Chapter objectives
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Summarize the importance of thorough knowledge of the biomechanical factors associated with rehabilitation.
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Describe the best rehabilitation exercises to elicit recruitment of the glenohumeral and scapulothoracic musculature.
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Explain the normal function of the supraspinatus and deltoid musculature during various ranges of motion.
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Identify which shoulder exercises produce the most amount of supraspinatus activity with the least amount of deltoid activity.
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Explain the effect of pathologic changes on shoulder biomechanics.
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Identify which exercises produce cocontraction of the quadriceps and hamstrings musculature.
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Explain the tibiofemoral shear forces observed during open kinetic chain and closed kinetic chain exercises.
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Describe the in vivo forces on the anterior cruciate ligament during open kinetic chain, closed kinetic chain, bicycle, and stair-climbing exercises.
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Explain the normal arthrokinematics of the patellofemoral joint.
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Summarize the stress on the patellofemoral joint during open kinetic chain and closed kinetic chain exercises.
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Select safe and appropriate exercises for the glenohumeral musculature, scapulothoracic musculature, anterior cruciate ligament, posterior cruciate ligament, and patellofemoral joint.
Biomechanical analysis of rehabilitation exercises has gained recent attention in sports medicine and orthopedic practice. Several investigators have sought to quantify the kinematics, kinetics, and electromyographic (EMG) activity during common rehabilitation exercises in an attempt to fully understand the implications of each exercise on the arthrokinematics and soft tissues of the shoulder and knee. Advances in understanding the biomechanical factors associated with rehabilitation have led to enhancement of rehabilitation programs that place minimal strain on specific healing structures while returning the injured athlete to competition as quickly and safely as possible. The purpose of this chapter is to provide an overview of the biomechanical implications associated with rehabilitation of the athlete’s shoulder and knee.
Biomechanical implications of shoulder rehabilitation
The glenohumeral joint exhibits the greatest amount of motion of any articulation in the human body, although little inherent stability is provided by its osseous configuration. Functional stability is accomplished through the integrated functions of the joint capsule, ligaments, and glenoid labrum, as well as through neuromuscular control and dynamic stabilization of the surrounding musculature, particularly the rotator cuff muscles. The rotator cuff musculature maintains stability of the glenohumeral joint by compressing the humeral head into the concave glenoid fossa during upper extremity motion. Thus, the glenohumeral muscles play a vital role in normal arthrokinematics and asymptomatic shoulder function.
Rehabilitation programs for the shoulder joint often focus on restoring maximum strength and muscular balance, particularly of the rotator cuff and scapulothoracic joint. The majority of research on shoulder biomechanics has focused on quantifying the EMG activity of particular muscles during common rehabilitation exercises, the goal of which is to determine the most optimal exercise to recruit specific muscle activity and maximize shoulder strength and efficiency.
Electromyographic analysis of shoulder exercises
Townsend et al conducted one of the first comprehensive studies analyzing EMG activity in the shoulder musculature during rehabilitation exercises. Dynamic fine-wire EMG activity in the four rotator cuff muscles, pectoralis major, latissimus dorsi, and the three portions of the deltoid was studied in 15 healthy male subjects during 17 common shoulder exercises. The authors quantified the exercises that produced the most activity in each specific muscle ( Table 10-1 ).
