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Effect of exercise at light loads with manipulative resistance on infraspinatus, trapezius (upper fiber) and deltoid (middle fiber) muscle activities in shoulder joint elevation

[Purpose] To clarify rotator cuff muscular activity in the raised position of the closed kinetic chain (CKC) exercise. [Subjects] Twenty-nine cases were studied, 19 men and 10 women (average age 21.5 ±4.7 years old, average body weight 60.1 kg ±11.4). [Methods] To determine the effects of the closed kinetic chain exercise on the upper limb, we measured the surface EMG of the infraspinatus muscle, the trapezius (upper fiber) and the deltoid (middle fiber) with the arm elevated. [Results] Our results show that at an elevation angle of 150° in the scapular plane of the upper limb, with 5% body weight load, the EMG activities of the infraspinatus muscle are approximately 30% of maximum voluntary contraction (MVC). [Conclusion] The raised position of the CKC exercise is effective in physical therapy for functional recovery of the infraspinatus muscle.


INTRODUCTION
Rotator cuff injury is a dysfunction disease of the shoulder joint. There are two main surgical methods of rotator cuff repair: by direct incision, and arthroscope. Physical therapy is an important component in the recovery of upper limb function, post-surgery. The rotor cuff consists of four muscles, the supraspinatus, infraspinatus, teres minor, and subscapularis. These muscles act as a depressor of the glenohumeral joint during elevation of the upper limbs. The deltoid and trapezius muscles are the outer muscles. These muscles are used during upper limb movement and act synergistically with the rotator cuff. The principal method of muscle strength rehabilitation of the rotator cuff muscles is using a light load, with the arms hanging by the sides generated by a thin rubber belt or light weights applied to the upper limbs 1,2) , however, the goal of physical therapy is the rehabilitation of activity of daily living (ADL) ability in the elevated upper limb position. We developed a closed kinetic chain exercise to incorporate the elevated position with load in the scapular plane (SP) to facilitate early performance of ADL. The SP faces forward at an angle of 35 degrees from that of the frontal plane, and it aligns with the body of the scapula 3) . We chose the infraspinatus in order to evaluate the activities of the inner muscles, and the trapezius (upper fiber) and deltoid (middle fiber) to evaluate the activities of outer muscles. This study aimed to clarify rotator cuff muscular activity in the raised position and the optimal amount of load during the CKC exercise.

SUBJECTS AND METHODS
Twenty-nine healthy adult cases were studied, 19 men and 10 women (average age 21.5 ±4.7 years old, average body weight 60.1 kg ±11.4, average body height 164.2±6.2 cm, average grip strength 34.35 ±6.9 kg). Our subjects voluntarily provided their informed consent after receiving an explanation of this research and guidelines, in accordance with the principles of the Declaration of Helsinki, from a medical attendant. This study received approval from the Ethics Committee of the Department of Physical Therapy of Matsue General Medical College.
Each subject was requested to sit back on a chair to prevent unexpected motion, with their back against the backrest. The subjects' arms were extended to maximum height using a manual muscle sensor and a therapist pushed down with a force. Each subject's muscle activities were measured under Conditions 1-9.

Original Article
Condition 1: 150° flexion in the SP of the right shoulder joint of the subject, with the elbow joint extended and the wrist at 85° dorsiflexion. A force equal to 5% of the patient's body weight was applied to the patient's open palm to exert load on the scapula glenoid.
Condition 2: A load equaling 10% of the subject's body weight was applied to the subject's limb and the muscle activities were measured with the limb in the same position as in Condition 1.
Condition 3: The right shoulder was positioned at 120° flexion in the SP and the muscle activities were measured with a load equal to 5% of body weight.
Condition 4: A load 10% of body weight was applied to the limb and the muscle activities were measured with the limb in the same position as in Condition 3.
Condition 5: The right shoulder joint was positioned at 90° flexion in the SP and the muscle activities were measured with a load of 5% body weight.
Condition 6: A load of 10% body weight was applied to the limb and the muscle activities were measured with the limb in the same position as in Condition 5.
Condition 7: The right shoulder joint of the subject was positioned at 45° of external rotation with the right upper arm hanging down by the side, the right elbow at 90° flexion and the forearm in the neutral position. The examiner added manipulative resistance to the right distal radius of the subject. Maximum voluntary contraction (MVC) of the infraspinatus and external rotation muscle activity of the shoulder was measured.
Condition 8: An examiner added downwards manipulative resistance just above the center of the top border of both scapula. The examiner measured MVC of the trapezius muscle (upper fibers).
Condition 9: The right shoulder joint of the subject was positioned at 90° abduction, with elbow flexion of 90°, and the forearm was in the neutral position. These conditions were chosen in order to investigate the muscle activities and to determine the %MVC of the upper limb elevated position.
The muscle activity was analyzed using integrated electromyograms (IEMG), measured and computed by the Neuro pack system (Nihon Koden). The surface electrodes used were Blue sensors M − 00 − S (diameter 40.8×34 mm) produced by Mets Ltd. The sampling frequency of EMG was 1,050 Hz, and data were recorded on a computer using the Windows XP operating system.
The skin over the target muscles was cleaned with Skin Pure (Nihon Koden). We set the electrodes to the appropriate positions on the trapezius, infraspinatus, pectoralis major and deltoid in accordance with the method and location indicated by Shimono 4) . We examined the muscles activity at rest for 7 seconds, and the middle 5 seconds, (after discarding the first and last second) was used in the analysis.
We measured the amount of the manipulative loadings using a manual muscle sensor (GT-310 and ISOFORCE, OG-Giken, Ltd.) and 3-minute rest periods were given between each condition. We used the multiple comparison test of Dunnett and to compare the activities of the muscles among in Conditions 1-6. For the analysis, we used the statistical processing software SPSS 14.0 Amos 6.0 for Windows.

