Modelling human power and endurance

A generalised three component hydraulic model has been proposed to represent the human bioenergetic processes relating internal energy stores to performance during exercise, and into recovery. Further development of the model allows testable predictions to be made. In particular in this paper I examine certain hypotheses of chemical fuel shortage as a subgroup of the potential causes of fatigue, and their implications for maximal power and for endurance. The assumption that the limitation to sustainable power is direct proportionality to the glycogen store remaining, appears the most feasible. Based on this assumption, equations for the decline in maximum attainable power over time, the endurance at fixed workrates and the endurance at incremental tests (as a function of the increment slope) are obtained. Using published data for fit males, the maximum exertable power declines after about 6 s at 972 W to very low levels after about 2 min. For constant powers selected between 208 and 927 W, endurance declines from ad infinitum to only 6 s. Endurance at VO2max is predicted to be about 9 min. For incremental exercise tests of slope ranging from 30 W/min to 60 W/min, endurance lessens from 14 to 9 min. In these tests the anaerobic threshold is reached in times between 6 and 3 min. Although the power at termination of a test increases with incremental slope, terminal oxygen consumption is effectively constant. Almost all these model predictions are observed to correspond well with published experimental findings. These results suggest that the model can be used to represent an adequate overview of the operation of the human bioenergetic system.     

Recovery time affects immunoendocrine responses to a second bout of endurance exercise

The purpose of this study was to examine the effect of different durations of rest between two bouts of exercise on immunoendocrine responses during and after the second bout of exercise. Nine endurance athletes participated in three 25-h trials: 1) complete bed rest (REST), 2) two bouts of exercise separated by 3 h of rest (SHORT), and 3) two bouts of exercise separated by 6 h of rest (LONG). Each cycle ergometer exercise bout lasted 75 min at 75% of maximal O(2) uptake. We observed a more pronounced increase in epinephrine, norepinephrine, adrenocorticotropic hormone, and cortisol, but not in growth hormone, and a larger neutrophilia and lymphocytopenia in connection with the second bout of exercise in trial SHORT compared with trial LONG. Lymphocyte activation was unaltered by the difference in rest protocol. In conclusion, a second bout of exercise elicited more pronounced change in neuroendocrine factors and leukocyte counts when preceded by 3 h of rest as opposed to 6 h of rest after the first bout of exercise.     

Estimation of time of divergence from phylogenetic studies.

Recent studies with comparative data on base sequences of homologous DNA's or amino acid sequences of homologous proteins indicate that simultaneous estimation of phylogenetic structure and time of divergence is often cumbersome and time consuming. On the other hand, when the topology of an evolutionary tree is known, it is shown in this paper that the least squares theory may be applied to obtain simple estimates of the relative time lengths for each segment of the tree under the assumption of uniform random substitutions in each segment. The method is illustrated with amino acid sequence data on various globin molecules and cytochrome c. The evolutionary significance of some of the estimates is also discussed.     

Determinants of time trial performance and maximal incremental exercise in highly trained endurance athletes

Human endurance performance can be predicted from maximal oxygen consumption (Vo(2max)), lactate threshold, and exercise efficiency. These physiological parameters, however, are not wholly exclusive from one another, and their interplay is complex. Accordingly, we sought to identify more specific measurements explaining the range of performance among athletes. Out of 150 separate variables we identified 10 principal factors responsible for hematological, cardiovascular, respiratory, musculoskeletal, and neurological variation in 16 highly trained cyclists. These principal factors were then correlated with a 26-km time trial and test of maximal incremental power output. Average power output during the 26-km time trial was attributed to, in order of importance, oxidative phosphorylation capacity of the vastus lateralis muscle (P = 0.0005), steady-state submaximal blood lactate concentrations (P = 0.0017), and maximal leg oxygenation (sO(2LEG)) (P = 0.0295), accounting for 78% of the variation in time trial performance. Variability in maximal power output, on the other hand, was attributed to total body hemoglobin mass (Hb(mass); P = 0.0038), Vo(2max) (P = 0.0213), and sO(2LEG) (P = 0.0463). In conclusion, 1) skeletal muscle oxidative capacity is the primary predictor of time trial performance in highly trained cyclists; 2) the strongest predictor for maximal incremental power output is Hb(mass); and 3) overall exercise performance (time trial performance + maximal incremental power output) correlates most strongly to measures regarding the capability for oxygen transport, high Vo(2max) and Hb(mass), in addition to measures of oxygen utilization, maximal oxidative phosphorylation, and electron transport system capacities in the skeletal muscle.     

