Lab 2 Flashcards

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1
Q

Introduction

A

Decades ago, it was recognized that the time one can tolerate a given exercise intensity (i.e., time-to-exhaustion, texhaustion) varies hyperbolically with the constant power output or speed at which the exercise is performed. As power output or speed is reduced from very high intensities, texhaustion becomes longer (i.e., a hyperbola) and, eventually, even tiny reductions in power output yield infinitely longer exercise tolerance times (i.e., there appears to be an asymptote). The power output corresponding to this asymptote is known as the critical power (CP), which, in mathematical terms, represents the highest power output that can be sustained indefinitely (i.e., without exhaustion). In other words, below CP, exercise duration can continue indefinitely, whereas above CP exercise duration is limited.

Traditionally, the CP is determined by quantifying the power versus time-to-exhaustion
relationship from multiple (usually 3 or more) exhaustive exercise bouts (performed on separate
days) at power outputs estimated to result in ‘exhaustion’ in ~2 to 20 min. The plot of power output (y-axis) vs time (time-to-exhaustion; x-axis) (or texhaustion) can be modelled with a hyperbolic function with an asymptote on the power output (y-axis) corresponding to the CP (see Figure 1). This power–time relation is described as:
texhaustion = W′ / (PO - CP) (1)
where PO is the power output (in Watts = J/s) (y-axis), texhaustion is the time-to-exhaustion (in s) (x-axis), CP is the critical power (in Watts) and W′ is the “curvature constant” (in J), equivalent to a fixed amount of work that can be performed above CP (≈ finite amount of stored energy) and is associated with a progressive rise in V ̇ O2 and the accumulation in muscle of fatigue-related metabolites, independent of power output. Therefore, if exercise demand requires a power output in excess of CP, exercise can be tolerated until W′ is reduced to zero. The further above CP, the more rapidly W′ is reduced and the shorter texhaustion will be!

A linear version of the power–time relationship is formed by plotting power output (y-axis) vs the inverse of texhaustion (1/texhaustion) (x-axis) (see Figure 2). The power – time relationship is described as:
PO = [ W′ ∙ (1/texhaustion) ] + CP (2)
where PO is the power output (in Watts or J/s) (y-axis), 1/texhaustion (in s-1) is the inverse of the time-to exhaustion, CP (in Watts) is the intercept on the PO (y-axis), W′ (in J) is the slope of the linear power – duration relationship.

In 1988, Poole et al. identified CP in 8 male participants and measured their physiological
responses during 24 minutes of cycling exercise at CP versus 5% above CP. With exercise at CP,
it was discovered that the V ̇ O2 and blood [lactate-] responses to exercise attained a delayed steady state, whereas above CP, they continued to rise until exhaustion ensured (Figure 3).

Adopting a similar experimental design (i.e., comparing physiological responses above versus
below CP), numerous studies have since corroborated that, with exercise above the CP: 1) V ̇ O2
never reaches a steady-state but increases progressively towards its maximum (V ̇ O2max); 2) there is a precipitous fall in muscle [phosphocreatine (PCr)], and [glycogen] – indicating increased reliance on substrate-level phosphorylation; and 3) there is a progressive accumulation of metabolites implicated in the development of fatigue including: intramuscular [lactate-], [H+],
[ADP] and [inorganic phosphate (Pi-)]. In contrast, below CP, all these variables eventually attain
a steady state and exercise may be tolerated for 30 minutes or longer. Based on these observations, CP is, for many, considered the gold standard marker separating the heavy- from severe-intensity domains and is defined, in physiological terms, as the highest power output where V ̇ O2, muscle and blood [lactate-], and [H+], and intramuscular [PCr] and [Pi-] can attain a “steady state”.

Theoretically, below CP, exercise duration can continue indefinitely, whereas above CP exercise
duration is limited. In reality, exercise below CP cannot be sustained indefinitely because factors
other than (and different from) those described above will limit one’s ability to continue the
exercise (e.g., muscle glycogen depletion, hyperthermia, dehydration, motivation, pain and tissue
damage). In addition, it is important to consider that fitting a hyperbolic model to data will always yield an asymptote whether one exists or not. Importantly, it is likely that the “true” power-time relationship continues to fall as texhaustion increases (albeit at a progressively slower rate as power output is reduced).

Nevertheless, CP provides a means of non-invasively quantifying the critical intensity, the
maximal metabolic steady state, and the boundary between heavy and severe-intensity domains.

To eliminate the need to perform repeated fatiguing exercise bouts for estimation of CP and W′, a protocol has been proposed (Burnley et al., Med. Sci. Sports Exerc. 38:1995-2003, 2006;
Vanhatalo et al., Med. Sci. Sports Exerc. 39:548-555, 2007 – see links below) that requires only a
single 3-min “all-out” exercise test.

In theory, if “all-out” exercise were performed for an extended period of time, the value of W′
would reduce to zero, at which point the highest possible power output that could be attained by a “willing participant” would equal CP because equation 2 above would reduce to PO = CP. 3-min is chosen because this duration has demonstrated a reproducible leveling out of power output after approximately 2.5 min has elapsed (i.e., within the last 30 s of the 3 min sprint).

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