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The Future is NIRS: Muscle Oxygen Saturation as an Estimation of The Power-Duration Relationship

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  • Emergent Performance Lab

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The relationship between power output and exercise tolerance is well described by a hyperbolic equation, establishing the critical power (CP) and the curvature constant (W'). Critical power represents the greatest metabolic rate that results in 'wholly-oxidative' energy provision. Wholly-oxidative considers the active organism in toto and means that energy supply through substrate-level phosphorylation reaches a steady-state, indicated by a lack of progressive intramuscular phosphocreatine breakdown. W' was initially described as an anaerobic work capacity but has subsequently been shown to be associated with the depletion of intramuscular energy stores as well as to be sensitive to alterations in oxygen delivery. Taken together, these two findings are consistent with the hypothesis that muscle oxygen saturation (SmO2), as measured by near-infrared spectroscopy (NIRS), is a viable means of estimating the power-duration relationship, and subsequently, time to exhaustion at a fixed pace.
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The Future is NIRS: Muscle Oxygen Saturation as an
Estimation of The Power-Duration Relationship
Received Date: November 02, 2020
Published Date: November 10, 2020
Anatomy & Physiology:
Open Access Journal
Open Access
Research Article Copyright © All rights are reserved by Evan Peikon
This work is licensed under Creative Commons Attribution 4.0 License
APOAJ.MS.ID.000503.
*Corresponding author: Evan Peikon, Emergent Performance Lab, Providence, USA.
Evan Peikon*
Emergent Performance Lab, Providence, USA
Abstract
The relationship between power output and exercise tolerance is well described by a hyperbolic equation, establishing the critical power
(CP) and the curvature constant (W’). Critical power represents the greatest metabolic rate that results in ‘wholly-oxidative’ energy provision.
Wholly-oxidative considers the active organism in toto and means that energy supply through substrate-level phosphorylation reaches a steady-
state, indicated by a lack of progressive intramuscular phosphocreatine breakdown. W’ was initially described as an anaerobic work capacity but
has subsequently been shown to be associated with the depletion of intramuscular energy stores as well as to be sensitive to alterations in oxygen


Introduction
For something so familiar that everyone has experienced it,
 
  

the underlying processes that lead to fatigue. For much of the time
      
fatigue was used to describe what occurs when an athlete reaches
the absolute limit of their physical performance. Proponents of this
model assert that the body either runs out of key nutrients or is
‘poisoned’ due to metabolite accumulation in the working skeletal
muscles. However, as early as the dawn of the twentieth century,
some individuals challenged these assumptions, one such example
         
appear an imperfection of our body, is on the contrary one of its
most marvelous perfections. The fatigue increases more rapidly
than the amount of work done saves us from the injury which lesser

is an immensely complex derivative of a number of functions,
behaviors, and psychological processes. As a result, exercise
limitations involve a wide range of systems working together in

While these descriptive views of fatigue and exercise limitations
can be useful, they do not improve practitioners’ ability to predict
or manage fatigue in athletes. As a result, the link between fatigue
and performance has always been elusive. However, in recent years
compelling evidence has indicated that the relationship between
fatigue and performance is enshrined in the concept of critical
power (CP). At its essence, this concept describes the tolerable
   
that can be done above critical power, which is represented by the
curvature constant, W’. W’ has traditionally been described as an

           
Anatomy & Physiology: Open Access Journal Volume 1-Issue 1
Citation: Evan Peikon. The Future is NIRS: Muscle Oxygen Saturation as an Estimation of The Power-Duration Relationship. Anat & Physiol
Open Access J. 1(1): 2019. APOAJ.MS.ID.000503. Page 2 of 8
power and W’ through the lens of oxygen delivery and utilization,
two of the major determinants of exercise capacity, we can gain
a new perspective that better allows us to describe and predict

This paper reviews the evidence that critical power is
         
working muscles and that the curvature constant, W’, is related
to regional oxygen saturation in said muscles. As a result, near-
infrared spectroscopy may be a viable means of estimating the

in live time.
Critical Power: An Important Fatigue Threshold
What is critical power and how is it calculated?
       
asymptote of the hyperbolic relationship between power output
         
         
in the severe exercise intensity domain, and physiologically critical
power represents the boundary between steady-state and non-
  

    
equation which describes the relationship between power output
and exercise tolerance within the severe exercise intensity domain
is as follows: t=W’/(P-CP), where (t) stands for time, (P) and
(CP) stands for power and critical power respectively, and (W’)
       
a two-parameter model where critical power represents the

         
two parameters can be used to predict the tolerable duration of
exercise above critical power. The hyperbolic equation describing
the power-duration relationship is rigorous and conserved across
different forms of exercise, individual muscles, and modalities, thus
establishing it as an important fatigue threshold for athletes in a

