Christian Froyd

The development of peripheral fatigue and short-term recovery during self-paced high-intensity exercise

In this study we describe the time course of fatigue development during and after an intense bout of self‐paced, high‐intensity dynamic exercise using various electrical stimulation parameters to assess neuromuscular function (NMF) changes. Most of the decrease in muscle function occurs within the first 40% of the exercise bout, and substantial recovery in muscle function occurs within 1–2 min after exercise termination. Decreases in muscle function varied greatly with different methods of stimulation, suggesting that the extent to which muscle fatigue is documented during exercise depends upon NMF assessment methodology. Measurements of muscle function must be performed as soon as possible after exercise termination, and previous studies may have underestimated the extent to which muscle fatigue develops during exercise.

Conventional testing methods produce submaximal values of maximum oxygen consumption

Background This study used a novel protocol to test the hypothesis that a plateau in oxygen consumption (VO2max) during incremental exercise testing to exhaustion represents the maximal capacity of the cardiovascular system to transport oxygen. Methods Twenty-six subjects were randomly divided into two groups matched by their initial VO2max. On separate days, the reverse group performed (i) an incremental uphill running test on a treadmill (INC1) plus verification test (VER) at a constant workload 1 km h−1 higher than the last completed stage in INC1; (ii) a decremental test (DEC) in which speed started as same as the VER but was reduced progressively and (iii) a final incremental test (INCF). The control group performed only INC on the same days that the reverse group was tested. Results VO2max remained within 0.6 ml kg−1 min−1 across the three trials for the control group (p=0.93) but was 4.4% higher during DEC compared with INC1 (63.9±3.8 vs 61.2±4.8 ml kg−1 min−1, respectively, p=0.004) in the reverse group, even though speed at VO2max was lower (14.3±1.1 vs 16.2±0.7 km h−1 for DEC and INC1, respectively, p=0.0001). VO2max remained significantly higher during INCF (63.6±3.68 ml kg−1 min−1, p=0.01), despite an unchanged exercise time between INC1 and INCF. Conclusion These findings go against the concept that a plateau in oxygen consumption measured during the classically described INC and VER represents a systemic limitation to oxygen use. The reasons for a higher VO2 during INCF following the DEC test are unclear.

Central Regulation and Neuromuscular Fatigue during Exercise of Different Durations.

PURPOSE The aim of this study was to determine if exercise time trials (TT) of different durations would cause different levels of peripheral and central fatigue during exercise. METHODS Twelve trained subjects (11 men, one woman) performed TT lasting 3, 10, and 40 min with repetitive self-paced concentric right knee extension at 60°·s on an isokinetic dynamometer. Neuromuscular function was assessed before, during, and immediately after the TT using voluntary and electrically evoked forces. RESULTS Maximal voluntary contraction force, evoked peak force for single stimulus, and rating of perceived exertion reached similar levels at termination of all TT. Evoked peak force for paired stimuli of 100 Hz decreased more for the 40-min TT compared with the 3-min TT (-42% ± 15% vs -37% ± 13%, P < 0.05), and central fatigue was significant for the 40-min TT and 10-min TT but not for the 3-min TT. Single stimulus and paired stimuli of 100 Hz decreased, whereas voluntary electromyography normalized to M-wave for self-paced contractions increased during the end-spurt in all TT. CONCLUSIONS These data demonstrate that the extent of peripheral and central fatigue that contribute to reductions in force of single-limb dynamic contractions depend on the duration and intensity of self-paced exercise. There was no evidence for a critical threshold in peripheral fatigue that was common to all TT.

Potentiation Increases Peak Twitch Torque by Enhancing Rates of Torque Development and Relaxation

Abstract The aim of this study was to measure the extent to which potentiation changes in response to an isometric maximal voluntary contraction. Eleven physically active subjects participated in two separate studies. Single stimulus of electrical stimulation of the femoral nerve was used to measure torque at rest in unpotentiated quadriceps muscles (study 1 and 2), and potentiated quadriceps muscles torque in a 10 min period after a 5 s isometric maximal voluntary contraction of the quadriceps muscles (study 1). Additionally, potentiated quadriceps muscles torque was measured every min after a further 10 maximal voluntary contractions repeated every min (study 2). Electrical stimulation repeated several times without previous maximal voluntary contraction showed similar peak twitch torque. Peak twitch torque 4 s after a 5 s maximal voluntary contraction increased by 45±13% (study 1) and by 56±10% (study 2), the rate of torque development by 53±13% and 82±29%, and the rate of relaxation by 50±17% and 59±22%, respectively, but potentiation was lost already two min after a 5 s maximal voluntary contraction. There was a tendency for peak twitch torque to increase for the first five repeated maximal voluntary contractions, suggesting increased potentiation with additional maximal voluntary contractions. Correlations for peak twitch torque vs the rate of torque development and for the rate of relaxation were r2= 0.94 and r2=0.97. The correlation between peak twitch torque, the rate of torque development and the rate of relaxation suggests that potentiation is due to instantaneous changes in skeletal muscle contractility and relaxation.

No Critical Peripheral Fatigue Threshold during Intermittent Isometric Time to Task Failure Test with the Knee Extensors

It has been proposed that group III and IV muscle afferents provide inhibitory feedback from locomotor muscles to the central nervous system, setting an absolute threshold for the development of peripheral fatigue during exercise. The aim of this study was to test the validity of this theory. Thus, we asked whether the level of developed peripheral fatigue would differ when two consecutive exercise trials were completed to task failure. Ten trained sport students performed two exercise trials to task failure on an isometric dynamometer, allowing peripheral fatigue to be assessed 2 s after maximal voluntary contraction (MVC) post task failure. The trials, separated by 8 min, consisted of repeated sets of 10 × 5-s isometric knee extension followed by 5-s rest between contractions. In each set, the first nine contractions were performed at a target force at 60% of the pre-exercise MVC, while the 10th contraction was a MVC. MVC and evoked force responses to supramaximal electrical femoral nerve stimulation on relaxed muscles were assessed during the trials and at task failure. Stimulations at task failure consisted of single stimulus (SS), paired stimuli at 10 Hz (PS10), paired stimuli at 100 Hz (PS100), and 50 stimuli at 100 Hz (tetanus). Time to task failure for the first trial (12.84 ± 5.60 min) was longer (P < 0.001) than for the second (5.74 ± 1.77 min). MVC force was significantly lower at task failure for both trials compared with the pre-exercise values (both P < 0.001), but there were no differences in MVC at task failure in the first and second trials (P = 1.00). However, evoked peak force for SS, PS100, and tetanus were all reduced more at task failure in the second compared to the first trial (P = 0.014 for SS, P < 0.001 for PS100 and tetanus). These results demonstrate that subjects do not terminate exercise at task failure because they have reached a critical threshold in peripheral fatigue. The present data therefore question the existence of a critical peripheral fatigue threshold during intermittent isometric exercise to task failure with the knee extensors.