Effects of fatiguing, submaximal high- versus low-torque isometric exercise on motor unit recruitment and firing behavior

Abstract

The purpose of this investigation was to evaluate the effects of repeated, high- (HT: 70% MVIC) versus low-torque (LT: 30% MVIC) isometric exercise performed to failure on motor unit (MU) recruitment and firing behavior of the vastus lateralis. Eighteen resistance-trained males (23.1 ± 3.8 years) completed familiarization, followed by separate experimental sessions in which they completed either HT or LT exercise to failure in random order. LT exercise resulted in a greater time to task failure and a more dramatic decline in the muscle's force capacity, but the total work completed was similar for HT and LT exercise. An examination of the firing trains from 4670 MUs recorded during exercise revealed that firing rates generally increased during HT and LT exercise, but were higher during HT than LT exercise. Furthermore, recruitment thresholds (RT) did not significantly change during HT exercise, whereas the RT of the smallest MUs increased and the RT for the moderate to large MUs decreased during LT exercise. Both HT and LT exercise resulted in the recruitment of additional higher threshold MUs in order to maintain torque production. However, throughout exercise, HT required the recruitment of larger MUs than did LT exercise. In a few cases, however, MUs were recruited by individuals during LT exercise that were similar in size and original (pre) RT to those detected during HT exercise. Thus, the ability to achieve full MU recruitment during LT exercise may be dependent on the subject. Consequently, our data emphasize the task and subject dependency of muscle fatigue.

Keywords: Firing Rate; Motor Unit Action Potential Amplitude; Muscle Fatigue; Recruitment Threshold; sEMG Decomposition.

Figure 1

Example trapezoidal torque tracing (solid black line) and the accuracies, recruitment, and derecruitment thresholds (indicated by colored circles), and spike trains (rows of colored vertical lines under the torque tracing) of individual motor units for one subject from a repetition during the low‐torque exercise bout (i.e., 30% MVIC).

Figure 2

The individual responses and mean (orange filled diamonds) ± 95% confidence intervals (orange rounded bars) for total repetitions and total work completed during the high‐torque vs. low‐torque fatiguing work bouts.

Figure 3

(A–B) Relationships between maximal voluntary isometric contraction (MVIC) strength and time to task failure during the (A) high‐torque and (B) low‐torque exercise. (C–D) Relationships between the predicted original recruitment threshold (RT; %MVIC) of the largest detected MU (maxMUAP_PP) and time to task failure during the (C) high‐torque and (D) low‐torque exercise.

Figure 4

(A–B) The (A) slopes (pps μV⁻¹) and (B) y‐intercepts (pps) of the mean firing rate (MFR) versus motor unit action potential amplitude (MUAP_PP) relationships during the first, early, middle, late, and last repetitions of the high‐torque (solid circles, black dotted line) and low‐torque (open circles, gray dotted line) exercise conditions. (C–D) The average linear regression lines for the MFR versus MUAP_PP relationship for the first, early, middle, late, and last repetitions during the (C) high‐torque and (D) low‐torque exercise. *indicates a significant main effect for torque (HT > LT); ^†indicates a significant increase from the first repetition, independent of torque

Figure 5

(A–B) The (A) slopes (pps·μV⁻¹) and (B) y‐intercepts (pps) of the recruitment threshold (RT) versus motor unit action potential amplitude (MUAP_PP) relationships during the first, early, middle, late, and last repetitions of the high‐torque (solid circles, black dotted line) and low‐torque (open circles, gray dotted line) exercise conditions. (C–D) The average linear regression lines for the RT versus MUAP_PP relationship for the first, early, middle, late, and last repetitions during the (C) high‐torque and (D) low‐torque exercise. *indicates a significant main effect for torque (HT > LT); ^†indicates significant differences between repetitions in the LT condition, only.

Figure 6

(A–B) The mean (±95% confidence interval) maximal detected MUAP_PP and corresponding RT, respectively, in the high‐torque (black open circles, black dotted line) and low‐torque (orange open circles, gray dotted line) exercise conditions. *indicates a significant main effect for torque (HT > LT); ^†indicates significant differences between repetitions, independent of torque.

Figure 7

The maximal detected (largest) motor unit action potential amplitude (maxMUAP_PP; μV) during HT versus LT exercise, the predicted original recruitment threshold (RT; %MVIC) of the maxMUAP_PP during HT versus LT exercise, and the maxMUAP_PP expressed relative to the predicted amplitude of the maximal (largest) MU (%Max_PRED) during HT versus LT exercise. The orange diamonds and shaded regions represent the mean ± SD values, respectively, whereas the individual lines connect the data points of individual subjects.

Figure 8

(A–B) The average, normalized motor unit action potential amplitude (MUAP_PP) during the first, early, middle, late, and last repetitions for motor units recruited within the 0–10%, 10–20%, 20–30%, 30–40%, etc. MVIC force range during the (A) high‐torque and (B) low‐torque exercise conditions. The numbers below each bar indicate the number of motor units analyzed in the corresponding recruitment threshold bin for that repetition. The relationship between average MUAP_PP and repetition was fit with linear regression lines (orange lines).