Physiological correlates of endurance time variability during constant-workrate cycling exercise in patients with COPD

Rationale

The endurance time (T_end) during constant-workrate cycling exercise (CET) is highly variable in COPD. We investigated pulmonary and physiological variables that may contribute to these variations in T_end.

Methods

Ninety-two patients with COPD completed a CET performed at 80% of peak workrate capacity (W_peak). Patients were divided into tertiles of T_end [Group 1: <4 min; Group 2: 4–6 min; Group 3: >6 min]. Disease severity (FEV₁), aerobic fitness (W_peak, peak oxygen consumption [ _peak], ventilatory threshold [ _VT]), quadriceps strength (MVC), symptom scores at the end of CET and exercise intensity during CET (heart rate at the end of CET to heart rate at peak incremental exercise ratio [HR_CET/HR_peak]) were analyzed as potential variables influencing T_end.

Results

W_peak, _peak, _VT, MVC, leg fatigue at end of CET, and HR_CET/HR_peak were lower in group 1 than in group 2 or 3 (p≤0.05). _VT and leg fatigue at end of CET independently predicted T_end in multiple regression analysis (r = 0.50, p = 0.001).

Conclusion

T_end was independently related to the aerobic fitness and to tolerance to leg fatigue at the end of exercise. A large fraction of the variability in T_end was not explained by the physiological parameters assessed in the present study. Individualization of exercise intensity during CET should help in reducing variations in T_end among patients with COPD.     

The effect of measurement time when evaluating static muscle endurance during sustained static maximal gripping

The purpose of this study was to examine the useful measurement time when evaluating static muscle endurance by comparing various parameters during sustained static gripping for 1, 3 and 6 min. Fifteen males (mean +/- SD age 20.8 +/- 1.3 yr, height 172.9 +/- 4.6 cm, body mass 67.7 +/- 5.7 kg) and fifteen females [mean +/- SD age 20.2 +/- 0.9 yr, height 158.5 +/- 3.2 cm, body mass 55.9 +/- 4.6 kg] volunteered to participate in this study. The subjects performed the sustained static maximal grip test with a sagittal and horizontal arm position for 1, 3 and 6 min on different days. Eleven force-time parameters were selected to evaluate static muscle endurance. The trial-to-trial reliability of each measurement time of sustained static maximal gripping was very high (rxy = 0.887-0.981 (1 min), 0.912-0.993 (3 min), 0.901-0.965 (6 min)). The errors of exertion values between trials were very small (below 10%). A significant correlation was found in the following parameters: the final strength and the exponential function between 1 min and 3 min, all parameters except for the time required to reach 80% of maximal grip, the regression coefficient at post-inflection between 3 min and 6 min, and the decreasing rate between all measurement times (1 min, 3 min, and 6 min). Significant differences between the measurement times were found in all parameters except for the time to 60, 70, and 80% force decreases, and the regression coefficient of pre-inflection. There was a tendency that the longer the measurement time, the larger the decreasing force. It is suggested that for the 6 min measurement, the subjects unconsciously restrained the maximal gripping force, influenced by a psychological factor as the pain became greater. The 1 min measurement may evaluate only the remarkable decreasing phase of the decreasing force, and not evaluate the phase of an almost steady state.     

Aerobic endurance, anatomical factors and time properties of laser Doppler recorded skin postocclusive hyperaemia

Aerobic endurance capacity is partly dependent on blood supply to and metabolic capacity of the active muscles. Recordings of lower limb skin postocclusive hyperaemia with laser Doppler flowmetry can differentiate between patients with lower limb atherosclerosis and healthy controls. In this study, we investigated the relationship between aerobic endurance, calf volume, common femoral artery diameter and time properties of the postocclusive laser Doppler curve. A group of 16 healthy male subjects with values for aerobic endurance which varied from those of untrained men to elite endurance trained athletes were examined. Duration of laser Doppler recorded skin postocclusive hyperaemia was significantly correlated to both aerobic power and anaerobic threshold (P less than 0.01). Hyperaemia in subjects with large common femoral artery diameter was of shorter duration (P less than 0.05). The peak and mean body mass related blood flow during hyperaemia was correlated to anaerobic threshold (P less than 0.05). These results were in agreement with previous studies indicating an effect of endurance training on the blood supply to the muscles concerned.     

