High-volume intermittent maximal intensity isometric exercise caused great stress, although central motor fatigue did not occur

Abstract

To establish whether very high-volume, high-intensity isometric exercise causes stress to the body and how it affects peripheral and central fatigue. Nineteen physically active healthy male subjects (21.2 ± 1.7 years; height - 1.82 ± 0.41 m, body weight - 79.9 ± 4.5 kg; body mass index - 24.3 ± 2.1 kg/m2) volunteered to participate in this study. They participated in two experiments 3-5 days apart. Each experiment comprised six series of 60-s maximum voluntary contraction (MVC) force (knee extension) achieved as rapidly as possible. This very high-volume, high-intensity exercise (HVHIE) was performed at different quadriceps muscle lengths: short (SL) and long (LL). The MVC and the electrically stimulated contractile properties of the muscle were measured prior to HVHIE, immediately after and 3 min after each series, and at 3, 10, and 30 min after the end of HVHIE. We found that HVHIE caused high levels of stress (cortisol levels approximately doubled, heart rate and the root mean square successive difference of interval (RMSSD) decreased by about 75%); lactate increased to 8-11 mmol/L, voluntary and 100 Hz stimulation-induced force (recorded immediately after HVHIE) decreased by 55% at LL and 40% at SL. However, the central activation ratio during MVC did not change after either exercise. Isometric HVHIE performed using one leg caused high levels of stress (RMSSD decreased, cortisol increased after HVHIE equally at SL and LL; La increased more while exercising at LL) and the voluntary and electrostimulation-induced muscle force significantly decreased, but muscle central activation during MVC did not decrease.

Keywords: High-intensity isometric exercise; High-volume; Muscle length; Peripheral and central fatigue; Physiological stress; Rate of force development.

FIG. 1

Study design. HRV – heart rate variability; ES – electrostimulation induced torque at 1 Hz (P1) and 100 Hz (P100); MVC+DS – electrostimulation during maximal voluntary contraction. S1, S2, S3. S4, S5 and S6 – 6 series of maximal voluntary contraction (MVC) of 60 s each. The rest interval between each series was 3 min, i.e. the time period from ES after MVC 60 s to ES before the next MVC.

FIG. 2

Changes in torque integral during each 10 s of 6 series (S1, S2, S3, S4, S5, S6) of MVC-60 s at short (A) and long (B) quadriceps muscle lengths and changes in torque integral during 60 s of MVC (C) and decrease in torque integral of 10 s of each 6 series (D) (S1, S2, S3, S4, S5, S6) of MVC-60 (e.g. MVC-50– 60 s/MVC-10 s x 100 percent in each series) at short and long quadriceps muscle lengths. Data are presented as mean ± SD. * – p < 0.01 compared to the first 10 s of the first series of MVC-60 s; # – p < 0.01 10 s torque integral significant decrease during MVC-60 s; & – p < 0.01 compared to the short length; $ – p < 0.01 compared to the first series of MVC-60 s.

FIG. 3

Changes in maximal voluntary contraction torque (MVC) (A), rate of force development (RFD) during MVC (B), central activation ratio (CAR) (C) and MVC/P100 before and after each 6 series (S1, S2, S3, S4, S5, S6) of MVC-60 s at short and long quadriceps muscle lengths. MVC, RFD, CAR and MVC/P100 values are given at the beginning of each series and immediately after the end of each series. P100 – electrostimulation induced torque at 100 Hz. R3, R10 and R30 – recovery after 3, 10 and 30 min after exercise respectively. Data are presented as mean ± SD. * – p < 0.01 compared to before of S1.

FIG. 4

Changes in EMGrms (A and B) and median frequency of EMG (C) at the beginning of each series and after each 6 series (S1, S2, S3, S4, S5, S6) of MVC-60 at short and long quadriceps muscle lengths (100 percent is the beginning of S1). R3, R10 and R30 – recovery after 3, 10 and 30 min after exercise respectively. Data are presented as mean ± SD. * – p < 0.01 compared to before of S1.

FIG. 5

Changes in electrically induced torque at 1 Hz (A), 100 Hz (B) and P1/P100 before and after each 6 series (S1, S2, S3, S4, S5, S6) of MVC-60 s at short and long quadriceps muscle lengths. R3, R10 and R30 – recovery after 3, 10 and 30 min after exercise respectively. Data are presented as mean ± SD. * – p < 0.01 compared to before of S1.

FIG. 6

Changes in rate of force development during electrically induced 100 Hz stimulation (RFDs) (A) and RFDv/RFDs before and after each 6 series (S1, S2, S3, S4, S5, S6) of MVC-60 s at short and long quadriceps muscle lengths. RFDv – rate of force development during maximal voluntary contraction. R3, R10 and R30 – recovery after 3, 10 and 30 min after exercise. Data are presented as mean ± SD. * – p < 0.01 compared to before of S1.

FIG. 7

Changes in lactate (A) and cortisol (B) concentration after 6 series of MVC-60 at short and long quadriceps muscle lengths. Data are presented as mean ± SD. * – p < 0.01 compared to before.