| Muscle | Exercise | Peak (% MMT ± SD) | Duration (% Exercise) | Peak Arc Range (°) |
|---|---|---|---|---|
| Anterior deltoid | Scaption IR | 72 ± 23 | 50 | 90-150 |
| Scaption ER | 71 ± 39 | 30 | 90-120 | |
| Flexion | 69 ± 24 | 31 | 90-120 | |
| Military press | 62 ± 26 | 50 | 60-90 | |
| Abduction | 62 ± 28 | 31 | 90-120 | |
| Middle deltoid | Scaption IR | 83 ± 13 | 70 | 90-120 |
| Horiz. abd. IR | 80 ± 23 | 38 | 90-120 | |
| Horiz. abd. ER | 79 ± 20 | 57 | 90-120 | |
| Flexion | 73 ± 16 | 31 | 90-120 | |
| Scaption ER | 72 ± 13 | 58 | 90-120 | |
| Rowing | 72 ± 20 | 43 | 90-120 | |
| Military press | 72 ± 24 | 38 | 90-120 | |
| Abduction | 64 ± 13 | 31 | 90-120 | |
| Deceleration | 58 ± 20 | 27 | 90-60 | |
| Posterior deltoid | Horiz. abd. IR | 93 ± 45 | 63 | 90-120 |
| Horiz. abd. ER | 92 ± 49 | 57 | 90-120 | |
| Rowing | 88 ± 40 | 57 | 90-120 | |
| Extension | 71 ± 30 | 44 | 90-120 | |
| External Rot. | 64 ± 62 | 43 | 60-90 | |
| Deceleration | 63 ± 28 | 27 | 60-90 | |
| Supraspinatus | Military press | 80 ± 48 | 50 | 0-30 |
| Scaption IR | 74 ± 33 | 40 | 90-120 | |
| Flexion | 67 ± 14 | 31 | 90-120 | |
| Scaption ER | 64 ± 28 | 25 | 90-120 | |
| Subscapularis | Scaption IR | 62 ± 33 | 22 | 120-150 |
| Military press | 56 ± 48 | 50 | 60-90 | |
| Flexion | 52 ± 42 | 23 | 120-150 | |
| Abduction | 50 ± 44 | 23 | 120-150 | |
| Infraspinatus | Horiz. abd. ER | 88 ± 25 | 71 | 90-120 |
| External rot. | 85 ± 26 | 43 | 60-90 | |
| Horiz. abd. IR | 74 ± 32 | 38 | 90-120 | |
| Abduction | 74 ± 23 | 31 | 90-120 | |
| Flexion | 66 ± 15 | 23 | 90-120 | |
| Scaption ER | 60 ± 21 | 38 | 90-120 | |
| Deceleration | 57 ± 17 | 27 | 90-60 | |
| Push-up (hands together) | 54 ± 31 | 38 | 90-60 | |
| Teres minor | External rot. | 80 ± 14 | 57 | 60-90 |
| Horiz. abd. ER | 74 ± 28 | 57 | 60-90 | |
| Horiz. abd. IR | 68 ± 36 | 43 | 90-120 | |
| Pectoralis major | Press-up | 84 ± 42 | 75 | 1/2 pk-pk |
| Push-up (hands apart) | 64 ± 63 | 50 | 60-30 | |
| Latissimus dorsi | Press-up | 55 ± 27 | 50 | pk-1 sec |
For the anterior deltoid, exercises involving elevation of the shoulder, such as scaption with internal rotation (empty can), scaption with external rotation (full can), and forward flexion, produced the greatest amount of activity at approximately 70% manual muscle test activity (70% MMT). This was also consistent with results for the middle deltoid, although exercises in the prone position involving horizontal abduction produced approximately 80% MMT. The posterior deltoid showed the greatest amount of activity in the prone position during such exercises as horizontal abduction and rowing at approximately 90% MMT.
Similar to the anterior deltoid, the supraspinatus muscle was most active during shoulder elevation movements, although the military press (from 0° to 30°) produced the greatest amount of supraspinatus activity at 80% MMT. Comparing the empty can and full can exercises, the authors found 74% MMT during the empty can and 64% MMT during the full can.
The exercises with the most activity in the subscapularis muscle also included those involving shoulder elevation, though at a moderate intensity of approximately 55% MMT. Interestingly, the side-lying internal rotation exercise was not found to produce significant activity in the subscapularis, although the similar exercise of internal rotation at 0° abduction with exercise tubing was shown to have 52% maximal voluntary contraction by Hintermeister et al. Others have recommended motions involving lifting the hand off the lower part of the back and motions that replicate a tennis forehand with internal rotation and horizontal adduction.
The infraspinatus and teres minor muscles showed similar results with high activity during the side-lying external rotation exercise (85% for the infraspinatus and 80% for the teres minor). Exercises in the prone position involving horizontal abduction also produced high activity in the external rotators, with up to 88% MMT (range, 68% to 88% MMT) in the infraspinatus during prone horizontal abduction with external rotation.
The press-up exercise was found to elicit the most activity in the latissimus dorsi (55% MMT) and the pectoralis major (84%). This is consistent with the prime function of shoulder depression for each of these muscles. In comparison, the push-up produced 64% MMT in the pectoralis major.
Townsend et al studied 17 exercises and recommended inclusion of the empty can exercise, shoulder flexion, prone horizontal abduction with external rotation, and the press-up in shoulder rehabilitation programs based on the high activity in each muscle examined during these exercises.
Several research studies have since expanded on the work of Townsend et al. In particular, researchers have sought to compare the effectiveness of several exercises for the external rotators, supraspinatus, deltoid, and scapulothoracic musculature. The following sections discuss each one in detail.