Figure 1: The power-duration curve denes the limit of tolerance for whole-body exercise and individual muscle exercise. The curve is
constructed by the subject exercising at constant power or speed to the point of exhaustion, represented by points 1–3 on the chart. Typically,
these bouts are performed on dierent days and result in exhaustion within between two and twenty minutes. Two parameters dene this
hyperbolic relationship: the asymptote for power and the curvature constant W′ denoted by the rectangular boxes above critical power. Note
that critical power denes the upper boundary of the heavy intensity domain and represents the highest power sustainable without drawing
continuously upon W′. Above critical power, exhaustion occurs when W′ has been expended.
There are currently two validated methods for determining

critical power, termed W’. Traditionally critical power and W’ were
calculated after having an individual perform three to seven all-out

These test results are then plotted on a chart where the x, y variables
represent time to failure and power for each trial. Critical power is
then determined as the slope of the work-time relationship, whereas
   
investigators have introduced a 3-minute all-out exercise test,
known as the 3MT, that has enabled the determination of critical

the 3-minute all-out test is that when a subject exerts themselves
fully and expends W’ wholly, their power output equals their critical
 
following equation: P=(W’/t lim)+CP.
   
        
         
mainly theoretical as no bout of exercise can be sustained for an
          
better understand critical power as the highest power output that
can be sustained for a very long period of time without fatigue.
Citation: Evan Peikon. The Future is NIRS: Muscle Oxygen Saturation as an Estimation of The Power-Duration Relationship. Anat & Physiol
Open Access J. 1(1): 2019. APOAJ.MS.ID.000503. Page 3 of 8
Anatomy & Physiology: Open Access Journal Volume 1-Issue 1
         
considered to represent the greatest metabolic rate that results
in ‘wholly oxidative’ energy provision, where wholly-oxidative
considers the active organism as a whole. This means that energy
supply through substrate-level phosphorylation reaches a steady-
state and that there is no progressive accumulation of blood lactate
or progressive breakdown of intramuscular phosphocreatine
 
spectroscopy, approximates phosphocreatine kinetics measured
with magnetic resonance spectroscopy, we can conclude that a
       

are two of the major determinants of exercise performance, can
be used as a means of understanding critical power and W’. The
balance of oxygen supply and demand can be understood through
near-infrared spectroscopy, also known as NIRS.
What is NIRS and what does it measure?
NIRS stands for near-infrared spectroscopy. NIRS is a technology
that allows one to measure in vivo oxidative metabolism in human
skeletal muscle. A NIRS device consists of a light source emitting
         
  
source. Since near-infrared light can penetrate biological tissues
with less scattering and absorption than visible light, it offers many
advantages for imaging and quantitative measurements. These
quantitative measurements depend on the physics principle of
         
states that certain materials attenuate the transmission of light at
 
the properties of human muscle, it allows one to measure changes
in oxygenated and deoxygenated hemoglobin concentrations within
a given muscle. This is made possible because the chromophores
hemoglobin and myoglobin are oxygen carriers in the blood and
skeletal myocytes, respectively, and their absorbance of near-
infrared light depends on whether they are in an oxygenated or

          
        
tissue oximeters can measure regional oxygenated hemoglobin and
myoglobin saturation, which represents tissue reserve capacity
         
useful tool for assessing two of the major determinants of exercise

What is the relationship between critical power and
oxygen delivery?
As stated previously, critical power is the maximal power
output at which a metabolic steady state characterized by stable
intracellular levels of adenosine triphosphate, phosphocreatine,
hydrogen ions, inorganic phosphate, and blood lactate are reached
        

can be done above critical power. W’ was initially described as an
anaerobic work capacity but has subsequently been shown to be
associated with the depletion of intramuscular energy stores and


phosphates, and blood lactate will progressively increase until


       
measured during intense exercise, and it represents the maximum
integrated capacity of the pulmonary, cardiovascular, and muscle

be measured in absolute liters of oxygen consumed per minute
(L/min) or relative to weight in milliliters of oxygen consumer

a physiological characteristic bounded by the parametric limits of
      


        
        


above critical power is performed, and as W’ is depleted, it would
indicate that there may be a causal relationship between critical
power and oxygen delivery and extraction. If this were the case, then
  
will come with a concomitant upward shift in critical power.
According to Hill, Long, and Lupton, “In running the oxygen
requirement increases continuously as the speed increases,
attaining enormous values at the highest speeds; the actual oxygen
intake; however, reaches a maximum beyond which no effort can
drive it. The oxygen intake may attain its maximum and remain
constant merely because it cannot go any higher owing to the
        
then, there has been additional evidence supporting the belief that
the pulmonary system can be a limiting factor in maximal effort
exercise. For example, in elite athletes with very high maximal
cardiac outputs, the decreased transit time of red blood cells in the
pulmonary capillaries can lead to a pulmonary diffusion limitation.

holder Peter Snell performed a maximal treadmill step test, where


showed that arterial oxygen desaturation occurs in some highly
trained endurance athletes and when these subjects breathe
Anatomy & Physiology: Open Access Journal Volume 1-Issue 1
Citation: Evan Peikon. The Future is NIRS: Muscle Oxygen Saturation as an Estimation of The Power-Duration Relationship. Anat & Physiol
Open Access J. 1(1): 2019. APOAJ.MS.ID.000503. Page 4 of 8

         
duration curve asymptote was shifted upwards when exercise was
         
Dekerle et al. demonstrated that it was shifted downwards when
        
experiments involved the within-subject manipulation of arterial
oxygen content via hypoxic and hyperoxic inspired gas, inferring
  
critical power was increased with hyperoxia and decreased with
hypoxia. These lend support for the relationship between critical
power and oxygen transport given that oxygen transport to the
skeletal muscle is a product of both cardiac output and arterial


transport, through breathing hypoxic air, result in a downward shift

substantiate the relationship between oxygen transport and critical


Figure 2: The relationship between oxygen delivery, cardiac output, arterial oxygen saturation, and hemoglobin concentration. This gure
shows the factors that contribute to oxygen delivery, as well as selected constituents that comprise these dierent factors.
        
