Competition Between Desired Competitive Result, Tolerable Homeostatic Disturbance, And Psychophysiological Interpretation Determines Pacing Strategy Part 1

Scientific interest in pacing goes back >100 years. Contemporary interest, both as a feature of athletic competition and as a window into understanding fatigue, goes back >30 years. Pacing represents the pattern of energy use designed to produce a competitive result while managing fatigue of different origins. Pacing has been studied both against the clock and during head-to-head competition. Several  models have been used to explain pacing, including the teleoanticipation model, the central governor model, the anticipatory-feedback rating of perceived exertion model, the concept of a learned template, the affordance concept, the integrative governor theory, and as an explanation for “falling behind.” Early studies, mostly using time-trial exercise, focused on the need to manage homeostatic disturbance. More recent studies, based on head-to-head competition, have focused on an improved understanding of how psychophysiology,  beyond the gestalt concept of rating of perceived exertion, can be understood as a mediator of pacing and as an explanation for falling behind. More recent approaches to pacing have focused on the elements of decision-making during sport and have expanded the role of psychophysiological responses including sensory-discriminatory, affective-motivational, and cognitive-evaluative dimensions. These approaches have expanded the understanding of variations in pacing, particularly during head-to-head competition.

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Keywords: home hemostasis, fatigue, psychophysiology

The concept of pacing, that is, distributing energetic resources throughout a task, is not new. Historical examples remind us of the necessity for pacing, including Aesop’s fable of the tortoise and the hare; Emil Zatopek asking Jim Peters (1952 Olympic marathon) in midrace if “they were running fast enough”; Vladimir Kuts (1956 Olympic 5- and 10-km) using an interval pacing pattern to defeat world-record (WR) holder Gordon Pirie; Kipchoge Keino using a “go out fast” strategy in the altitude of Mexico City to defeat WR holder Jim Ryun (1968 Olympic 1500 m); David Wottle, coming from 20 m behind after the first 200 m to win (1972 Olympic 800 m); and WR holder Steven Jones (European Championships marathon,1986), 2 minutes ahead of the field at 20 miles, who faded and finished 13th place. In all these cases, pacing (good or bad) helped define the competitive result.

Pacing is the process of using the resources available at the start, in an anticipatory manner based on experience,1 or in response to internal and external stimuli, and 2 to achieve the desired result. Often the goal is to finish as quickly as possible, particularly against the clock rather than head to head. Pacing represents the balance between energy availability, technique, and fatigue. Energy availability depends on energy-producing systems, which depend on the physiologic capacity and the duration and mode of the event. Technique depends on neuromuscular performance, which is of modest importance in running, but crucial in other activities (skating, cycling, cross-country skiing, rowing, and swimming), and may deteriorate with fatigue. For example, in cycling and skating, athletes can continue to glide or roll toward the finish even after considerable losses of power output; whereas, in running and swimming, there is a rapid deceleration with loss of power output. Fatigue, which has become better understood,3–6 depends upon the depletion of substrates (adenosine triphosphate, creatine phosphate, glucose, and glycogen), the accumulation of metabolites (inorganic phosphate and hydrogen ions) and heat, and functioning as control processes via afferent nerves, as well as the interpretation of what these changes mean.

Historical Evidence of Interest in Pacing

The concept of pacing is not new. The first report was by Tripplet in 1898.7 He evaluated why drafting improved performance. While describing performance improvements when following a pacer, he reported distance–velocity relationships that anticipated the critical speed (CS)/critical power (CP) concept.8 He also developed theories (suction, shelter, encouragement, and hypnotic suggestion) anticipating concepts of reduced wind resistance9 and the ergogenic effect of a competitor riding just a little faster than an athlete's personal best.10 Other studies by Kennelly11 and Hill,12 performed a century ago, described the distance–velocity relationship (for running, walking, The concept of pacing is not new. The first report was by Tripplet in 1898.7 He evaluated why drafting improved performance. While describing performance improvements when following a pacer, he reported distance–velocity relationships that anticipated the critical speed (CS)/critical power (CP) concept.8 He also developed theories (suction, shelter, encouragement, and hypnotic suggestion) anticipating concepts of reduced wind resistance9 and the ergogenic effect of a competitor riding just a little faster than an athlete's personal best.10 Other studies by Kennelly11 and Hill,12 performed a century ago,  described the distance–velocity relationship (for running, walking,