External Rotators
An overhead throwing athlete requires the rotator cuff to maintain an adequate amount of glenohumeral joint congruency for asymptomatic function. Strength of the infraspinatus and teres minor is integral during the overhead throwing motion to develop a compressive force equal to body weight (BW) at the shoulder joint to prevent distraction. Andrews and Angelo found that overhead throwers most often have rotator cuff tears located from the mid–supraspinatus posterior to the midinfraspinatus area, which they believed to be a result of the compressive force produced to resist distraction, horizontal adduction, and internal rotation at the shoulder during arm deceleration. Thus, the external rotators are muscles that often appear weak and are affected by different pathologic shoulder conditions, such as internal impingement, joint laxity, labral lesions, and rotator cuff lesions, particularly in overhead throwing athletes. Consequently, many authors have advocated emphasis on strengthening of external rotation during rehabilitation or athletic conditioning programs to enhance muscular strength, endurance, and dynamic stability in overhead throwing athletes.
Several studies have documented the EMG activity in the glenohumeral musculature during specific shoulder exercises. Variations in experimental methodology have resulted in conflicting outcomes and controversy in the selection of exercises. As discussed previously, Townsend et al evaluated infraspinatus and teres minor activity during 17 shoulder exercises. The authors determined that the exercise eliciting the most EMG activity in the infraspinatus muscle was prone horizontal abduction with external rotation (88% MMT) and that the most effective exercise for the teres minor muscle was side-lying external rotation (80% MMT).
Similarly, Blackburn et al performed EMG analysis of the rotator cuff muscles in 28 healthy subjects during a series of 23 common posterior rotator cuff–strengthening exercises. The authors reported high levels of EMG activity in the infraspinatus (80% EMG activity) and teres minor (70% EMG activity) when the prone horizontal abduction movement at 90° and 100° of abduction with full external rotation was performed by 28 healthy subjects.
Conversely, Greenfield et al compared shoulder rotational strength in the scapular and frontal planes during isokinetic testing in 20 healthy subjects. The authors reported that no significant differences were found in internal rotation strength between positions; however, external rotation strength was significantly higher in the scapular plane, thus suggesting that the plane of the scapula may be a more effective position to exercise the external rotators.
Ballantyne et al compared EMG activity in the external rotators during side-lying external rotation and during external rotation in the prone position at 90° of abduction in 40 subjects. The authors reported similar EMG findings for the infraspinatus and teres minor during both exercises, with approximately 50% normalized activity for each muscle. Conversely, in the previously cited study by Blackburn et al, the authors compared side-lying external rotation and prone external rotation exercises and noted greater EMG activity during prone external rotation in the infraspinatus (prone, 80%; side lying, 30%) and teres minor (prone, 88%; side lying, 45%).
Reinold et al analyzed several different exercises commonly used to strengthen the external rotators to determine the most effect exercise and position to recruit muscle activity in the posterior rotator cuff. Integrated EMG activity in the infraspinatus, teres minor, supraspinatus, posterior deltoid, and middle deltoid in 10 asymptomatic subjects (5 male and 5 female subjects; mean age, 28.1 years; range, 22 to 38 years) was analyzed during seven exercises: prone horizontal abduction at 100° of abduction, full external rotation and prone external rotation at 90° of abduction, standing external rotation at 90° of abduction, standing external rotation at 45° in the scapular plane, standing external rotation at 0° of abduction, standing external rotation at 0° of abduction with a towel roll, and side-lying external rotation at 0° of abduction.
Based on the results of this study, the exercise that elicited the most combined EMG activity in the infraspinatus and teres minor was side-lying external rotation (infraspinatus, 62% maximum voluntary isometric contraction [MVIC]; teres minor, 67%), followed closely by external rotation in the scapular plane (infraspinatus, 53%; teres minor, 55%), and finally prone external rotation in the 90° abducted position (infraspinatus, 50%; teres minor, 48%) ( Table 10-2 ).