             
       
relationship between oxygen delivery and critical power, Monod
        
would reduce critical power to zero watts without altering the
         
        

this case, it was found that the reduction in oxygen delivery with
        
    
         
demonstrate that the amount of work that can be done above
critical power can vary across conditions. Moreover, the amount
of work that can be done above critical power appears to be a
consequence of the depletion of intramuscular energy stores and
the accumulation of fatigue-inducing metabolites, limiting exercise
    
        
delivery, lower critical power, resulting in the utilization of W’ and
fatigue at lower relative intensities. Additionally, the larger body of
evidence suggests that any alteration in oxygen delivery, whether
       
changes, will directly affect critical power and W’ depletion

The impact of oxygen delivery on the curvature constant,
W’, and it’s reconstitution
The curvilinear relationship between power output and the
time for which it can be sustained is a fundamental and well-known
feature of high-intensity exercise performance. This relationship
‘levels off’ at a ‘critical power’ that separates power outputs
that can be sustained with stable values of, for example, muscle
phosphocreatine, blood lactate, and pulmonary oxygen uptake, from
power outputs where these variables change continuously with time
until their respective minimum and maximum values are reached
and exercise intolerance occurs. The amount of work that can be
done during exercise above critical power is known as the W’. The W’
is constant but may be utilized at different rates depending on the
exercise power output’s proximity to critical power. As a result, the


of the W’ are complex and remain ambiguous. The W’ was originally
 
shown that W’ is sensitive to manipulations in oxygen delivery and
     
        
Citation: Evan Peikon. The Future is NIRS: Muscle Oxygen Saturation as an Estimation of The Power-Duration Relationship. Anat & Physiol
Open Access J. 1(1): 2019. APOAJ.MS.ID.000503. Page 5 of 8
Anatomy & Physiology: Open Access Journal Volume 1-Issue 1
amount of work that can be performed above critical power does
not appear to be a determinant of W’, but rather a consequence of
the depletion of intramuscular energy stores like phosphocreatine
and glycogen, and oxygen, as well as the accumulation of fatigue-
inducing metabolites like Pi and H+, which limit exercise tolerance

W’ is expended when power output exceeds critical power
and it is reconstituted when the power output is below critical
       
  
a net depletion of W’ will occur and when W’ is fully depleted task
          
        
of relaxation. While W’ will be used in its entirety for exercise
intensities above critical power, the proportion of W’ that contributes
directly to external work is not constant across all power outputs.
For example, at rest under occlusion W’ will be used it its entirety
to support factors distinct from external work like resting cellular
processes and ion handling, which is demonstrated by resting


As power output increases, a greater proportion of W’ would
be utilized for external work, though it appears that some of the
energy derived from the utilization of W’ still contributes to factors

  
   
      
that hemoglobin and myoglobin deoxygenation occur during
resting occlusion as well, which subsequently phosphocreatine as
  
that W’ is tightly correlated with oxygen delivery and availability
   

          
       
values of deoxyhemoglobin and oxyhemoglobin respectively were
correlated with a loss of force production, this appears to be the

Oxygen Delivery as a Rate Limiting Factor for
Critical Power and W’
VO2max as a measure of integrated capacity
       
        
absolute liters of oxygen consumed per minute (L/min) or relative
to weight in milliliters of oxygen consumer per kilogram of body
mass per minute (mL/Kg/min). The concept that there exists
          
mitochondria of exercising muscles began with Archibald Hill and
          

is a physiological characteristic bounded by the parametric limits




       
cycle ergometer ramp test completed to exhaustion, though other
modalities can be used effectively. This test involves exercising at
an intensity that increases every few minutes until the participant
reaches volitional failure at a maximal exertion point. During this
test, a participant will wear a face mask to measure the volume of
gas concentrations of inspired and expired air. It’s important to
note that an individual’s maximum attainable oxygen consumption

with and the modality they are tested on. As a result, it’s important

discrete number for each individual.
  
on convective oxygen transport to the working muscles. However,
we cannot neglect the fact that oxygen delivery is a product of both

be limited by multiple different physiological variables, including
pulmonary diffusion capacity for oxygen, maximal cardiac output,
peripheral circulation, and skeletal muscle’s metabolic capacity.
         
