Chronic shoulder pain is a common consequence of repetitive overhead activity, such as serving and smashing in tennis, freestyle or butterfly swimming, javelin throwing, or bowling in cricket. Usually such pain is a result of damage to the rotator cuff muscles. The rotator cuff group consists of four muscles, subscapularis, teres minor, infraspinatus and supraspinatus, which are positioned around the shoulder joint. These muscles are small, but their function is very important. Specifically, they act to keep the shoulder joint stable during movements of the arm by, in layman's terms, keeping the 'ball' in the 'socket'. There are three classic exercises for strengthening the rotator cuff: these are the lateral raise with internal rotation, the internal rotation exercise and the external rotation exercise, which can all be performed using a resistance band or dumbbells.
The lateral raise with internal rotation (LRIR) involves standing on the band with hands by your sides holding the end of the band in each hand, or standing holding dumbbells in each hand. The movement involves internally rotating your arms so that your thumbs point downwards, and then raising your arms out to the side until your elbows are just below the shoulder, keeping your thumbs pointing down as you raise. It is as if you are emptying a can of drink on the floor. This exercise is very popular, and is commonly recommended as part of a shoulder-injury rehabilitation programme. The LRIR is specifically thought to train the supraspinatus, as this muscle is positioned above the shoulder joint and should be involved during arm abduction movements (lifting the arm out to the side). In fact, researchers Jobe and Moynes in 1982 claimed it was the most effective exercise for the supraspinatus.
A variation of the LRIR exercise is to perform the arm abduction movement while lying on one's side. This exercise has been advocated in a recent study by Joseph Horrigan, a chiropractor in Los Angeles, who has observed good clinical results for the treatment of rotator cuff problems with the use of the side lying abduction (SLA) exercise (Horrigan et al (1999) Medicine & Science in Sports & Exercise, 31 (10) pp. 1361-1366). Horrigan states that the exercise should be performed while lying on one side and holding a dumbbell with your palm facing into your leg. You then raise your arm straight up to an angle of 45 degrees. He recommends that men use a dumbbell weighted to 8% of body weight and women use 5% of body weight.
Horrigan and his study team decided to examine the muscle activity of the SLA and compare it with the activity of the LRIR, to determine if there is any advantage in using one exercise over the other. To do this they used a slightly uncommon method of measuring muscle activity, Magnetic Resonance Imaging (MRI). MRI scans are used routinely as diagnostic tools for examining injuries, but less regularly used in other areas of sports science. The reason MRI can be used to measure muscle activity is that the signal intensity of the muscle image increases after it has been exercised. This is known as an exercise-induced enhancement. Thus, by comparing MRI scans before and after a particular exercise, researchers can tell which muscles have been active. More importantly, MRI can also quantify the amount of muscle activity involved in the exercise by analysing the strength of the signal intensity. The bigger the change in signal intensity between before and after the exercise, then the more the muscle has been working. Comparisons with EMG analysis, which is the more traditional method for measuring muscle activity, has shown that the MRI is a very sound method. In fact, one could argue that it is superior since it is easy to compare different muscle groups or different exercises by simply comparing the percentage change in signal intensity. For example, if one performed an exercise and the signal intensity in the biceps muscle increased 69% whereas the signal intensity in the triceps muscle increased only 5%, one could confidently conclude that the exercise works the biceps much more than the triceps.
Which exercise did best?
Horrigan and his team used this type of comparison in their experiment. They measured the signal intensity of the trapezius, deltoid, supraspinatus, subscapularis, infraspinatus and teres minor before and after three different shoulder exercises, the LRIR, the SLA and the traditional military press exercise (MP). Five subjects were used and all had no shoulder problems. The chart shows the percentage change in signal intensity for each muscle group for all three exercises.
The supraspinatus, subscapularis and infraspinatus muscles are all most active during the SLA exercise compared to the other two. This supports Horrigan's observation that it is a very effective rotator cuff exercise. The SLA produces no activity in teres minor, the fourth rotator cuff muscle, but neither LRIR and MP produce very much either. Teres minor is situated behind the shoulder and acts as a lateral rotator, and perhaps requires a specific lateral rotation exercise to be active.
The deltoid is also most active during the SLA, suggesting that it is a very effective shoulder exercise in general, not just for the rotator cuff. Interestingly, the SLA involves much less trapezius activity than the LRIR and MP, presumably because the exercise is performed lying down and so there is less shoulder elevation. This reduction in trapezius activity is not necessarily a disadvantage because the trapezius is often strong, especially in the upper fibres, and can be too dominant. This muscle imbalance can contribute to a hunched and rounded shoulder posture during shoulder movements which can exasperate shoulder injuries.
The SLA is also ideal in that it only involves 45 degrees of abduction, compared to the LRIR which involves almost 90 deg. When the arm is abducted between 70 deg and 120 degrees the sub-acromial space, which is the area within the shoulder joint, is compressed. This can cause impingement of the soft tissue within that joint space, which can lead to pain or injuries. Therefore, as well as being more effective, the SLA also seems to be a safer exercise than the LRIR.
In conclusion, based on this MRI comparison, the SLA exercise seems to be a superior rotator cuff and shoulder exercise to the LRIR. During the SLA the deltoid, supraspinatus, infraspinatus and subscapularis can all be trained effectively without any danger of joint impingement.