The anaerobic processes involved in muscular activity

Heart Rate Variability After Sprint Interval Training in Cyclists and Implications for Assessing Physical Fatigue

ABSTRACT

Hebisz, RG, Hebisz, P, and Zatoń, MW. Heart rate variability after sprint interval training in cyclists and implications for assessing physical fatigue. J Strength Cond Res 36(2): 558-564, 2022-This study evaluated the time- and frequency-domain indexes of heart rate variability (HRV) during sprint interval exercise test (SIXT) and identify the onset of fatigue by HRV concurrent with changes in average (Pavg) and peak (Ppeak) power output, total oxygen uptake (V̇o2tou), and blood hydrogen (H+) and lactate (La-) concentrations. Twenty-seven cyclists performed 4 sets of SIXT in which each set consisted of four 30-second maximal sprints interspersed with 90 seconds of low-intensity cycling. Each set was separated by 25-40 minutes of recovery. Before beginning each set, HRV was analyzed by time (mean normal-to-normal RR intervals [RRNN], SD of normal-to-normal RR intervals [SDNN], and square root of the mean squared difference between successive normal-to-normal RR intervals [RMSSD]) and frequency (total spectral power [T] and very low- [VLF], low- [LF], and high-frequency [HF] spectral power) domain methods. Pavg, Ppeak, and V̇o2tou were recorded in each set, and H+ and La- were measured after each set. RRNN, SDNN, and VLF decreased in the second set, whereas all time and frequency indexes of HRV decreased in the third and fourth set. Pavg and H+ decreased, while V̇o2tou increased in the fourth set. Ppeak decreased in the second, third, and fourth set. Correlations were found between the changes in the time and frequency indexes of HRV with H+, La-, and V̇o2tou. The results indicate that HRV does not reflect the onset of physical fatigue in SIXT as was observed in Pavg and no correlation was found between the changes in HRV with Pavg and Ppeak.

'Aerobic' and 'Anaerobic' terms used in exercise physiology: a critical terminology reflection

The purpose of this Current Opinion article is to focus on the appropriate use of the terms 'aerobic'- and 'anaerobic'-exercise in sports medicine, in order to try to unify their use across coaches/athletes and sport scientists. Despite the high quality of most of the investigations, the terms aerobic/anaerobic continue to be used inappropriately by some researchers in exercise science. Until late 2014, for instance, 14,883 and 6,136 articles were cited in PubMed, in the field of 'exercise science', using the words 'aerobic' or 'anaerobic', respectively. In this regard, some authors still misuse these terms. For example, we believe it is wrong to classify an effort as 'anaerobic lactic exercise' when other metabolic pathways are also simultaneously involved. It has extensively been shown that the contribution of the metabolic pathways mainly depends on both exercise intensity and duration. Therefore, it is our intent to further clarify this crucial point and to simplify this terminology for coaches and sports scientists. In this regard, several research articles are discussed in relation to the terminology used to describe the predominant metabolic pathways active at different exercise durations and the oversimplification this introduces. In conclusion, we suggest that sports scientists and field practitioners should use the following terms for all-out ('maximal') efforts based on exercise duration: (a) 'Explosive Efforts' (duration up to 6 s, with preponderance of the 'phosphagens' metabolic pathway'); (b) 'High Intensity Efforts' (efforts comprised between >6 s and 1 min, with preponderance of the 'glycolytic pathway'), and

Cerebral glycolysis: a century of persistent misunderstanding and misconception

Since its discovery in 1780, lactate (lactic acid) has been blamed for almost any illness outcome in which its levels are elevated. Beginning in the mid-1980s, studies on both muscle and brain tissues, have suggested that lactate plays a role in bioenergetics. However, great skepticism and, at times, outright antagonism has been exhibited by many to any perceived role for this monocarboxylate in energy metabolism. The present review attempts to trace the negative attitudes about lactate to the first four or five decades of research on carbohydrate metabolism and its dogma according to which lactate is a useless anaerobic end-product of glycolysis. The main thrust here is the review of dozens of scientific publications, many by the leading scientists of their times, through the first half of the twentieth century. Consequently, it is concluded that there exists a barrier, described by Howard Margolis as “habit of mind,” that many scientists find impossible to cross. The term suggests “entrenched responses that ordinarily occur without conscious attention and that, even if noticed, are hard to change.” Habit of mind has undoubtedly played a major role in the above mentioned negative attitudes toward lactate. As early as the 1920s, scientists investigating brain carbohydrate metabolism had discovered that lactate can be oxidized by brain tissue preparations, yet their own habit of mind redirected them to believe that such an oxidation is simply a disposal mechanism of this “poisonous” compound. The last section of the review invites the reader to consider a postulated alternative glycolytic pathway in cerebral and, possibly, in most other tissues, where no distinction is being made between aerobic and anaerobic glycolysis; lactate is always the glycolytic end product. Aerobically, lactate is readily shuttled and transported into the mitochondrion, where it is converted to pyruvate via a mitochondrial lactate dehydrogenase (mLDH) and then is entered the tricarboxylic acid (TCA) cycle.

The recovery heat-production in oxygen after a series of muscle twitches

Experiments determining the recovery heat-production in oxygen after a short tetanic stimulus have been described on various occasions, e. g. , by Hartree and hill (1) (2) and by Hartree and Liljestrand (3). In employing the results of these experiments to calculate the "oxidative quotient for lactic acid" as defined by Meyerhof (see, for example (4). p. 567), viz.: (Lactic acid removed in recovery) / (Lactic acid-or carbohydrate equivalent-oxidised) a complication arises in respect of the anaerobic delayed heat (see 1, 5 and 6).