cardiovascular, and muscular systems’ maximum integrated

The impact of hypoxia and heavy exercise on oxygen
delivery, VO2max, critical power, and W’
During exercise, the circulatory system is challenged to
       
and diffusive factors regulate oxygen delivery. Convective oxygen
transport refers to the bulk movement of oxygen in the blood and
depends on active, energy-consuming processes that generate
        
oxygen transport refers to the passive movement of oxygen
down its concentration gradient across tissue barriers, including
the alveolar-capillary membrane and the extracellular matrix
between the tissue capillaries and individual cells mitochondria

system is challenged to facilitate oxygen delivery to the working
tissues, which ultimately impacts the development of fatigue and
performance.
As altitude increases, there is an expected and logical
       
Anatomy & Physiology: Open Access Journal Volume 1-Issue 1
Citation: Evan Peikon. The Future is NIRS: Muscle Oxygen Saturation as an Estimation of The Power-Duration Relationship. Anat & Physiol
Open Access J. 1(1): 2019. APOAJ.MS.ID.000503. Page 6 of 8
is a decreased peak oxygen uptake in the pulmonary system and
systemic circulation and a reduction in pulse oxygen saturation at
         
    
combination of central factors, like stroke volume and heart rate, as
well as peripheral factors like arterial oxygen saturation, it is logical

       
       
           
          
heavy exertion level or close to a maximal power output, there is
a progressive decrease in muscle oxygen saturation to a minimal
         
hypoxic conditions, this plateau or bottoming out in oxygenated
hemoglobin concentration is considered an indication of maximal
skeletal muscle oxygen extraction as a product of reduced oxygen
      
can be observed non-invasively with near-infrared spectroscopy in

Consistent with the data on hypoxia’s dose-response effect
         
          
     

        
where a ‘metabolic stability’ can be achieved, where metabolic
stability is characterized by minimal disturbances to intramuscular

supporting the idea that phosphocreatine is not only reconstituted
via oxidative means but dependent on oxygen availability. In
hypoxia, convective oxygen transport to the working muscles
        

metabolic stability and has been shown to correlate with critical
         
kinetics may impair metabolic stability and thus explain why

Since critical power is lower under conditions of hypoxia,
then it holds that a given absolute intensity will cause a faster
depletion of W’ under these conditions. According to the critical
power model, faster depletion of W’ will cause a reduction in time
    
This would be associated with an exacerbated rate of fatigue
development in hypoxia, which has been demonstrated by Romer,

mechanism which exacerbates fatigue, it is the effect of hypoxia on
decreasing critical power that leads to a more rapid onset of fatigue

output. However, there is also evidence that W’ will be reduced at
altitude and that changes in W’ are related to changes in critical
 
       
effect of hypoxia on W’ appears to display a threshold characteristic
       

has suggested that the decrease in W’ at altitude is consistent with

given the decreased peak oxygen uptake that occurs at altitude,
which results in lower arterial oxygen saturation and as well as


that oxygen uptake and transport can be rate-limiting factors that
determine critical power and W’.
Modeling Time to Exhaustion
Using muscle oxygen saturation as estimation of the
power duration relationship
The critical power concept describes the relationship between
sustainable power output and severe intensity exercise duration
above the ‘critical power’. A simple hyperbolic, two-parameter model
was proposed to explain this concept: t lim=(W’)/(P-CP) where t
lim is the time until task failure occurs, P is power output, CP is the
  
below which no expenditure of W’ occurs, and W’ is the total work
accumulate above critical power until task failure. In this model,
the value W’ is progressively depleted during exercise whenever
power output exceeds critical power and reconstitutes when
   
depleted to zero, then t lim also reaches zero, and hence task failure
ensues. Since the total amount of work accumulated above critical
power is terminated at the moment of task failure, the mechanisms
leading to task failure also determine W’s value. It has previously
been observed that as exercise increases up to a heavy exertion
level, or close to a maximal power output, there is a progressive
decrease in muscle oxygen saturate to a minimal point or plateau

the relationship between oxygen delivery, critical power, and W’
it stands to reason that muscle oxygen saturation can be used as
an indicator of proximity to failure. Previous investigations have

          
        

           
change can be used to potentially predict failure during isometric
muscle contraction and similar results were reported during
sustained gripping tasks where maximum and minimum values
for deoxyhemoglobin and oxyhemoglobin respectively were
Citation: Evan Peikon. The Future is NIRS: Muscle Oxygen Saturation as an Estimation of The Power-Duration Relationship. Anat & Physiol
Open Access J. 1(1): 2019. APOAJ.MS.ID.000503. Page 7 of 8
Anatomy & Physiology: Open Access Journal Volume 1-Issue 1

into this area is needed before this concept can be extrapolated to
cyclic activities such as running, it seems promising given that the
hyperbolic form of the power duration relationship is rigorous and
highly conserved across forms of exercise, individual muscles, and
  


the idea that tissue oximeters provide information on the balance
between oxygen supply and oxygen demand and that regional
oxygenated hemoglobin saturation represents tissue reserve
        
equation t lim= W’/P-CP can be reconceptualized as t lim= (regional
       



order to further elucidate the incompletely understood relationship
between oxygen delivery, critical power, and W’.
Conclusion
        
relationship between oxygen delivery and critical power, such that
muscle oxygen saturation may be used to approximate the power-
duration relationship non-invasively and in real-time. While critical

power-output above CP, one of its major limitations is its sensitivity
to environmental factors like altitude and prior heavy exercise,
fatigue, and glycogen depletion. As a result, this requires an athlete
to test critical power in all of these conditions, which may not be
    
rate of change of muscle oxygen saturation, it’s possible that these
issues are voided. However, further research is needed to examine
the relationship between critical power and oxygen delivery, W’
and regional oxygenated hemoglobin saturation, as well as time to
exhaustion, minimum muscle oxygen saturation, and rate of change
of muscle oxygen saturation.
Acknowledgement
None.


