Activation characteristics of shoulder muscles during maximal and submaximal efforts

Abstract

Determination of individual maximum voluntary contraction (MVC) force is used as the gold standard for normalising surface EMG (SEMG) data. Assuming a linear amplitude-force relationship, individual strain levels are defined according to percentage rates of the measured MVC levels. The purpose of the study was to investigate if the assumed force-strain relationship can be applied without qualification. Therefore, healthy volunteers (nine men, ten women) were investigated during isometric exercises of shoulder muscles at MVC and 50% levels. Tasks were performed at four different angular positions in frontal, sagittal and horizontal planes. In each plane, both possible force directions were investigated. SEMG was taken simultaneously from 13 muscles of the shoulder and upper arms from both sides of the body. At a force level of 50% MVC, SEMG amplitude levels were compared to the expected 50% level. Differences in muscular co-ordination patterns were also determined. During retroversion and horizontal flexion, amplitude levels significantly remained at levels below 50%. This was seen for all the muscles investigated, independent of relative contribution to force production. During horizontal extension and abduction, the main force-producing muscles showed amplitude levels that significantly exceeded the expected 50% level. Co-ordination patterns differed significantly between MVC and submaximal conditions for anteversion, retroversion and horizontal extension. Specifically, four shoulder muscles showed higher proportions at the 50% level compared to MVC. Therefore, certain percentage rates of MVC force levels exhibit quite different strain rates, as identified by SEMG. Depending on force direction, differences in co-ordination patterns exist between MVC and submaximal test conditions. Both findings have implications for therapeutic and training applications.

Is muscle co-activation a predisposing factor for low back pain development during standing? A multifactorial approach for early identification of at-risk individuals

“…Disposable pre-gelled EMG Ag-AgCl electrodes (Blue Sensor, Medicotest, Inc., Olstykke, Denmark) with a 2 cm centre-to-centre inter-electrode distance were applied over 8 bilateral muscle groups: Thoracic Erector Spinae (5 cm lateral to T9 spinous process) (Callaghan et al, 1998), Lumbar Erector Spinae (above and below L1 spinous process) (Danneels et al, 2001), Latissimus Dorsi (upper 1/3 of a line connecting the posterior shoulder crease and L1) (Anders et al, 2005), Rectus Abdominus (1 cm above umbilicus and 2 cm lateral to midline) (Ng et al, 1998), Internal Oblique (1 cm medial to ASIS and beneath a line joining bilateral ASIS) (Ng et al, 1998), External Oblique (below the rib cage, along a line connecting the inferior costal margin and the contralateral pubic tubercle) (Ng et al, 1998), Gluteus Medius (1 inch distal to the midpoint of the iliac crest) (Zipp, 1982), and Gluteus Maximus (midway between the greater trochanter and the sacrum) (Zipp, 1982). All electrode placements were confirmed through palpation and manual resistance.…”

The influence of providing feedback on force production and within-participant reproducibility during maximal voluntary exertions for the anterior deltoid, middle deltoid, and infraspinatus

“…Smoothing techniques and 'peak' extraction from an MVE trial are controversial (Anders et al, 2005). Mathiassen et al (1995) found few studies that reported their methods for extracting peak values from the MVE recordings.…”

Normalization of EMG Signals: To Normalize or Not to Normalize and What to Normalize to?

“…Lifting an absolute load of say 1 kg mass might require 10% of the maximum muscle capacity in a strong individual compared to say 40% of the maximum muscle capacity in another person who is not as strong. It is not possible to estimate maximum muscle activity from a relative submaximal contraction by linear extrapolation because the torque/EMG relationship is nonlinear [96]. Additionally, the lengths of muscle moment arms in individuals vary and since the EMG signal is related to the force produced by the muscle and not the torque produced by the limb, the force required by the muscle to produce a given torque would be different between individuals.…”