The Concept of Pacing Strategy Emerges

The first contemporary studies of pacing emerged from groups in the Netherlands and the United States.14–19 These studies demonstrated that: (1) there was a range of advantageous pacing strategies in cycling events of 1000 to 4000 m (or even longer); (2) an all-out strategy was better in shorter events; (3) longer events favored a brief high-intensity start which was then “dialed back” after ∼10 to 15 seconds; and (4) more even, or U shaped, pacing patterns were seen in longer events. These studies, particularly the frequent observation of an end-spurt, also established the concept that high speed at the finish was essentially wasted kinetic energy that might have been better used to go faster earlier and arrive at the finish sooner. Trying to improve performance (particularly in events <4 min) required an athlete to take a “calculated risk” of starting faster than normal, to achieve a performance that they had never previously achieved.20

Teleoanticipation Model

By the mid-1990s, the first conceptual model of pacing emerged. Ulmer1 suggested that energy output was governed by central control mechanisms designed to: (1) avoid early fatigue, (2) not waste time with a slow start, (3) use learned behavior as a template for current activity, and (4) anticipate the time required to finish. Thus, the teleoanticipation model was conceptualized as a closed-loop, feedback-dependent, anticipatory regulation of the energetic output. At about this same time, evidence emerged of a replicable pattern of pacing strategy and that elite athletes used the same pacing as recreational athletes.21 Beyond single efforts, there was evidence of pacing in the Grand Tours of cycling, in which general classification competitors would only exert themselves heavily on the days when significant time gains were possible.22 On other days, teammates would keep them near the front of the peloton. These findings reinforced Ulmer's concept of anticipating stresses across an entire event. Less than a decade later, evidence emerged of a consistent pattern in the pacing of races where the goal was to defeat other competitors head-to-head.23,24 It also became evident that pacing displayed a consistent pattern, evolving toward less of the fast–slow–slower–fast pattern observed in the early 20th century.23,24 The concept also emerged that the pacing strategy, in attempts to improve best performance, was consistent over time.25 Supporting Ulmer's concept, there was evidence that different events had unique pacing patterns, suggesting that the anticipation of muscular power output was very strongly grounded.26–29

Pacing Versus Fatigue (Central Governor Model)

Early concepts of fatigue were based on observations of the progressive reduction in force/power output (to near zero values) in isolated skeletal muscle despite supramaximal stimulation.30 It was thought that muscle failure was related to factors including the level of stimulation, blood flow, availability of O2, and the ability to buffer changes in pH. Observations by Noakes31 that humans rarely exercise to the point of total muscular failure suggested that fatigue was not solely related to absolute levels of muscular substrates or metabolites. While there is evidence that homeostatic disturbances are profound during severe exercise and that exercise endpoints occurred at similar levels of homeostatic disturbance regardless of the task,32–35 complete muscle, cardiac, or organ system failure rarely occurred. This evolved to the understanding that fatigue acts to prevent cellular damage related to severe homeostatic disturbance.36 Even demanding tasks such as the Wingate test (30 s in duration) can be extended to as long as 3 minutes, with the power output only falling as low as the CP.37 These data suggested the presence of bidirectional signaling between the efferent neural output and afferent signals from peripheral receptors, rather than unidirectional unresponsiveness by the muscle. Noakes et al38–40 called this bidirectional signaling the central governor model. This concept was expanded by St Clair Gibson and Foster41 suggesting that pacing involved competition between the psychological drive to perform a task and managing homeostatic disturbances. Thus, although catastrophic collapses of ambulatory ability are possible, they are comparatively rare.42 Studies of exercise in the presence of afferent blockade43 supported the role of afferent signaling as an obligatory feature in pacing. Evidence in support of bidirectional signaling was provided by studies where warm-up was manipulated to induce fatigue before a time trial.44 The lesson from the central governor model was that pacing, far from being an epiphenomenon of athletic competition, was a window into how fatigue was experienced and managed.