References
            
         

 
exercise behavior to ensure the protection of whole body homeostasis.

3.    

           
hyperoxia on muscle metabolic responses and the power-duration work


5. 
in near-infrared spectroscopy (NIRS) for the assessment of local skeletal
muscle microvascular function and capacity to utilise oxygen. Artery Res

 

     
constant which determines the duration of tolerance to high intensity

           
 

              
curvature constant parameter of the power-duration curve for varied-


Power: An Important Fatigue Threshold in Exercise Physiology. Med Sci



           
determine peak oxygen uptake and the maximal steady state. Med Sci




by isometric contractions in exercising humans and in mouse isolated

            
Simultaneous in vivo measurements of HbO saturation and PCr kinetics


in relation to muscle oxygenation. Clinical physiology and functional

          

          
          


metabolic responses to exercise above and below the “critical power”
         




 
Force Test: A New Method to Establish Forearm Aerobic Metabolic


Paradigms of Maximal Exercise Performance. Anat & Physiol Open




volume expansion does not explain the increase in peak oxygen uptake
Anatomy & Physiology: Open Access Journal Volume 1-Issue 1
Citation: Evan Peikon. The Future is NIRS: Muscle Oxygen Saturation as an Estimation of The Power-Duration Relationship. Anat & Physiol
Open Access J. 1(1): 2019. APOAJ.MS.ID.000503. Page 8 of 8


    
    

            
Structural and functional assessments of a champion runner: Peter

          

      


     

30.             
       
characteristics and the parameters of the power-duration relationship. J

            

central fatigue during small muscle mass handgrip exercise. J. Physiol.

     
measure of physical work capacity and anaerobic threshold. Ergonomics

33.             


           
         

35.              
oral creatine supplementation on the curvature constant parameter of
the power-duration curve for cycle ergometry in humans. Jpn J Physiol


depletion on the curvature constant parameter of the power-duration


Effects of prior very-heavy intensity exercise on indices of aerobic
       

    
expenditure and reconstitution during severe intensity constant power
exercise: mechanistic insight into the determinants of W’. Physiol Rep


Noninvasive measures of oxidative metabolism on working human


metabolism from near infrared spectroscopy during rhythmic handgrip

      In vivo ATP
       

   


  



   

   
Cerebral Deoxygenation and Perfusion during Repeated Sprints in




           
effect of acute simulated moderate altitude on power, performance and

55.
          
    

50.           
lactate accumulation and muscle deoxygenation during incremental

         


Legrand R, Ahmaidi S, Moalla W, Chocquet D, Marles A, Prieur F, et al.
 arterial desaturation in endurance athletes increases muscle

53.           
       
    




55. 
The mechanistic bases of the power-time relationship: muscle metabolic
         

         
submaximal exercise in hyperoxia, normoxia, and hypoxia. Can J Appl

          
     

          
Prediction of Critical Power and W’ in Hypoxia: Application to Work-
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Romer LM, Haverkamp HC, Amann M, Lovering AT, Pegelow DF, et
          
endurance capacity in healthy humans. Am J Physiol Regul Integr Comp
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            
          

        


