238 The Systematic Approach to Shoulder Rehabilitation

Phillips B and Lucas S

distance is defined as the length of a line drawn perpendicular to the line of action of the force and the axis of rotation (Figure 4).9 This is a crucial concept that all rehabilitation providers must under-stand and use as they formulate exercise programs. Changes can be made to the amount of force applied (ie, using a weight) and to the distance or length of the lever arm (ie, shoulder abduction with the elbow flexed to 90° or the elbow fully extended) to make an exer-cise easier or harder for a patient.

Figure 4 demonstrates how a change in the perpendicular length of the lever arm alters the amount of resistance that the muscles of the shoulder girdle must overcome as the upper extremity is moved through a ROM (in this example, abduction). With the up-per extremity resting by the side, no effective lever arm is created between the center of rotation of the shoulder and the center of mass of the extremity, thus no torque is generated. As the upper extremity begins to move away from the side, the length of the lever arm that is created by the perpendicular distance of the line of action around the shoulder joint increases. This increases the amount of torque that the shoulder musculature must generate to overcome the downward resistance force to raise the upper extremity. At 90° of abduction, the extremity is at a perpendicular angle to the downward force of gravity, creating the greatest distance between the center of mass of the upper extremity and the axis of rotation. This increase in distance makes 90° of shoulder elevation the angle producing the longest lever arm and consequently the largest amount of resistance

torque acting on the glenohumeral joint (Figure 5). As the upper extremity contin-ues into abduction above shoulder level, the length of the lever arm acting perpen-dicular to the downward force of gravity begins to decrease, leading to a decrease in the resistance torque generated. This decrease in torque continues as the up-per extremity continues toward terminal elevation. (Figure 6). Therefore, if the de-sire were to strengthen the supraspinatus, having the patient stop at shoulder level rather than abducting the upper extremity through a full ROM when performing an active elevation exercise would be more challenging to the muscle.

ISOTONIC DUMBBELL VS. ELASTIC-RESISTANCE IN SUPINE

Rehabilitation providers commonly use both elastic-resistance bands and free weights during strengthening exercises for the shoulder girdle. A common miscon-ception regarding the use of free weights and resistance bands is that they both pro-vide the same amount of resistance to the patient throughout an exercise. Hughes et al1 showed the amount of resistance that free weights and resistance bands provide varies as the extremity moves through a given ROM. Examining the differences in the amount of resistance provided by these 2 types of resistance exercises dur-ing overhead strengthening in the supine and upright positions will help to better explain this concept.

For the patient who cannot actively elevate his or her upper extremity against gravity while standing or for the patient who has substantial pain when lifting the upper extremity, the use of gravity-mini-mized strengthening exercises is appropri-ate to reduce the resistance effect of grav-ity on elevating the extremity overhead. Supine forward-elevation exercises can be

Figure 4. The effect of increasing the lever arm of the upper extremity during shoulder abduction.

239The St. Francis Orthopaedic Institute & HPRC at St. Francis Rehabilitation Center

Biomechanical Principles of Selected Shoulder Girdle Exercises

used to strengthen the anterior deltoid muscle to enable a patient to raise his or her upper extremity overhead.

When using free weights during overhead strength-ening exercises in the supine position, the force of gravity against the upper extremity plays an important role in both the amount of resistance and the direction in which the resistance is applied to the shoulder girdle muscles. To perform the exercise, the patient begins by resting the upper extremity at the side while holding a hand weight. As the patient lifts the extremity off the table, the downward force of gravity on the long lever arm of the extended extremity produces a substan-tial amount of resistance to the extremity (Figure 7). This requires the shoulder muscles to generate a large amount of torque to continue to lift the upper extremity upward against this downward resistive force.

As the upper extremity reaches shoulder level (90° of forward elevation), the activity of the anterior shoul-der muscles decreases as the perpendicular distance from the line of action to the center of rotation of the shoulder decreases the length of the lever arm in rela-tion to the downward resistance of gravity.10 This point of upper extremity elevation (approximately 90°-115° of forward elevation) is considered to be a “bal-ance point” for the shoulder because of the minimal muscle activation needed by the rotator cuff to keep

Figure 5. Shoulder abduction at 90° of elevation.

Figure 6. Shoulder abduction atterminal elevation.

Figure 7. Supine forward elevation with dumbbell resistance. Purple shaded region represents arc of motion requiring activation of the shoulder flexors. Non-shaded region represents the “balance point” of the shoulder where minimal muscle activation is needed to stabilize the shoulder. Green shaded region represents the arc of motion requiring activation of the shoulder extensors.

