Resistance equipment is often restricted to a single plane of movement; however, multiplanar movements may be more effective, by facilitating the development of motor strategy and intermuscular coordination. Due to their moving axis cam technology, the MuJo™ External Shoulder Machine and Internal Shoulder Machine enable such movement. Furthermore, the range of motion (ROM) through which the shoulder travels can be adjusted to target specific muscles, which may have useful implications within a rehabilitation environment. However, little is currently known about the consequences of varying the ROM whilst using the devices. Therefore, the purpose of this study was to determine the effect of altering ROM on shoulder muscle activity during exercises performed on the MuJo™ Shoulder Machines.
Following institutional ethics approval, nine recreationally-active male participants (means ± SDs: age: 25 ± 5 years; body mass: 77.06 ± 11.06 kg; height: 1.76 ± 0.09 m) performed abduction and external rotation, and adduction and internal rotation of the shoulder at twelve different ranges of motion, in a randomised, counterbalanced order. Surface electromyography (EMG) was collected from the upper trapezius, anterior and posterior deltoids, infraspinatus, pectoralis major, and latissimus dorsi. Muscle activity was normalised to the peak activity from a one repetition maximum test, also performed on the machines. The effect of abduction/adduction and rotation angle on normalised peak EMG was analysed via a two-way analysis of variance with repeated measures for each muscle; effect sizes were calculated using partial eta squared (η2p).
For the External Machine, a significant main effect for abduction in the upper trapezius was found (F(1.1, 8.7)=15.34, P=0.003, η2p=0.66). Electromyography amplitude was significantly higher at 90° of abduction than at 60° and 30°, and significantly higher at 60° than at 30°. For the anterior deltoid, EMG amplitude was significantly higher at 90° than at 60° of abduction (F(2,16)=7.17, P=0.006, η2p=0.47). A significant main effect for rotation in the latissimus dorsi was found (F(3,24)=7.96, P=0.001, η2p=0.50), with EMG amplitude significantly lower at 0° than at both 90° and 60°. For the Internal Machine, a main effect for rotation in the pectoralis major was observed (F(3, 24)=6.98, P=0.002, η2p=0.47), with EMG amplitude significantly lower at 60° than 30° of rotation. No significant interactions or main effects were observed in the remaining muscles on either machine.
In conclusion, altering the ROM results in some changes in muscle activity during abduction and rotation, perhaps indicating a greater requirement for stabilisation during the less constrained repetitions. Further studies incorporating kinematics and inverse dynamics may provide deeper understanding into the effects on motor strategy that may occur when exercising with this device