Moxy Muscle Oxygenation Research Summit, USA.
... Today, near-infrared spectroscopy (NIRS) has positioned itself in the field of physical activity and health as a valid, reliable and inexpensive wireless instrument [1][2][3][4][5][6][7]. In addition, this technology is capable of evaluating, in real time, the balance between muscle oxygen supply and its demand during physical exercise [8]. Technically, these devices illuminate the skeletal muscle with infrared light and detect the light reflected through it as a consequence of the amount of light absorbed by the tissue, recognizing variables such as oxyhemoglobin (O 2 Hb) and deoxyhemoglobin (HHb), as well as other derivatives such as total hemoglobin (tHb = O 2 Hb + HHb) and muscle oxygen saturation expressed in percentage (%) (SmO 2 = [O 2 Hb]/[tHb] × 100), among others [9]. ...
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Background: This study aimed to report, through a systematic review of the literature, the baseline and final reference values obtained by near-infrared spectroscopy (NIRS) of muscle oxygen saturation (SmO2) during resistance training in healthy adults. Methods: Original research studies were searched from four databases (Scopus, PubMed, WOS, and SportDiscus). Subsequently, three independent reviewers screened the titles and abstracts, followed by full-text reviews to assess the studies’ eligibility. Results: Four studies met the inclusion criteria, data were extracted and methodological quality was assessed using the Downs and Black scale. Muscle oxygen saturation (% SmO2) during reported muscle strength exercises showed a decreasing trend after a muscle strength protocol; that is, before the protocol (range = 68.07–77.9%) and after (range = 9.50–46.09%). Conclusions: The trend of the SmO2 variables is to decrease after a muscle strength protocol. Studies are lacking that allow expanding the use of these devices during this type of training.
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It is widely believed that peak cardiac output and total body hemoglobin content are the dominant and deterministic pathways that account for the vast majority of interindividual variability in VO2max. This article presents a case that VO2max represents the maximum integrated capacity of the cardiovascular, pulmonary, and muscular system, and that ‘limiting’ factors for VO2max can vary between individuals.
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The aim of this study was to measure muscle oxygen saturation (SmO2) dynamics during a climbing specific task until failure in varying conditions. Our prediction was that SmO2 should be a good marker to predict task failure. Eleven elite level climbers performed a finger-hang test on a 23 mm wooden rung under four different weighted conditions, 1. body weight (BW), 2. body weight +20% (BW +20), 3. body weight −20% (BW −20) and 4. body weight −40% (BW −40), maintaining half crimp grip until voluntary exhaustion. During each trial SmO2 and time to task failure (TTF) were measured. TTF was then compared to the minimally attainable value of SmO2 (SmO2min) and time to SmO2min (TTmin). There is a considerable degree of agreement between attainable SmO2min at high intensity conditions (MBW = 21.6% ± 6.4; MBW+20 = 24.0% ± 7.0; MBW−20 = 23.0% ± 7.3). Bland-Altman plot with an a priori set equivalency interval of ±5% indicate that these conditions are statistically not different (MBW-BW + 20 = −2.4%, 95% CI [1.4, −6.2]; MBW−Bw−20 = −1.3, 95% CI [2.5, −5.1]). The fourth and lowest intensity condition (MBW −40 = 32.4% ± 8.8) was statistically different and not equivalent (MBW-BW −40 = −8.8%, 95% CI [−5.0, −12.6]). The same agreement was found between TTF and TTmin for the high intensity conditions plotted via Bland-Altman. While the rate with which oxygen was extracted and utilised changed with the conditions, the attainable SmO2min remained constant at high intensity conditions and was related to TTF.
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Background: Since the introduction (in 2006) of commercially available portable wireless muscle oximeters, the use of muscle near-infrared spectroscopy (NIRS) technology is gaining in popularity as an application to observe changes in muscle metabolism and muscle oxygenation during and after exercise or training interventions in both laboratory and applied sports settings. Objectives: The objectives of this systematic review were to highlight the application of muscle oximetry in evaluating oxidative skeletal muscle performance to sport activities and emphasize how this technology has been applied to exercise and training. Methods: Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed in a systematic fashion to search, assess and synthesize existing literature on this topic. The Scopus and MEDLINE/PubMed electronic databases were searched to 1 March 2017. Potential inclusions were screened against eligibility criteria relating to recreationally trained to elite athletes, with or without training programs, who must have assessed physiological variables monitored by commercial oximeters or NIRS instrumentation. Results: Of the 14,609 identified records, only 57 studies met the eligibility criteria. This systematic review highlighted a number of key findings in 16 sporting activities. Overall, NIRS information can be used as a marker of skeletal muscle oxidative capacity and for analyzing muscle performance factors. Conclusions: Although NIRS instrumentation is promising in evaluating oxidative skeletal muscle performance when used in sport settings, there is still the need for further instrumental development and randomized/longitudinal trials to support the detailed advantages of muscle oximetry utilization in sports science.