240 The Systematic Approach to Shoulder Rehabilitation

Phillips B and Lucas S

the humeral head centered in the glenoid. This is ac-complished by the downward axial compressive force of gravity on the glenohumeral joint and a medially directed force vector of the deltoid on the humerus.11

As the upper extremity continues to travel over-head, the amount of downward force from gravity on the extremity again increases, but the direction of the force from gravity is now on the posterior aspect of the extremity. The weight of the patient’s upper extremity, the downward force of gravity, and the resistance from the free weight are no longer providing resistance to the muscles that flex the shoulder, such as the anterior deltoid and supraspinatus, but instead are assisting in forward elevation of the extremity. If this force is large because of the weight of the upper extremity or a heavier dumbbell, then the shoulder extensors, such as the latissimus dorsi and the long head of the triceps, would become active to resist the torque that is moving the extremity into flexion. Therefore, supine isotonic strengthening from approximately 100° to the over-head position is not a biomechanically advantageous way to strengthen the muscles that actively elevate the upper extremity in standing.

The use of resistance bands for overhead strength-ening in the supine position provides both a differ-ent amount and direction of resistance to the anterior shoulder muscles compared to a free weight. In the starting position with the upper extremity by the side, the band initially provides no resistance. As the ex-tremity is lifted away from the body, the band tightens and begins to exert resistance to the shoulder muscles (Figure 8). As the extremity reaches 90° of forward

elevation, the band applies its largest amount of re-sistance to the anterior shoulder muscles.1 This is the opposite of the resistance provided by the free weight at this same position. As with the free weight when the upper extremity moves beyond 90° of elevation, the direction of resistance from gravity changes from pulling the extremity into extension to assisting the extremity into flexion. With the upper extremity now above shoulder level, the amount of resistance to the shoulder flexors decreases somewhat because of the change in the resistance from gravity and a decrease in the band-to-arm angle; however, the resultant force of this exercise still pulls the upper extremity into exten-sion throughout the entire ROM. The band-to-arm angle (Figure 9) is defined as the angle created by the long axis of the resistance band and the longitudinal axis of the upper extremity.1 As the upper extremity moves away from the body, the angle at the glenohu-meral joint between the trunk of the patient and the ex-tended extremity constantly increases as the extremity continues in an overhead direction. This change in the angle of motion at the shoulder causes a simultaneous change in the band-to-arm angle at the hand. As the angle of motion at the shoulder increases, the band-to-arm angle at the hand simultaneously decreases. This decrease in the band-to-arm angle contributes to the slight decrease in the amount of resistance that the

Figure 8. Supine forward elevation with elastic-band resistance.

Figure 9. Band-to-arm angle during shoulder flexion. The yellow angle represents the band-to-arm angle. The red angle represents ROM of the glenohumeral joint.

241The St. Francis Orthopaedic Institute & HPRC at St. Francis Rehabilitation Center

Biomechanical Principles of Selected Shoulder Girdle Exercises

band provides in the supine position after the upper ex-tremity moves above 90° of flexion. Despite this slight decrease in resistance from the band, the tension of the band continues to increase as the upper extremity con-tinues overhead. The band continues to provide a force pulling the extremity into extension and requiring the shoulder flexors to be active as the extremity moves through its full ROM, but the amount of resistance is less than when the extremity was at 90° of forward elevation and the amount of resistance from the band was at its largest.1 The amount of rotator cuff and del-toid activation required to perform this exercise is also much less than that required to perform standing ac-tive elevation of the upper extremity.12 The continued resistance from the band into extension beyond 90° of elevation is vastly different from what is seen when us-ing the free weights above shoulder level in the supine position, where the resistance provides a force assist-ing the upper extremity into flexion. Therefore, when using resistance to strengthen the shoulder flexors in the supine position, the rehabilitation provider must consider that the use of resistance bands can provide resistance throughout the full ROM of the upper ex-tremity, whereas the use of free weights cannot.

ISOTONIC DUMBBELL VS. ELAS-TIC RESISTANCE IN STANDING

The effect of gravity on the shoulder changes the biomechanics of overhead strengthening as the patient progresses from supine to standing exercises. When using a dumbbell for overhead strengthening in the standing position, the patient begins the exercise with the upper extremity at the side. As the upper extremity is lifted away from the body, the downward resistance from gravity steadily increases until the extremity is at 90° of flexion (Figure 10). At this point in the exercise, the resistance from gravity and from the dumbbell is at its largest because of the long lever arm created by the perpendicular distance between the downward resistive force and the center of rotation at the glenohumeral joint (Figure 4). This is a sharp contrast to the amount of resistance noted with the extremity at shoulder level in the supine position when using the free weight. The torque required to elevate the upper extremity begins to decrease as the extremity continues to move over-head and the effective lever arm begins to decrease,

Figure 10. Standing forward elevation with dumbbell resistance.

Figure 11. Standing forward elevation with elastic-band resistance.