| Muscle | Prone Horizontal Abduction (100°, Full ER) | Prone ER (90° Abduction) | Standing ER 90° (90° Abduction) | Standing ER Scapular Plane | Standing ER 0° | Standing ER 0° with Towel | Side-Lying ER | Significance * |
|---|---|---|---|---|---|---|---|---|
| Supraspinatus | 82 ± 37 | 68 ± 33 | 57 ± 32 | 32 ± 24 | 41 ± 39 | 41 ± 37 | 51 ± 47 | a, c, d, e |
| Middle deltoid | 82 ± 32 | 49 ± 15 | 55 ± 23 | 38 ± 19 | 11 ± 7 | 11 ± 6 | 36 ± 23 | a, b, c, f |
| Posterior deltoid | 88 ± 33 | 79 ± 31 | 59 ± 33 | 43 ± 30 | 27 ± 27 | 31 ± 27 | 52 ± 42 | a, c, d, f, g |
| Infraspinatus | 39 ± 17 | 56 ± 30 | 59 ± 38 | 53 ± 24 | 40 ± 15 | 51 ± 14 | 62 ± 13 | h |
| Teres minor | 41 ± 24 | 41 ± 22 | 33 ± 18 | 64 ± 51 | 34 ± 14 | 46 ± 22 | 62 ± 31 |
Exercises in the 90° abducted position are often incorporated to simulate the position and strain on the shoulder during similar overhead activities such as throwing. This position produced moderate activity in the external rotators but also increased activity in the deltoid and supraspinatus to stabilize the shoulder. It appears that the amount of infraspinatus and teres minor activity progressively decreases as the shoulder moves into an abducted position whereas activity in the supraspinatus and deltoid increases. This may imply that as the arm moves into a position of less shoulder stability, the supraspinatus and deltoid are active to assist in the external rotation movement while providing some degree of glenohumeral stability through muscular contraction.
In the standing position, external rotation at 90° of abduction may have a functional advantage over 0° of abduction, and in the scapular plane, because of the close replication of this position in sporting activities the combination of abduction and external rotation places strain on the shoulder’s capsule, particularly the anterior band of the inferior glenohumeral ligament. When the arm is not in an abducted position, external rotation places less strain on this portion of the joint capsule. Therefore, although muscle activity was low to moderate during external rotation at 0° of abduction, this rehabilitation exercise may be worthwhile when strain of the inferior glenohumeral ligament is a concern. Side-lying external rotation may be the most optimal exercise to strengthen the external rotators based on the highest amount of EMG activity observed during this study.
Theoretically, external rotation at 0° of abduction with a towel roll provides both low capsular strain and also good balance between the muscles that externally rotate the arm and the muscles that adduct the arm to hold the towel. Our clinical experience has shown that adding a towel roll to the external rotation exercise provides assistance to the patient by ensuring that proper technique is observed without muscle substitution ( Fig. 10-1 ). With the addition of a towel roll to the exercise, a tendency toward higher activity in the posterior rotator cuff was consistently seen as well. An approximately 20% to 25% increase in infraspinatus and teres minor EMG activity was noted with the use of a towel roll.
Furthermore, external rotation in the scapular plane may be an effective exercise during rehabilitation because of the moderate amount of muscular activity in each of the muscles tested, with a moderate amount of capsular strain occurring in the 45° abducted position ( Fig. 10-2 ). This exercise may offer a compromise between strengthening and stabilization.
The exercises that produced the greatest amount of activity in the external rotators were side-lying external rotation, external rotation in the scapular plane, and external rotation at 90° of abduction (both standing and in the prone position).
As the angle of arm elevation increases during shoulder external rotation exercises, the amount of supraspinatus and deltoid activity increases, whereas the amount of infraspinatus and teres minor activity decreases.
Adding a towel roll to external rotation at 0° of abduction results in higher EMG activity in the infraspinatus and teres minor.
Supraspinatus and Deltoid
Numerous investigators have studied EMG activity in the supraspinatus during rehabilitation exercises. Controversy exists regarding the optimal exercise to elicit muscle activity. Jobe and Moynes were the first to recommend elevation in the scapular plane with internal rotation, or the empty can exercise. The authors recommended this exercise for strengthening the supraspinatus because of the high EMG activity observed during this movement. The recommendation for the empty can exercise was further strengthened by the work of Townsend et al, who reported greater amounts of EMG activity in the supraspinatus and deltoid muscles during the empty can exercise than during exercises consisting of elevation in the scapular plane with external rotation, or the full can exercise, although statistical analysis was not performed to compare the exercises (see Table 10-1 ).
In clinical studies, numerous authors have suggested that the empty can exercise may provoke pain in many patients by encroaching on soft tissue within the subacromial space during this impingement-type maneuver. Numerous authors have since compared the empty can exercise with several other common supraspinatus exercises to determine whether exercises that place the shoulder in less of a disadvantageous position elicit similar amounts of supraspinatus activity.
Blackburn et al compared EMG activity in the rotator cuff during several exercises and reported no significant differences in supraspinatus activity during the empty can and full can exercises. However, the authors did report a statistically significant increase in supraspinatus activity during prone horizontal abduction at 100° with full external rotation ( Fig. 10-3 ).
