Muscle activation patterns in shoulder impingement patients

Introduction: Shoulder impingement is one of the most common presentations of shoulder joint problems 1. It appears to be caused by a reduction in the sub-acromial space as the humerus abducts between 60o -120o – the 'painful arc'. Structures between the humeral head and the acromion are thus pinched causing pain and further pathology 2. Shoulder muscle activity can influence this joint space but it is unclear whether this is a cause or effect in impingement patients. This study aimed to observe muscle activation patterns in normal and impingement shoulder patients and determine if there were any significant differences. Method: 19 adult subjects were asked to perform shoulder abduction in their symptomatic arm and non-symptomatic. 10 of these subjects (age 47.9 ± 11.2) were screened for shoulder impingement, and 9 subjects (age 38.9 ± 14.3) had no history of shoulder pathology. Surface EMG was used to collect data for 6 shoulder muscles (Upper, middle and lower trapezius, serratus anterior, infraspinatus, middle deltoids) which was then filtered and fully rectified. Subjects performed 3 smooth unilateral abduction movements at a cadence of 16 beats of a metronome set at 60bpm, and the mean of their results was recorded. T-tests were used to indicate any statistical significance in the data sets. Significance was set at P<0.05. Results: There was a significant difference in muscle activation with serratus anterior in particular showing a very low level of activation throughout the range when compared to normal shoulder activation patterns (<30%). Middle deltoid recruitment was significantly reduced between 60-90o in the impingement group (30:58%).Trends were noted in other muscles with upper trapezius and infraspinatus activating more rapidly and erratically (63:25%; 60:27% respectively), and lower trapezius with less recruitment (13:30%) in the patient group, although these did not quite reach significance. Conclusion: There appears to be some interesting alterations in muscle recruitment patterns in impingement shoulder patients when compared against their own unaffected shoulders and the control group. In particular changes in scapula control (serratus anterior and trapezius) and lateral rotation (infraspinatus), which have direct influence on the sub-acromial space, should be noted. It is still not clear whether these alterations are causative or reactionary, but this finding gives a clear indication to the importance of addressing muscle reeducation as part of a rehabilitation programme in shoulder impingement patients.

The effects of cadence and power output upon physiological and biomechanical responses to incremental arm-crank ergometry

The aim of this study was to examine the effects of cadence and power output on physiological and biomechanical responses to incremental arm-crank ergometry (ACE). Ten male subjects (mean +/- SD age, 30.4 +/-5.4 y; height, 1.78 +/-0.07 m; mass, 86.1 +/-14.2 kg) undertook 3 incremental ACE protocols to determine peak oxygen uptake (VO2 peak; mean of 3 tests: 3.07 +/- 0.17 L.min-1) at randomly assigned cadences of 50, 70, or 90 r.min-1. Heart rate and expired air were continually monitored. Central (RPE-C) and local (RPE-L) ratings of perceived exertion were recorded at volitional exhaustion. Joint angles and trunk rotation were analysed during each exercise stage. During submaximal power outputs of 50, 70, and 90 W, oxygen consumption (VO2) was lowest for 50 r.min-1 and highest for 90 r.min-1 (p < 0.01). VO2 peak was lowest during 50 r.min-1 (2.79 +/-0.45 L.min-1; p < 0.05) when compared with both 70 r.min-1 and 90 r.min-1 (3.16 +/-0.58, 3.24 +/-0.49 L.min-1, respectively; p > 0.05). The difference between RPE-L and RPE-C at volitional exhaustion was greatest during 50 r.min-1 (2.9 +/- 1.6) when compared with 90 r.min-1 (0.9 +/- 1.9, p < 0.05). At VO2 peak, shoulder range of motion (ROM) and trunk rotation were greater for 50 and 70 r.min-1 when compared with 90 r.min-1 (p < 0.05). During submaximal power outputs, shoulder angle and trunk rotation were greatest at 50 r.min-1 when compared with 90 r.min-1 (p < 0.05). VO2 was inversely related to both trunk rotation and shoulder ROM during submaximal power outputs. The results of this study suggest that the greater forces required at lower cadences to produce a given power output resulted in greater joint angles and range of shoulder and trunk movement. Greater isometric contractions for torso stabilization and increased cost of breathing possibly from respiratory-locomotor coupling may have contributed increased oxygen consumption at higher cadences.