Patterns of Pacing Strategy

Much of the early pacing research was dominated by observations during athletic competitions. Abbiss and Laursen45 identified basic pacing strategy variants. Subsequent work from several laboratories,14–19,21,22,27–29,45–72 identified physiological responses during variations in pacing strategy. These studies demonstrated that pacing could be understood in terms of the power balance model of van Ingen Schenau et al,18,19 with power production depending on the summation of aerobic and anaerobic energy provision and power losses related to summated resistive forces. The first clear evidence that pacing was related to homeostatic disturbances, primarily related to the substrate (creatine phosphate32–34 and glycogen46–48 depletion, and/or metabolite accumulation32–35 and hyperthermia),49–51 appeared during this period.

The pacing strategy follows general rules related to the distance/time taken to complete a task and displays differences related to the nature of the task, particularly the retarding medium.52 There is evidence of “reserve” built into pacing strategy 53,54 that can be disrupted by deception regarding distance feedback and influenced by another competitor (or avatar) that is slightly faster than an athlete's previous performances,60–65 but hindered if the other competitor is too much faster.65–69 These findings suggest that the reserve during exercise tasks can be manipulated, either by time/distance deception or the meaningfulness of the competition (club race vs. Olympic final). Furthermore, the most predictable strategy to improve performance is a faster-than-normal start. However, only about 50% to 80% of fast-start experiences will lead to improved performance.65–69,73,73,74 Head-to-head racing against a much superior opponent can lead to both an inappropriately rapid increase in rating of perceived exertion (RPE), and a negative effect and loss of self-efficacy during the race, leading to reductions in speed/power output (i.e., letting go of the leading competitors).73,75,76

The structure of the pacing pattern (Figure 1), at least against has been conceptualized as a “landscape” where the interaction of race distance and percentage of the race completed define momentary power output, regardless of whether the power output is attributable to aerobic or anaerobic energetic sources.77,78

Rating of Perceived Exertion

Several studies have shown that RPE grows systematically about the percentage of a task completed.25,28,29,79–89 This suggests a scaling of RPE to the overall level of homeostatic disturbance, regardless of the precise nature of the disturbance. The rate of RPE growth during an event appears to be tightly regulated, as blinded changes in inspired (O2) cause a rapid change in muscular power output while the rate of RPE growth barely changes.80,89–91 Similarly, while changes in pre-exercise muscle glycogen exert a consequential influence on power output, the growth of RPE normalized to endurance time hardly changes.92

The overriding importance of RPE as a way to express the sensation of both intensity and progressive fatigue is so powerful that the third major conceptual model of pacing, the anticipatory feedback-RPE model93,94 proposes that power output is regulated based on prior experience, anticipated completion time, and rate of growth of RPE. If the rate of growth of RPE is discordant with that anticipated, then power output is either upregulated or downregulated to return RPE to the anticipated growth curve (Figure 2). This concept has been supported in studies where power output was increased by mid-race tactical decisions81,91 or deception regarding the distance remaining.60,64

The growth of RPE relative to the percentage of an event remaining has been combined into a derived variable called the Hazard score (momentary RPE × fractional distance remaining), which seems to be able to inform athletes when to change power output during an event.82,84,95,96 An extension of this technique, the summated Hazard score, has been shown to allow appreciation of how taxing an event feels.96

For as important as the RPE has been to understanding pacing,  it has been recognized that RPE is a gestalt of several sensory inputs that reflect how a given power output, progress through an event, and homeostatic disturbance is interpreted. As such, RPE has been criticized as being a less-than-ideal psychophysiological marker, with other measures being regarded as potentially more discriminatory. Do Carmo et al66 and Renfree et al97,98 have demonstrated that another psychophysiological construct, the effect (or valence) toward a task (the degree to which momentary effort is viewed as pleasant or unpleasant) is more explanatory of when an athlete is having a good or bad performance, despite identical RPE growth? Thus, affect appears superior to RPE in the heuristic type of decision-making processes that athletes often use. Given the importance of head-to-head competition in augmenting performance,68,97–100 the ability of athletes to solve the performance challenges raised by their physiology, the capacity, and tactics of their opponents, and challenges presented by the course and environment requires a more granular psychophysiological tool than RPE.