Worrell et al compared the amount of supraspinatus activity during the empty can exercise recommended by Jobe and Moynes and the prone horizontal abduction exercise recommended by Blackburn and associates. The authors performed fine-wire EMG and handheld dynamometer measurements in 22 healthy subjects and reported greater supraspinatus activity during the prone exercise but less total force production than during the empty can exercise. The authors hypothesized that although supraspinatus activity was greater in the prone position, a greater amount of surrounding muscular activity was noted during the empty can exercise.
The effect of increased deltoid activity during arm elevation is a concern for the rehabilitation specialist, especially when a patient with subacromial impingement or pathologic rotator cuff conditions is being rehabilitated. Morrey et al examined the resultant force vectors of the deltoid and supraspinatus during arm elevation at various degrees of motion. Deltoid activity alone exhibited a superiorly orientated force vector from 0° to 90° and a compressive force on the glenohumeral joint at 120° to 150°. Conversely, the supraspinatus muscle produced a consistent compressive force throughout the range of elevation ( Fig. 10-4 ). In patients with subacromial impingement, weak posterior rotator cuff muscles, inefficient dynamic stabilization, or pathologic changes in the rotator cuff, exercises that produce high levels of deltoid activity may be detrimental because of the amount of superior humeral head migration observed when the rotator cuff does not efficiently compress the humeral head within the glenoid fossa. Superior humeral head migration may be disadvantageous to patients with rotator cuff pathology or decreased stability of the glenohumeral joint. Superior humeral head migration may result in subacromial impingement, subdeltoid bursa trauma, and bursal thickening. The previously mentioned pathologies can result in tendon degeneration and eventually failure of the rotator cuff. Therefore, exercises are often chosen that minimize the opportunity for the deltoid to overpower the rotator cuff musculature during arm elevation.
Based on the hypothesis of Worrell et al, Malanga et al examined EMG activity in the supraspinatus and deltoid muscles during the empty can and prone exercises in 17 healthy subjects. The authors reported no significant differences in supraspinatus EMG activity during the two exercises (empty can, 107%; prone, 94%). However, a statistically significant increase in posterior deltoid EMG activity was observed during the prone exercise (empty can, 76%; prone, 96%) and significantly greater anterior deltoid EMG activity during the empty can exercise (empty can, 96%; prone, 65%). Middle deltoid EMG activity was high during both exercises (empty can, 104%; prone, 111%).
In a similar study, Kelly et al compared isometric EMG activity in the supraspinatus and deltoid muscles during the full can and empty can exercises in 11 healthy subjects. The authors again reported no significant difference in supraspinatus activity; however, they did note that the least amount of surrounding muscle activity was observed during the full can position and therefore recommended this position for manual muscle testing of the supraspinatus.
Takeda et al examined the most effective exercise for strengthening the supraspinatus by comparing magnetic resonance imaging (MRI) T2 relaxation times in the shoulders of six healthy subjects. The authors reported an increase in relaxation time that correlated well with concentric and eccentric muscle contractions. Subjects performed the empty can, full can, and prone exercises, and MRI scans were obtained immediately before and after each exercise. The change in relaxation in the supraspinatus was significantly higher during the empty can (10.5 msec) and full can (10.5 msec) exercises than during the prone exercise (3.6 msec). The least amount of deltoid activity was observed during the full can exercise.
Reinold et al were the first to examine both supraspinatus and deltoid muscle activity dynamically during all three exercises—empty can, full can, and prone. The authors measured fine-wire EMG activity in the dominant shoulder of 22 healthy subjects (15 male and 7 female subjects; mean age, 27 ± 5 years) ( Table 10-3 ). EMG activity was normalized to MVIC and analyzed with a one-way repeated measures analysis of variance. The authors reported no significant difference in supraspinatus activity, which ranged from 62% to 67% MVIC during each exercise. However, post hoc analysis revealed significant differences in deltoid activity during the three exercises. The posterior deltoid showed the greatest activity during the prone full can exercise, whereas the middle deltoid displayed the greatest activity during the empty can and prone full can exercises. This information is important to consider when creating a strength program to maximize strength gains in the supraspinatus.
| Muscle | Full Can | Empty Can | Prone |
|---|---|---|---|
| Supraspinatus | 62.2 ± 39.9 | 62.7 ± 45.2 | 66.8 ± 50.1 |
| Middle deltoid | 52.2 ± 26.6 | 76.9 ± 43.9 * | 63.1 ± 30.8 * |
| Posterior deltoid | 37.8 ± 32.0 | 54.1 ± 28.3 * | 86.7 ± 52.7 * |
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