Research in Sport Medicine and Technology

The purpose of this study was to compare the effect of shoulder abduction angles, dominant and non-dominant arm and external loading on co-contraction ratio during arm abduction in scapular plane in overhead athletes and non-athletes. 10 swimmers, 10 handball players and 10 tennis players and ten non-athletes voluntarily participated in this study. Electromyography activity of shoulder muscles (middle and anterior deltoid; upper, middle and lower trapezius; infraspinatus, serratus anterior and latissimus dorsi) during dynamic and static arm abduction in 3 different angles (0-45º in 1second and holding it for 3 seconds, 0-90º in 2 seconds and holding it for 3 seconds, 0-135º in 1 second and holding it for 3 seconds) was recorded for dominant and non-dominant arm in loading and non-loading conditions. Co-contraction ratio of shoulder muscles was calculated for both groups. Findings indicated external loading, shoulder dominance and abduction angles had significant effect on shoulder muscles co-contraction ratio during (0-45º, 0-90º, 0-135º) and holding shoulder elevation in scapular plane (45º, 90º, 135º),  also there was significant difference in shoulder muscles co-contraction ratio among four groups during and holding elevation in scapular plane (p≤0.05). Significant differences in shoulder muscles co-contraction ratio among athletes group and between athletes and non-athletes may be related to sport demands and adaptation to exercises and extensive use of upper limb as well. 
 

It is necessary to examine the changes of muscle co-contraction in different percent of fatigue-induced force reduction. The present study aims to investigate the effect of fatigue-induced force reduction of knee extension on muscle co-contraction. Ten healthy and active women aged 26.10±2.99 years performed fatigue protocol including 5 seconds of maximal voluntary isometric contraction (MVIC) of the knee extension and a 5-second rest. Muscle co-contraction was calculated for each subject in the repetitions in which the MVIC force was reduced by 10, 20, 30, 40 and 50% as compared to the pre-fatigue MVIC force. Knee extension force and muscle activity measured by Isokinetic and electromyography apparatus, respectively. The difference between co-contraction of vastus lateralis and medialis in different percentages of force reduction was not statistically significant. The co-contraction of rectus femoris and biceps femoris increased by 40 and 50% of force reduction compared to pre-fatigue co-contraction (P<0.01). Co-contraction of biceps femoris and vastus lateralis increased by 20, 30, and 40%, as well as co-contraction of biceps femoris  and vastus medialis increased 40% of the force reduction, compared to pre-fatigue co-contraction (P <0.05). The results, overall, showed that the change of co-contraction during fatigue is influenced by the selected muscle and the rate of force reduction. This finding can help researchers decide how neuromuscular fatigue effects on muscle co-contraction