Venhorst et al73,75,76 have shown that effect (valence) and RPE grow differently during head-to-head competition and reflect the degree to which an athlete is “winning” or “losing” a competition. In particular, changes in effect (valence) reflect the point in a  competition when athletes first begin to fall behind and then “disengage” from their competitors (action crisis).73,75,76 They suggest that psychophysiological regulation of exercise behavior can be viewed in 3 dimensions. The first is perceived physical and mental strain, reflecting sensory-discriminatory processes akin to homeostatic disturbances. The second is affect and arousal reflecting the interpretation of effort as pleasant–unpleasant and the momentary level of arousal. This can be viewed as interpreting whether the increasing level of discomfort is worth continued effort. The third is a cognitive-evaluative process, what they term an “action crisis” or “letting go” of their opponent in mid-race. Their model accounts for traditional homeostatic challenges provided by a task, how pleasant or unpleasant the task is, and how willing they are to continue to compete.

The Pacing Template (Self-Regulation Model)

One striking element of pacing is how difficult it is to disrupt freely chosen patterns. Monetary incentives to improve performance by going out faster have little effect.101 Conscious prerace decisions to select different strategies have small effects on the actual pacing pattern used, at least in against-the-clock events.81,91 Pairing with a  faster opponent can improve performance, but only when the opponent/avatar is seen as a realistic “rival” and “within reach” of the best current performance.68–72 Otherwise, the riders “let the  superior rider go.” This corresponds to the action crisis described by Venhorst et al.73,75,76 Apparently, the magnitude of “reserve” within the pacing strategy can be revised by changing the focus from anticipatory-internal monitoring (against the clock) to relative positional-external monitoring (head-to-head so long as homeostatic changes are not ignored.

Within race experimental manipulations, such as exposing participants to sudden onset episodes of hypoxia and hyperoxia, can rapidly change the pattern of power output.28,80,89,90,102 However, blinded exposure to simulated altitude in the minutes immediately before the start of an event does little to change the early pattern of power output.89,90 Even exposure to simulated altitude during the warm-up period, sufficient to result in increases in heart rate, blood [lactate], and RPE, does little to influence power output during the opening segment of time trials (Figure 3). Beyond this initial phase, with the opportunity for afferent feedback to express itself, there is a large negative effect consistent with that expected in hypoxia.102 There is a large negative effect of prerace glycogen depletion in events ranging from 1500 (∼2 min) to 4000 m (∼5 min)102 (Figure 3) to 1 hour.48 Power output in the early stages of a time trial is only modestly affected by glycogen depletion (Figure 4). During warmup, there is an increased heart rate, decreased blood [lactate], and increased RPE, expected with glycogen depletion. Similarly, strategies designed to increase muscle glycogen content, resulting in improved performance, do not exert an effect until later in an event.46,47 Evidence supports the presence of a pre-exercise template, which is a learned behavior, specific to competitive circumstances.103 Learning may take several trials and typically evolves at a faster early pace (e.g., less “reserve”). In time trial events, this learned strategy seems very hard to override, despite conditions in the warm-up that might be expected to reset the template.104 In head-to-head competitions, it is possible to reset the template. This supports data regarding the development of pacing strategies in youth athletes of the need for experience to develop self-regulating strategies.105,106

In fit people, with minimal time trial experience, there is evidence of modifications in the template with repeated time trials,103 that may take ≥6 trials. In athletes attempting to improve their best performance, the pacing pattern is more or less similar,  with the exception that the opening segment is slightly faster,  suggesting that improved performance is more attributable to improved physiologic capacity than to pacing.25 Empirical evidence suggests that competitive performance may improve when novel pacing strategies are employed during practice or less important competitions, to reset the template.16