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During supramaximal exercise, exacerbated at exhaustion and in hypoxia, the circulatory system is challenged to facilitate oxygen delivery to working tissues through cerebral autoregulation which influences fatigue development and muscle performance. The aim of the study was to evaluate the effects of different levels of normobaric hypoxia on the changes in peripheral and cerebral oxygenation and performance during repeated sprints to exhaustion. Eleven recreationally active participants (six men and five women; 26.7 ± 4.2 years, 68.0 ± 14.0 kg, 172 ± 12 cm, 14.1 ± 4.7% body fat) completed three randomized testing visits in conditions of simulated altitude near sea-level (~380 m, FIO2 20.9%), ~2000 m (FIO2 16.5 ± 0.4%), and ~3800 m (FIO2 13.3 ± 0.4%). Each session began with a 12-min warm-up followed by two 10-s sprints and the repeated cycling sprint (10-s sprint: 20-s recovery) test to exhaustion. Measurements included power output, vastus lateralis, and prefrontal deoxygenation [near-infrared spectroscopy, delta (Δ) corresponds to the difference between maximal and minimal values], oxygen uptake, femoral artery blood flow (Doppler ultrasound), hemodynamic variables (transthoracic impedance), blood lactate concentration, and rating of perceived exertion. Performance (total work, kJ; −27.1 ± 25.8% at 2000 m, p < 0.01 and −49.4 ± 19.3% at 3800 m, p < 0.001) and pulse oxygen saturation (−7.5 ± 6.0%, p < 0.05 and −18.4 ± 5.3%, p < 0.001, respectively) decreased with hypoxia, when compared to 400 m. Muscle Δ hemoglobin difference ([Hbdiff]) and Δ tissue saturation index (TSI) were lower (p < 0.01) at 3800 m than at 2000 and 400 m, and lower Δ deoxyhemoglobin resulted at 3800 m compared with 2000 m. There were reduced changes in peripheral [Δ[Hbdiff], ΔTSI, Δ total hemoglobin ([tHb])] and greater changes in cerebral (Δ[Hbdiff], Δ[tHb]) oxygenation throughout the test to exhaustion (p < 0.05). Changes in cerebral deoxygenation were greater at 3800 m than at 2000 and 400 m (p < 0.01). This study confirms that performance in hypoxia is limited by continually decreasing oxygen saturation, even though exercise can be sustained despite maximal peripheral deoxygenation. There may be a cerebral autoregulation of increased perfusion accounting for the decreased arterial oxygen content and allowing for task continuation, as shown by the continued cerebral deoxygenation.
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Purpose: Develop a prediction equation for critical power (CP) and work above CP (W′) in hypoxia for use in the work-balance (WBAL′) model. Methods: Nine trained male cyclists completed cycling time trials (TT; 12, 7, and 3 min) to determine CP and W′ at five altitudes (250, 1,250, 2,250, 3,250, and 4,250 m). Least squares regression was used to predict CP and W′ at altitude. A high-intensity intermittent test (HIIT) was performed at 250 and 2,250 m. Actual and predicted CP and W′ were used to compute W′ during HIIT using differential (WBALdiff′) and integral (WBALint′) forms of the WBAL′ model. Results: CP decreased at altitude (P < 0.001) as described by 3rd order polynomial function (R² = 0.99). W′ decreased at 4,250 m only (P < 0.001). A double-linear function characterized the effect of altitude on W′ (R² = 0.99). There was no significant effect of parameter input (actual vs. predicted CP and W′) on modelled WBAL′ at 2,250 m (P = 0.24). WBALdiff′ returned higher values than WBALint′ throughout HIIT (P < 0.001). During HIIT, WBALdiff′ was not different to 0 kJ at completion, at 250 m (0.7 ± 2.0 kJ; P = 0.33) and 2,250 m (−1.3 ± 3.5 kJ; P = 0.30). However, WBALint′ was lower than 0 kJ at 250 m (−0.9 ± 1.3 kJ; P = 0.058) and 2,250 m (−2.8 ± 2.8 kJ; P = 0.02). Conclusion: The altitude prediction equations for CP and W′ developed in this study are suitable for use with the WBAL′ model in acute hypoxia. This enables the application of WBAL′ modelling to training prescription and competition analysis at altitude.
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The sustainable duration of severe intensity exercise is well-predicted by critical power (CP) and the curvature constant (W′). The development of the W′BAL model allows for the pattern of W′ expenditure and reconstitution to be characterized and this model has been applied to intermittent exercise protocols. The purpose of this investigation was to assess the influence of relaxation phase duration and exercise intensity on W′ reconstitution during dynamic constant power severe intensity exercise. Six men (24.6 ± 0.9 years, height: 173.5 ± 1.9 cm, body mass: 78.9 ± 5.6 kg) performed severe intensity dynamic handgrip exercise to task failure using 50% and 20% duty cycles. The W′BAL model was fit to each exercise test and the time constant for W′ reconstitution (τW′) was determined. The τW′ was significantly longer for the 50% duty cycle (1640 ± 262 sec) than the 20% duty cycle (863 ± 84 sec, P = 0.02). Additionally, the relationship between τW′ and CP was well described as an exponential decay (r2 = 0.90, P < 0.0001). In conclusion, the W′BAL model is able to characterize the expenditure and reconstitution of W′ across the contraction–relaxation cycles comprising severe intensity constant power handgrip exercise. Moreover, the reconstitution of W′ during constant power severe intensity exercise is influenced by the relative exercise intensity, the duration of relaxation between contractions, and CP.
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Purpose of review: Continuous wave near infrared spectroscopy (CW NIRS) provides non-invasive technology to measure relative changes in oxy- and deoxy-haemoglobin in a dynamic environment. This allows determination of local skeletal muscle O2 saturation, muscle oxygen consumption ([Formula: see text]) and blood flow. This article provides a brief overview of the use of CW NIRS to measure exercise-limiting factors in skeletal muscle. Recent findings: NIRS parameters that measure O2 delivery and capacity to utilise O2 in the muscle have been developed based on response to physiological interventions and exercise. NIRS has good reproducibility and agreement with gold standard techniques and can be used in clinical populations where muscle oxidative capacity or oxygen delivery (or both) are impaired. CW NIRS has limitations including: the unknown contribution of myoglobin to the overall signals, the impact of adipose tissue thickness, skin perfusion during exercise, and variations in skin pigmentation. These, in the main, can be circumvented through appropriate study design or measurement of absolute tissue saturation. Summary: CW NIRS can assess skeletal muscle O2 delivery and utilisation without the use of expensive or invasive procedures and is useable in large population-based samples, including older adults.
