Lack of gender differences in the ability to relax between repetitive maximum isokinetic shoulder forward flexions: a population-based study among northern Swedes

The aim of the present study was to analyse the effects of gender and age i n a population-based sample of clinically healthy subjects on: (1) strength , endurance and perception of fatigue, (2) mean frequency (MNF) of the elec tromyogram (EMG), and (3) the ability to relax between active contractions (signal amplitude ratio; SAR) of the surface EMG of an isokinetic shoulder forward flexion test. With this aim we have analysed aspects of the validit y of MNF as an indicator of fatigue, using peak torque as a criterion varia ble. The subjects were 27 men and 28 women (age range: 20-60 years), who we re obtained by random sample from the official census lists (participation rate: 66%). The peak torque and surface EMG of two portions of the trapeziu s, deltoid and infraspinatus muscles were recorded throughout 100 repetitiv e maximal isokinetic shoulder forward flexions, No significant differences in the perception of fatigue and relative endurance levels of peak torque a nd work were found between males and females. Males were significantly stro nger than females and, on average, females produced approximately 60% of th e output of the males; 76% after normalisation for body mass. The men had s ignificantly lower MNF endurance levels for three of the investigated muscl es. When controlled for age and body mass, the men had a significantly high er MNF of the deltoid muscle than did the females. These differences were o nly found for the deltoid muscle. Significant correlations existed between the MNF of the four muscles and biomechanical output, indicating criterion validity for the MNF variable with respect to fatigue. There were no signif icant effects of gender or age on the ability to relax between repetitive c ontractions (SAR). The higher prevalence of musculoskeletal complaints of t he neck-shoulder region in females cannot be explained by a higher intrinsi c muscle tension. Age, gender, body mass index and biomechanical output can have significant effects upon MNF. These effects are important considerati ons in the interpretation of MNF, for instance in ergonomic situations.