend Line during Phase I.
100
120
140
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Therefore, it seems reasonable that a “training” effect is partially responsible for the similar, steeper,
logarithmic pattern of HR response produced by the trained cyclists in this study during Phase I.
Phase II also showed a similarity of pattern by trend line categorization in which polynomial trend
lines showed the highest r2 values for HR response. In Phase II, group mean HR response was best
fit with a polynomial trend line. During Phase II, HR began the slower, polynomial rise characteristic of
the steady state phase. The powerful and rapid logarithmic response of Phase I as the body
transitions from rest to exercise is not needed to meet the demands of the incremental increase in
workload during Phase II.
Phase II group mean HR response was best fit by a polynomial trend line (r2 = 0.99). Linear (r2 = 0.96)
and exponential (r2 = 0.96) categorizations did not fit the data nearly as well as a polynomial. Phase II
HR response can be seen in Figure 3. Previous studies have classified the Phase II HR response
primarily as linear (5,27). However, the results of this study indicate that a polynomial trend line
should be considered as a viable alternative to the traditional linear classification.
Phase II
170
165
R² = 0.99
160
155
150
145
Phase II
140
Poly. (Phase II)
135
130
125
120
0
100
200
300
400
500
600
700
800
Figure 3. Heart Rate Trend Line during Phase II.
The gradual, upward slope of the Phase II group mean HR response in this study reflected the
proportional increase in intensity and in sympathetic activity on the heart (1,4,5,8,10,12,15,16,19,24,
27,29). The group mean Phase III HR response was best fit by a polynomial trend line (r2 = 0.99).
Figure 4 indicates the HR response in Phase III.
The final pattern of this response in cyclists has been highly controversial in the literature.
Researchers (3) have debated whether a HR deflection or plateau is a normal occurrence with
cyclists as they near volitional fatigue. This controversy has led to several classifications of the Phase
III response including constant, upward, and downward deflections (3).
In the present study, linear (r2 = 0.9937) and logarithmic (r2 = 0.9928) trend lines were also statistically
significant (P = 0.01) fit for the group mean. When using a group mean response for a moderately
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small sample of cyclists, it is not surprising that a polynomial trend line would be the most appropriate
fit in this phase due to the unique physiological response of each cyclist to approaching volitional
fatigue. This type of a trend line can accommodate a small group mean average that might contain
constant, upward, or downward deflections in pattern that are most prevalent in Phase III. The
polynomial pattern response was most certainly a reflection of the many ways that the central
command and the exercise pressor reflex can respond to allow for patterned HR responses in spite of
impending volitional fatigue (10,12,14,15,19,22,24,25,31).
Phase III
185
R² = 0.99
180
Phase III
175
Poly. (Phase III)
170
165
160
0
100
200
300
400
500
600
700
800
Figure 4. Heart Rate Trend Line during Phase III.
The combination of central command and the exercise pressor reflex response differed for each of
the 10 cyclists as their biological systems attempted to address increasing metabolite concentrations,
the inability of the baroreceptor to operate at a higher point, and fatiguing cardiovascular mechanism
(28,29,36). In relation to the current study, the Phase III group mean HR response was a steep, rapid
rise and ended with a slight downward deflection. This explains why the response was most precisely
fit by a polynomial (r2 = 0.9941) trend line while still recognizing the significant linear (r2 = 0.9937) and
logarithmic (r2 = 0.9928) trend lines. From this, each segment associated with a distinct trend line
establishing the HR change “trend” for that segment.
Moreover, these changes for each stage are physiologically distinct in the following manner: (a)
Phase I rate change as a result of parasympathetic neural withdrawal allowing for a rapid increase in
HR with an established logarithmic trend line; (b) Phase II rate change as a result of sympathetic
neural increase matching workload increase, allowing for a gradual increase in HR response with a
2nd degree polynomial trend line; and (c) Phase III rate change as a result of peripheral metabolite
increase with work stimulating a direct afferent signal increasing HR with a 4th degree polynomial
trend line.
The findings of similarity in group mean pattern response for cyclists during the AIS test occurred in
Phases I and II. In these phases, trend line fitting for each variable led to a similar categorization of
the trend line response. In Phase I, HR was best fit by logarithmic trend line, and the ratio comparison
of the derivatives of these trend lines produced a constant value. Heart rate was best fit by a 2nd
degree polynomial trend line in Phase II. In this phase, the combination of actions by central
command and the exercise pressor reflex to maintain HR dynamic constancy were not as rapid and
Exercise and Venous Occlusion
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powerful as Phase I, which was primarily shaped by a strong parasympathetic withdrawal and
hyperventilation.
However, Phase II responses showed a similar steady, polynomial increase throughout the phase in
response to the increasing workload. In Phase III, HR, which has been shown to deflect or plateau as
volitional fatigue approaches, was best fit by a 4th degree polynomial pattern. Thus, this research has
established distinct phases of HR response to increased work and provided rate change trend lines
for each of these phases.
CONCLUSIONS
Heart rate response during increased work has been shown to follow distinct patterns (5,27). More
recent research indicates noted HR variation in response to both steady state conditions and altered
intensities of work (2,12,18). In addition, influences on HR variation with exercise show multifaceted
signaling patterns from the central nervous system, mechanical properties of the neuromuscular
system, and the chemical and flow characteristics of blood (8-10,12,14-16,22,24,28, 36).
The current research identified three phases of response to increasing workloads (Phase I, Phase II,
and Phase III). Distinct patterns of HR response were analyzed through regression trend lines with
associated physiological influences. This closer analysis of HR response allows for more specific
investigations of HR response when comparing groups of different demographics. While the current
research utilized a fit population, it is suggested by the investigators that comparisons with different
demographic groups (i.e., unfit, pathological) is warranted.
By establishing these patterns and associated physiological influences of HR response with fit
samples, noted deviations would allow clinical diagnostics of neurological or metabolic pathologies.
Lastly, this research allows for analysis of adaptation responses to training in the athlete. Pre-post
testing protocols in athletes would indicate proper adaptations in each phase and the altered
physiological mechanisms resulting in part from proper training protocols.
Address for correspondence: Frank B. Wyatt, Department of Athletic Training and Exercise
Physiology, 3410 Taft Blvd., Ligon Hall 209, Midwestern State University, Wichita Falls, TX 763082099, Phone: 940-397-4829, Email: frank.wyatt@mwsu.edu
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