Specific attempts to influence the pacing strategy, such as by mid-race “break away” efforts,81,91 support the concept of a template, in that upward speed departures from a normal template in 10- to 20-km time trials are marked by a subsequent reduction of power output until homeostatic disturbances (heart rate, blood [lactate], RPE, and muscle O2 saturation) return toward normal, at which time the template is resumed (Figure 5). Similarly,  attempts to force starting ∼5% faster or slower over the first 30% of a time trial show a rapid return to the “best race” template as soon as the experimental constraints are removed.96

Pacing Strategy Versus Racing Strategy

Early research on pacing was mostly conducted on events where performance was against the clock, the competitive pattern in pursuit cycling, 1-hour cycling, metric-style speed skating, and swimming. Many events where pacing might be important are decided based on relative placing rather than absolute time, leading to a more stochastic pacing pattern.67,104,107–111 These events demonstrate evidence of variations in starting strategy and of an end-spurt. In addition, they display evidence of intentional variations in speed or power output. Within a single elite athlete, WR or best performances are often characterized by small variations in momentary speed (e.g., low coefficient of variation). Championship races are often characterized by frequent, potentially preplanned, variations in momentary speed, and high speed during the end spurt, high coefficient of variation. Variations in pacing seem designed to drop weaker competitors from the leading group and reduce the number of competitors in contention before the end-spurt occurs.67,104,107–111

Hettinga et al68 discussed the role of opponents in pacing,  using ecological principles and the affordance hypothesis. They explored mechanisms of interactive behavior, proposing a pacing framework to understand head-to-head competition in which both internal (e.g., fatigue) and external (e.g., opponent) factors interact. Support for this model was obtained through a series of laboratory and field studies67,68 pacing behaviors of other exercisers69 and different competitive circumstances. In addition to a preplanned template, interactions with competitors and other environmental aspects play roles that have been described as the affordance concept, wherein the actions of the opponents afford the athlete a range of possibilities to modify preplanned strategies.67–69,74

St Clair Gibson et al112 proposed the Integrative Governor Theory proposing a continuous oscillation between psychological drives (e.g., competitive goals) and homeostatic disturbances that serve to regulate momentary power output. Both concepts highlight the complexity of the processes regulating momentary power output and highlight the meaningfulness of competition and actions of opponents are drivers of competitive strategy. In addition, since slower starting strategies reduce feelings of effort during competition,96 there is a tendency in head-to-head competition to start slower than the best performance strategy, insert competitive “surges,” and recovery sections, and rely on the end-spurt to win the race. This is true unless the athlete perceives that their end spurt might be inadequate to match other competitors, whereupon higher intensity segments might be inserted to neutralize the end spurt of other athletes or to force them to drop off mid-race. This is an example of the concept of affordances. Head-to-head races use the best performance strategy until the actions or perceived capabilities of opponents allow for using stochastic pacing. This is particularly true in aerodynamic (cycling, speed skating) or hydrodynamic (rowing, swimming) events where the cost of locomotion can be influenced by pacing or where the pacing of teammates (cycling, pack style skating, or team pursuit skating) or adversaries (Grand Tours, open water swimming) can influence energy cost. It is even possible that an athlete may go to the front, to slow the pace, if they perceive that they cannot effectively complete the pace their opponents have adopted. In other words, starting with the best performance strategy as a default, pacing in head-to-head competitive events can be modified almost infinitely depending on the real or potential behavior of competitors. However, the overriding need to limit the magnitude of homeostatic disturbances remains, causing competitors to change from the externally monitored competitive strategy back to the internally monitored best performance (e.g., survival) strategy. Opponents have thus been called social placebos/nocebos, influencing expectations regarding successful/unsuccessful pacing and performance.113