Condition 1 showed significantly higher infraspinatus and
trapezius muscle activities (p <0.05) than Conditions 2-6 ( Table 1). In the case of the CKC exercise with a load at 5% body weight load and 150° shoulder joint elevation in the SP, the highest muscular activities were shown by the trapezius and the infraspinatus. The muscle activity ratio of Condition 7 and Condition 1 was 0.298, and the muscle activity of the infraspinatus was approximately 30% MVC (Table 2). With the shoulder joint elevation at 150° in the SP and a 10% body load (Condition 2), the activity of the deltoid muscle was at its highest among conditions 1-6 ( Table 3).

DISCUSSION
Tsutsui, Yanaguchi et al. showed that as load increases, greater outer muscle activity is facilitated and inner muscle activity is inhibited 1,2) . Kozuka et al. reported that in the case of low load (3 Nm), the activities of the deep (inner) muscles, i.e. the rotator cuff, increase; however, those of the surface (outer) muscles do not. Furthermore, high loads (9 Nm, 15 Nm) increased the activities of the outer muscles 5) . These findings are in agreement with the results of our study. Stability between the humeral head and glenoid of the scapula is reduced, and the internal pressure of the shoulder joint changes to positive pressure from negative pressure as the elevation angle of the shoulder joint increases [6][7][8][9] . As a result of this pressure, the infraspinatus contracts strongly in order to increase the glenohumeral joint stability, and the activity of the trapezius muscle increases with a corresponding increase of infraspinatus activity. For these reasons, the deltoid muscle activity of Condition 2 was higher than that of Condition 1. Also, the activity of the deltoid muscle was altered in direct proportion to the strength of the load. The activity of the deltoid muscle was the highest in Condtion 9. Ohizumi and, Ogi reported that, when the shoulder joint is at 90° abduction, the activity of the deltoid muscle is at the maximum and the activity of infraspinatus muscle is decreased 10,11) . Their results agree with our findings.
The ratio of the activity of the infraspinatus muscle in Condition 1 was 0.298 (≈30%MVC). Using EMG, Jasvholm reported that isometric contraction of the infraspinatus has a near linear correlation in a healthy shoulder 12) . Basmajian suggested that the EMG is a useful indicator of muscle output in isometric contraction 13) . Using IEMG to analyze 30-70%MVC muscle activities, Kuroki revealed that there is a significant correlation between %IEMG and %muscle contraction 14) . We verified this correlation between %IEMG and %muscle contraction during the CKC exercise. With a load of 5% body weight ratio and 150° shoulder joint elevation in the SP, the infraspinatus muscle activity was about 30%MVC.
Min reported the relationship of the muscle activity of the posterior deltoid muscle fibers and the infraspinatus muscle in CKC exercise 21) , and demonstrated that, it is necessary to evaluate the muscle activities of each of the three types of deltoid muscle fiber. Jinhwa showed a relationship between the improvement of the endurance capacity of the infraspinatus muscle and the stability of the scapula 22) . We need to study the EMG activities of the muscles of the scapula in the raised position of CKC exercise.
The results of the present study suggest that closed kinetic chain exercise (CKC) by the elevated upper limb results in improvement and functional maintenance of the infraspinatus muscle. Our results strongly suggest that it can be applied to physical therapy for the shoulders with inner muscle dysfunction.