The surface-tension theory of muscular contraction

It was shown by Bernstein (1) in 1908 that the maximum mechanical response in a muscle twitch is greater at a lower temperature. Since surface tension decreases as the temperature is raised, this observation was regarded as strong evidence in favour of the theory that “ changes in surface tension are a controlling factor in the development of the energy of muscular contraction ” (Bayliss (2), p. 448); other physical effects such as osmotic pressure and “ Quellung ” were, according to Bernstein, excluded since these increase as the temperature is raised. If it had been shown at the same time that the total energy liberated in a muscular contraction was independent of temperature, the mechanical energy alone varying, this might indeed have been regarded as in favour of a surface-tension theory. Actually, however, the total heat set free in a twitch decreases as the temperature is raised, in just the same way as does the tension; indeed, there is a very constant relation between the two, so that for a given liberation of total energy , i. e., for a given chemical change , the tension energy set free is independent of the temperature . Bernstein’s observation, therefore, gives us no grounds for concluding that the development of the mechanical response in muscle is due in any way to changes of surface tension. To put the matter in terms of lactic acid, a given production of lactic acid is accompanied by the same rise of tension whatever the temperature. If further evidence be required against the deduction from Bernstein’s observations it is supplied by the fact that in a tetanic contraction the tension developed and the heat set free are both greater, and not less, at the higher temperature. When a frog’s muscle is maintained in a constant state of contraction by a succession of stimuli, the tension is not lower at a higher temperature, as it should be on the surface-tension theory, but appreciably higher. Another explanation of these facts has been given by Hartree and Hill (3, p. 141).

The absence of delayed anaerobic heat in a series of muscle twitches

In 1922, Hartree and Hill (1) described a delayed heat-production after a short tetanic stimulus in a muscle deprived of oxygen, amounting to about one-third of the recovery beat in oxygen. In 1923 the same authors (2) re-examined the matter, with greater precautions to exclude oxygen, and found a "most probable " value for this delayed anaerobic heat of about one-quarter of the initial, or one-sixth of the recovery, heat. The existence of this heat has remained an obscure phenomenon, a complication in an otherwise comparatively simple scheme, and a further attempt was made by Furusawa and Hartree (3) in 1926 to trace its source. In spite of all precaution to exclude oxygen, and to obviate physical effects (lack of uniformity in the muscle, etc.) which might produce the same apparent result, the delayed heat persisted, and Furusawa and Hartree concluded that its minimum value was about 12 per cent, of the initial heat ; in many cases. It was more, sometimes much more. It should be noted that all the investigations referred to dealt with a short tetanus, not with a single twitch. The increment produced by stimulation in the resting heat-rate of a muscle under anaerobic condition, described in a previous paper of the present series, is one of the factors responsible for the effect discussed. Unless the galvanometer-zero, and the temperature of the thermopile-chamber, be extremely constant, it is easy to misinterpret the permanent shift of position of the galvanometer alter stimulation in nitrogen, and to deduce the existence of a long-continued slow production of beat gradually diminishing to zero. The reactions underlying the increment in the resting beat-rate are not part of the process of activity itself, although induced by it, and it is incorrect to attribute the energy the liberate to the preceding contraction; this is obviously the case, since the increment in beat-rate we know now to be permanent, so that the energy liberated is not constant but proportional to the time during which one choose to follow the galvanometer defection. It will be shown, indeed, by Hartree and Hill in a later paper of this series that such anaerobic delayed heat as really exists, in the case of a tetanus, occupies only a minute or two alter the stimulus; the long-continued part of the delayed heat, described in each of the three papers cited above, is an error due to a misinterpretation of the facts, which could not then be observed with the same accuracy as is possible now.

The anaerobic delayed heat-production after a tetanus

In a previous paper of this series it was shown by one of us that, in the case of a regular succession of separate twitches, there is no considerable anaerobic delayed heat-production: the only important after-effect of such anaerobic stimulation is a permanent increase in the resting beat-rate. It was pointed out there that this increment, produced by activity, in the basal beat-rate, cannot be regarded as part of the contraction-process itself, though it may easily be misinterpreted; unless the galvanometer-sere and the temperature of the thermopile be very steady, this permanent increase may be mistaken for a long-continued delayed heat associated with contraction and ending in 10 or 15 minutes. The diagram of fig. 1 illustrates this point, At time zero the muscle is stimulated by a short tetanus and the galvanometer deflects, returning to a constant position—but not to its original one—in about 3 minutes. The displacement from the initial position is a measure of the increment in resting heat-rate. This increment must be supposed to occur at, or immediately after, the moment of contraction. There is no obvious reason why it should occur at any other moment. If so, the baseline from which the area of the deflection-time curve must be calculated is the continuation backwards of the line representing the final level attained by the galvanometer. Now let us imagine that owing to galvanometer instability, or to gradual temperature change in the thermopile, there is a slow, small and not quite regular creep of the spot of light on the scale. The only way to proceed is to allow a sufficient time to elapse after stimulating and to join the initial to the final position, so obtaining a base-line ( e. g ., B in the diagram) from which to measure the area of the deflection-time curve. It is obvious that a greater area will thus lie found than if the correct base-line A bad been adopted. His case illustrated in the diagram is exaggerated. It will made clear, however, how an error may occur unless extreme stability of galvanometer and thermostat be available. We have found, employing the present apparatus, in all cases, whether of short tetanic stimuli or of a series of single shocks, that with the thermopiles used the galvanometer becomes quite steady again in 3 or 4 minutes: there is no long-continued delayed anaerobic heat , such as was described by ourselves (1) (2) and by Furusawa and Hartree (3). The genuine anaerobic delayed heat is confined to the first 2 or 3 minutes after stimulation.

Observations on muscle-fibre structure: The swelling of muscle fibres by acids and alkalis

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