CS and Pacing

CS or CP is the speed /power associated with the highest sustainable metabolic rate.8 This is derived from the asymptote for the hyperbolic speed–time or power–time relationship, recognized for nearly 60 years,7,8 and anticipated before the turn of the 20th century.11 Although not the same, CS/CP approximates the physiological intensity of the maximal lactate steady state, the second ventilatory threshold, or the second lactate threshold.8,114 CS/CP is at least as explanatory of endurance performance as maximal oxygen consumption and ventilatory threshold. If the CS/CP  explains the upper limit of sustainable aerobic power, the concept of D′ (or W′) representing the curvature constant of the speed–time or power–time relationship accounts for additional nonoxidative energetic capacity during exercise above CS/CP. The momentary balance of W′/D′ can explain the likelihood of needing to decrease power output during severe exercise or the ability to increase power output in service of competitive goals.115,116 This “anaerobic”  energy can be used as needed to sustain metabolic rates over CS/CP in shorter events (<15 min), to make mid-race surges, or during the end-spurt. Using the direct measurement of anaerobically attributable energy supply, there is evidence78,117,118 that, within an individual, the magnitude of anaerobically attributable energy (e.g., D′), after adjustment for changes in gross efficiency, may be more or less constant.80 There is evidence supporting the concept that the D′/W′ may be reconstituted if, during the middle of an event, the speed/power output decreases below CS/ CP.115,116 Examining the pacing of elite runners during 10-km competitions, it is evident that WR performances are performed close to CS, whereas important races (Olympic finals) are contested with an average speed < CS, but with tactical bursts above CS (Figure 6).104,118 Pacing in groups of runners (first 3, middle 3, and last 3) in an Olympic final show that better runners run much of the early part of the event < CS, preserving D′ for the end-spurt, whereas less good runners run the early part of the event > CS to stay with the early pace, thus limiting energetic reserve (D′) to contest the last laps (Figure 7). This concept has been called the D′ balance.116 On this basis, it would be expected that the D′ balance would fall to very low values near the end of a race. Recent evidence from WR 1-mile races (entirely > CS) and high-level 800-m swimming races117,118 supports this expectation (Figure 8). Additional evidence from the 2008 Olympic men’s 10-km race indicates that the CS/D′ balance could predict how high-level races unfolded, including evidence that 80% of athletes falling out of contention before the end spurt do so, often by mid-race, when D′ reaches critically low levels and that D′ often increases during the remainder of the race as they are running < CS (eg, survival mode). However, in the 20% remaining in contention until the last 400 m, the magnitude of D′ falls to very low values only at the end of the race (Figure 8).118 Recent evidence suggests that the magnitude of the end-spurt was related to how well runners were able to preserve D′ until the last 400 m and that superior athletes might win or lose competitions based on good or poor management of D′. 108

The CS/CP and D′/W′ seem to be as definitional of performance level and pacing strategy as were prior candidates such as maximal oxygen consumption, lactate threshold/ventilatory threshold, and the O2 cost of running,8,119,120 While these metrics are still powerful predictors of the ability to move at a certain pace, the concept of an anaerobic capacity,121 and how it is deployed during an event, represented by the concept of D′ is useful for analysis of performance, for explaining why some athletes drop off the leading group during mid-race, and why some athletes have particularly effective end-spurts. 108

The CS/CP may also explain, at least in part, athletes’ predisposition to use a fast start strategy during shorter, high-intensity events. There is evidence that such an approach speeds VO2 kinetics, leading to a greater aerobic contribution in the early phase of exercise, thereby sparing D′/W′. This effect of a fast start strategy on VO2 kinetics also increases CP compared with that established using constant work rate protocols. The pattern of D′/W′ use during short-duration exhaustive exercise, where W′ starts at 100% and finishes near 0%, will also be altered by a U-shaped (relatively fast start and finish) compared with more even pacing. The regularly adopted U-shaped pacing strategy may be a behavioral evolution not only because is it likely to be performance-enhancing but also because it would result in a higher W′/D′ over a large fraction of the mid-race, potentially making the exercise feel more tolerable.

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