Estimation of time intervals during exposure to a static load

Ten healthy subjects from 17 to 23 years old participated in the study. The subjects had to hold the ergograph load with their right hand thus fulfilling a static work. The effort magnitude was 50 per cent of maximal value of voluntary strength. The subjects pressed the button on the ergograph handle with their thumb depending on the experimental conditions and held it for 0.8 or 2.5 s. The work with each interval included three conditions: interval estimation without static load (SL), the same with the SL and after SL. At the end of experiment the subjects worked with the interval of 2.5 s under the conditions of maximally long SL holding as far as it would go. An increase of reaction time (RT) was observed at the transition from simple button pressing to interval estimation. RT tended to increase with prolongation of a standard interval. SL did not influence significantly the RT value if it did not cause the general fatigue. A gradual increase of interval estimations was observed under the influence of SL the interval of 0.8 s being estimated more accurately. Estimation of various intervals was supposed to reflect different mechanisms of their perception. Estimation of the interval of 0.8 s was based on the memory trace processes and that of 2.5 s interval was connected with conditioned reflex activity. Apparently SL did not influence interval estimation directly but by changing the functional state of the subject's organism it predetermined a prolongation of the interval estimations.     

Effects of endurance training on three superoxide dismutase isoenzymes in human plasma

The effects of endurance training and acute exhaustive exercise on plasma levels of three superoxidedismutase (SOD) isoenzymes and the ability of superoxide generation in neutrophils were studied. Eighteen healthy male students, aged 17-22 years, who volunteered for this study, underwent three months of endurance training in swimming or running. Before and after the training course, they performed acute exercise and blood samples were collected before and after this exercise. The endurance training significantly increased maximal oxygen uptake (VO2max) in all subjects. Neither the endurance training nor the acute exercise affected the plasma CuZn-SOD level. Acute exercise after the training, but not before the training, increased both the plasma Mn-SOD and extracellular SOD (EC-SOD) levels by 33.6 and 33.5%, respectively. The training decreased the EC-SOD level at rest by 22.2%. Acute exercise after the training, but not before the training, increased the plasma lipid peroxide level, suggesting higher oxidative stress in trained subjects during exhaustive exercise. The ability of neutrophils to generate superoxide was increased by the acute exercise, but induction of the superoxide was suppressed after training. These results indicate that EC-SOD levels were changed in a different manner from the CuZn-SOD and Mn-SOD: it was decreased by training but was increased by acute exercise, suggesting that endurance training increases the reserve of EC-SOD in tissues. The results also suggest the possibility of plasma EC-SOD assay as a new index of endurance training.     

Influence of running pace upon performance: effects upon treadmill endurance time and oxygen cost.

Three possible patterns of pacing (type 1, fast/slow; type 2, slow/fast; and type 3, steady rate) were compared over a 1400 m, 4 min run. The subsequent running time to exhaustion at 370 m . min-1 was significantly longer with a type 1 than with a type 2 protocol (P less than 0.05). The steady rate pattern gave results intermediate between type 1 and type 2 pacing. Data for oxygen debt and recovery heart rate confirmed the superiority of type 1 pacing. Possible explanations included (1) a reduction of inefficient anaerobic work, and (2) a greater mechanical efficiency associated with the better matching of required effort to a tapering physiological power. Runners should aim at a steady physiological rather than a steady physical load.     

On a model of human bioenergetics. II. Maximal power and endurance

The properties of a proposed three component hydraulic model of human bioenergetics are examined in respect of the maximum exertable power and the endurance time for given workloads. An equation expressing the decline in maximal effort during an "all-out" exercise is derived, from which a VO2 time equation can be deduced. In addition a workload versus endurance time equation can be obtained, enabling the prediction of time to exhaustion. The resulting equations are illustrated graphically by means of a numerical example. Model predictions, when compared with experimental results, reveal discrepancies which confirm the rejection of the model, as was suggested in an earlier paper.