Muscular fatigue during repeated isokinetic shoulder forward flexions in young females

Abstract

Peak torque, work, mean power and electromyographic (EMG) activity were recorded for each of 150 repeated isokinetic maximal shoulder flexions (45 degrees-90 degrees) in 23 healthy females. From the EMG signals of trapezius, deltoid, infraspinatus and biceps brachii the mean power frequency and the signal amplitude were determined in real time. The mechanical output showed a steep decrease during the first 40 contractions, followed by a plateau maintained until the end. In all muscles, except the biceps brachii, significant decreases in mean power frequency occurred during the first 40 contractions, showing a tendency to stabilize around the same absolute frequency value. Signal amplitude increased in the trapezius, the deltoid and the infraspinatus, but was constant in the biceps brachii. For some individuals rather high EMG activity was recorded in the muscles during the time the arm was supposed to be passively extended to the starting position, and this was found to be associated with lower strength and endurance levels. Longitudinal analyses showed that the mean power frequencies correlated better than the signal amplitudes with the three mechanical variables. The results suggest that the initial steep decrease in mechanical performance and mean power frequency is caused by fatiguing of type 2 motor units.

Mechanical performance and electromyography during repeated maximal isokinetic shoulder forward flexions in female cleaners with and without myalgia of the trapezius muscle and in healthy controls

“…The methods have been reported in detail elsewhere Gerdle et al 1989;Karlsson et al 1994); the following is a summary. The subjects performed dynamic maximal shoulder exions using an isokinetic dynamometer (Biodex system 2, model 900-800 equipped with Advantage software version 4.5).…”

“…the ratio between rms of the passive phase and rms of the active phase of the contraction cycle during maximal isokinetic contractions (signal amplitude ratio, SAR), have been found in patients suering from ®bromyalgia (FMS), work-related myalgia and chronic whiplash associated disorders (WAD) (Elert et al 1992b;Fredin et al 1997). In clinically healthy subjects SAR has been correlated negatively with output Gerdle et al 1989) and positively with the proportion of type 1 ®bres . A high SAR level has been suggested to precede muscle pain (Elert 1991) but this has not been shown.…”

Worsening of neck and shoulder complaints in humans are correlated with frequency parameters of electromyogram recorded 1-year earlier

“…The preset angular velocity chosen was 1.05 rad s A1 . This velocity of 1.05 rad s A1 (60°s A1 ) was chosen after taking into consideration the results from a recent study on the relationship between EMG-variables and mechanical output at dierent angular velocities Gerdle et al 1989). The subjects were informed about the purpose of the experiment, but not about the number (100) of repeated maximal forward¯exions scheduled in the protocol.…”

Myosin heavy chain isoforms influence surface EMG parameters: a study of the trapezius muscle in cleaners with and without myalgia and in healthy teachers

“…Different pictures (both increases and decreases) have been reported for the signal amplitude of the EMG during repetitive isokinetic contractions (Gerdle et al 1989;Kellis and Baltzopoulos 1997;Wretling and Henriksson-Larse´n 1998). Different groups of subjects, muscle groups, and signal amplitude variables can partly account for these differences.…”

“…The electrodes of the trapezius were placed 2 cm lateral of the midpoint between the seventh process of the cervical spine and the lateral part of the acromion process. For a detailed description of the electrode positioning of the other three muscles see Fredin et al (1997) and Gerdle et al (1989). Before the electrodes were attached, the skin area had been dry shaved and rubbed with alcohol and ether (4:1).…”