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The hyperbolic form of the power-duration relationship is rigorous and highly conserved across species, forms of exercise and individual muscles/muscle groups. For modalities such as cycling, the relationship resolves to two parameters, the asymptote for power (critical power, CP) and the so-called W' (work doable above CP), which together predict the tolerable duration of exercise above CP. Crucially, the CP concept integrates sentinel physiological profiles - respiratory, metabolic and contractile - within a coherent framework that has great scientific and practical utility. Rather than calibrating equivalent exercise intensities relative to metabolically distant parameters such as the lactate threshold or V˙O2 max, setting the exercise intensity relative to CP unifies the profile of systemic and intramuscular responses and, if greater than CP, predicts the tolerable duration of exercise until W' is expended, V˙O2 max is attained, and intolerance is manifested. CP may be regarded as a 'fatigue threshold' in the sense that it separates exercise intensity domains within which the physiological responses to exercise can (<CP) or cannot (>CP) be stabilized. The CP concept therefore enables important insights into 1) the principal loci of fatigue development (central vs. peripheral) at different intensities of exercise, and 2) mechanisms of cardiovascular and metabolic control and their modulation by factors such as O2 delivery. Practically, the CP concept has great potential application in optimizing athletic training programs and performance as well as improving the life quality for individuals enduring chronic disease.
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For high-intensity muscular exercise, the time-to-exhaustion (t) increases as a predictable and hyperbolic function of decreasing power (P) or velocity (V). This relationship is highly conserved across diverse species and different modes of exercise and is well described by two parameters: the 'critical power' (CP or CV), which is the asymptote for power or velocity, and the curvature constant (W') of the relationship such that t = W'/(P-CP). CP represents the highest rate of energy transduction (oxidative ATP production, V? O2) that can be sustained without continuously drawing on the energy store W' (composed in part of anaerobic energy sources and expressed in kilojoules). The limit of tolerance (time t) occurs when W' is depleted. The CP concept constitutes a practical framework in which to explore mechanisms of fatigue and help resolve crucial questions regarding the plasticity of exercise performance and muscular systems physiology. This brief review presents the practical and theoretical foundations for the CP concept, explores rigorous alternative mathematical approaches, and highlights exciting new evidence regarding its mechanistic bases and its broad applicability to human athletic performance.
Article
Key points: The power-asymptote (critical power; CP) of the hyperbolic power-time relationship for high-intensity exercise defines a threshold between steady-state and non-steady-state exercise intensities and the curvature constant (W') indicates a fixed capacity for work >CP that is related to a loss of muscular efficiency. The present study reports novel evidence on the muscle metabolic underpinnings of CP and W' during whole-body exercise and their relationships to muscle fibre type. We show that the W' is not correlated with muscle fibre type distribution and that it represents an elevated energy contribution from both oxidative and glycolytic/glycogenolytic metabolism. We show that there is a positive correlation between CP and highly oxidative type I muscle fibres and that muscle metabolic steady-state is attainable CP. Our findings indicate a mechanistic link between the bioenergetic characteristics of muscle fibre types and the power-time relationship for high-intensity exercise. Abstract: We hypothesized that: (1) the critical power (CP) will represent a boundary separating steady-state from non-steady-state muscle metabolic responses during whole-body exercise and (2) that the CP and the curvature constant (W') of the power-time relationship for high-intensity exercise will be correlated with type I and type IIx muscle fibre distributions, respectively. Four men and four women performed a 3 min all-out cycling test for the estimation of CP and constant work rate (CWR) tests slightly >CP until exhaustion (Tlim ), slightly <CP for 24 min and until the >CP Tlim isotime to test the first hypothesis. Eleven men performed 3 min all-out tests and donated muscle biopsies to test the second hypothesis. Below CP, muscle [PCr] [42.6 ± 7.1 vs. 49.4 ± 6.9 mmol (kg d.w.)(-1) ], [La(-) ] [34.8 ± 12.6 vs. 35.5 ± 13.2 mmol (kg d.w.)(-1) ] and pH (7.11 ± 0.08 vs. 7.10 ± 0.11) remained stable between ∼12 and 24 min (P > 0.05 for all), whereas these variables changed with time >CP such that they were greater [[La(-) ] 95.6 ± 14.1 mmol (kg d.w.)(-1) ] and lower [[PCr] 24.2 ± 3.9 mmol (kg d.w.)(-1) ; pH 6.84 ± 0.06] (P < 0.05) at Tlim (740 ± 186 s) than during the <CP trial. The CP (234 ± 53 W) was correlated with muscle type I (r = 0.67, P = 0.025) and inversely correlated with muscle type IIx fibre proportion (r = -0.76, P = 0.01). There was no relationship between W' (19.4 ± 6.3 kJ) and muscle fibre type. These data indicate a mechanistic link between the bioenergetic characteristics of different muscle fibre types and the power-duration relationship. The CP reflects the bioenergetic characteristics of highly oxidative type I muscle fibres, such that a muscle metabolic steady-state is attainable below and not above CP.