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

Abstract

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.

Neuromuscular Fatigue at Task Failure and During Immediate Recovery after Isometric Knee Extension Trials

“…Studies using protocols more comparable to the present study found that the level of peripheral fatigue at task failure is higher following a pre-fatiguing isometric trial of the same muscle group using either intermittent [15] or sustained [23] contractions. Surprisingly, evoked peak forces for SS, PS100 and tetanus did not change significantly from task failure in the first compared to the second trial (Table 1), and thus contrast with the results from the first experimental visit [15], and with the reduction in evoked peak force from the first to the third trial. We believe that this might have happened because two of the ten subjects seemed to lose motivation and "give up" exercise prematurely during the second trial (subjects 1 and 3, Figure 4).…”

“…RMS during MVCs was divided by the M-wave peak to peak amplitude of the following SS response to estimate neuromuscular activation (RMS•M -1 ) [5]. In several studies RMS•M -1 has been used to calculate neuromuscular activation [14,15,26], whereas RMS has been used to calculate central motor drive [11].…”

“…After the familiarization visit, subjects made two visits to the laboratory 3-5 days apart. On the first experimental visit, subjects performed two trials to task failure separated by eight minutes of recovery, and results from these experiments are reported elsewhere [15]. On the second experimental visit, subjects performed three trials to task failure, with varying degrees of task failure.…”

“…Stimulation current was kept constant for the same subject for all types of electrical stimulation. These methods were the same as those used in our previous studies [14,15].…”

“…This model predicts that the level of peripheral fatigue that develops during any form of exercise is task specific and must always be the same at task failure -defined as the inability to maintain the target forcesince a critical threshold of peripheral fatigue was reached [9,10]. However some authors dispute the existence of a single critical threshold of peripheral fatigue [8,[12][13][14][15][16][17].…”