Electromyographic evaluation of muscle recovery after isometric fatigue

Recovery and Fatigue Behavior of Forearm Muscles during a Repetitive Power Grip Gesture in Racing Motorcycle Riders

Despite a reduction in the maximal voluntary isometric contraction (MVC_isom) observed systematically in intermittent fatigue protocols (IFP), decrements of the median frequency, assessed by surface electromyography (sEMG), has not been consistently verified. This study aimed to determine whether recovery periods of 60 s were too long to induce a reduction in the normalized median frequency (MF_EMG) of the flexor digitorum superficialis and carpi radialis muscles. Twenty-one road racing motorcycle riders performed an IFP that simulated the posture and braking gesture on a motorcycle. The MVC_isom was reduced by 53% (p < 0.001). A positive and significant relationship (p < 0.005) was found between MF_EMG and duration of the fatiguing task when 5 s contractions at 30% MVC_isom were interspersed by 5 s recovery in both muscles. In contrast, no relationship was found (p > 0.133) when 10 s contractions at 50% MVC were interspersed by 1 min recovery. Comparative analysis of variance (ANOVA) confirmed a decrement of MF_EMG in the IFP at 30% MVC_isom including short recovery periods with a duty cycle of 100% (5 s/5 s = 1), whereas no differences were observed in the IFP at 50% MVC_isom and longer recovery periods, with a duty cycle of 16%. These findings show that recovery periods during IFP are more relevant than the intensity of MVC_isom. Thus, we recommend the use of short recovery periods between 5 and 10 s after submaximal muscle contractions for specific forearm muscle training and testing purposes in motorcycle riders.

Effects of very short pauses on electromyographic variables measured during fatiguing isometric contractions

15 healthy men (26.6 ± 4.6 years old, weight of 70.7 ± 8.6 kg, and height of 1.750 ± 0.072 m) performed three 30-seconds isometric contractions at 60% MVC, with two 10-seconds resting intervals between them. The goal was to study the effect of the resting intervals on the variables that are most commonly used to analyze surface electromyographic (S-EMG) signals (conduction velocity [CV], root mean square [RMS], average rectified value [ARV], mean power frequency [MNF], and median power frequency [MDF]). For the first 30-second contraction, the S-EMG variables behaved exactly like described in the literature. However, after the first and second pauses, the CV variable ceased to behave like in the literature. In the first contraction, the conduction velocity had a statistically significant decreasing trend, in the second contraction, it had a statistically non-significant positive trend, and, in the third contraction, a statistically significant positive trend. These results suggest that short pauses between isometric constant-force contractions lead to changes in the recruiting strategies of the muscles involved in the contraction. The causes of these changes are not yet clear, and further work is needed in order to understand this effect.

Firefighters muscular recovery after a heavy work bout in the heat

Occasionally firefighters need to perform very heavy bouts of work, such as smoke diving or clearing an accident site, which induce significant muscle fatigue. The time span for muscular recovery from such heavy work is not known. The purpose of this study was to evaluate firefighters' force-, neural-, metabolic-, and structural-related recovery after task-specific heavy work in the heat. Fifteen healthy firefighters (14 males and 1 female) performed a 20-min heavy work bout that simulated smoke diving and the clearance of an accident site at 35 °C. After the work, muscular recovery was evaluated by wrist flexion maximal voluntary contraction (MVC), average electromyography during MVC and during 10%MVC, rate of force production, motor response and stretch reflex responses, muscle oxygen consumption and oxygenation level, and wrist flexor muscle pennation angle. Recovery was followed for 4 h. Each of the 12 measured parameters changed significantly (p < 0.05) from those at baseline during the follow-up. Muscle oxygen consumption and the wrist flexor pennation angle remained elevated throughout the follow-up (oxygen consumption baseline, 12.9 ± 1.7 mL O2·min(-1)·(100 g)(-1); 4-h value, 17.5 ± 1.6 mL O2·min(-1)·(100 g)(-1); p < 0.05 and pennation angle baseline, 15.7 ± 0.8°; 4-h value, 17.8 ± 0.8°; p < 0.05). Muscle reoxygenation rate was elevated for up to 2 h (baseline, 2.3 ± 0.4 μmol·L(-1)·min(-1); 2-h value, 3.4 ± 0.4 μmol·L(-1)·min(-1); p < 0.05). The other 9 parameters recovered (were no longer significantly different from baseline) after 20 to 60 min. We concluded that the recovery order in main components of muscle function from fastest to slowest was force, neural, metabolic, and structural.

Electromyographic study in 5 muscles during an isometric fatiguing protocol

In this study, 12 healthy men aging 22.8 ± 2.2 years old were submitted to a protocol of isometric resistance to fatigue contemplating elbow flexion on three different angles: 45°, 90° and 135°. The objective was to study electromyographic median frequency (MDF) in the following muscles: i) Biceps Brachialis Long Head (BBL), Brachioradialis (BRD), Flexor Digitorum Superficialis (FDS), Triceps Brachialis Long Head (TBL), and Extensor Digitorum (ED). It was verified that, for all muscles, including the muscles that act in opposition to the contraction, fatigue presence was verified by the decrease of MDF value.