Normal isometric strength of rotator cuff muscles in adults

Study group

A total of 400 healthy adult volunteers between the ages of 20 and 60 years were tested. The volunteers were recruited from the Rambam Medical Center and Faculty of Medicine staff. There were 50 men and 50 women volunteers for each decade within the age range.

None of the volunteers had a previous shoulder complaint or injury. On clinical examination performed by one of the authors (AC), they were confirmed as having no restriction of shoulder movements (active or passive), no pain in the shoulder girdle or in the cervical area and negative provocative tests for subacromial impingement or labral pathology. All the subjects had a normal observed rhythm of scapulothoracic movement.

Most of the clinically important pathological changes in the rotator cuff muscles (e.g. tears, partial or full, degeneration, such in different impingement syndromes, calcific tenditis, etc.) occur in supraspinatus (SS), infraspinatus (IS) and subscapularis (SSC) muscles. Accordingly, these structures were evaluated in the present study.

Assessment of isometric strength

Time curves of the tested muscles in both shoulders in every person were measured at standardised positions. The dominancy of the tested limb was recorded. The measurements of the isometric force were performed by a specialised dynamometer (1200 readings/sec, resolution 0.04 N; Myometer; Atlantech Medical, Nottingham, United Kingdom).

In general, the rotator cuff muscles act synergistically, but there are several testing positions when one of these muscles generates the main force.¹² We used the following optimal positions for the measurements of various muscles.

Supraspinatus muscle testing

While in seated position, the tested arm was held in 45° of abduction and 30° of forward flexion with the dynamometer cuff placed above the elbow. The elbow was flexed to 90°. The lever arm of abduction force was measured between the centre of the dynamometer cuff and the point 3 cm distal to the lateral edge of acromion. This position represents the centre of rotation of the humeral head.

Subscapularis and infraspinatus muscle testing

In a supine position, the tested extremity is 30° tilted forwards and fully adducted. This positioning is chosen in order to eliminate scapular movements and to evaluate the generated forces close to the scapular plane. The dynamometer cuff was placed on the forearm and the forces were produced by the internal and external rotation of the shoulder. The connection between the dynamometer cuff and the dynamometer was kept perpendicular to the forearm, in the opposite direction to the force vector, and therefore the lever arm in this position is negligible.

Each strength measurement was performed for a time period of five seconds. This period of test was chosen as it is empirically similarly to the widely accepted Constant’s shoulder assessment scoring method.¹³ Two measurements were performed for each muscle in five-minute intervals and the highest results were recorded. Characteristic force-time curves were produced (Fig. 1). For the further evaluation the values from the beginning of the generated force rise and in the duration of four seconds were used in order to eliminate the artifacts of the initiation and completion of measurements. Therefore the information was recorded as sequence of measurements lasting four seconds. Each value was multiplied by the lever arm and expressed as a moment of torque. The results were normalised to the lean body mass (LBM) in order to eliminate the effect of the variations in the overall fitness conditions of the examined subjects. The LBM was calculated according to the following formulae¹⁴:

Fig. 1

Sample profiles of force–time curves as recorded by a dynamometer.

LBM (men) = (1.10×Weight) – 128×(Weight²/(100×Height²))

LBM (women) = (1.07×Weight) – 148×(Weight²/(100×Height²))

The mean values for each group were recorded and the standard deviation calculated.

This project was approved by the Institutional Ethical Committee.

Statistical analysis

We studied the force–time curves. The corresponding values of force, according to the equal time intervals, were compared by one-way analysis of variance (ANOVA) followed by post-hoc Tukey test (for comparison of pairs of result groups) after a normal distribution of values was determined by the Kolmogorov–Smirnov test. If not a normal distribution, one-way ANOVA on ranks with Student’s Newman–Keuls post-hoc test was used. A p-value < 0.05 was considered to be statistically significant. The calculations of the statistical comparisons were performed by SigmaStat software v2 (SPSS Inc., Chicago, Illinois).

Supraspinatus muscle testing

In testing of supraspinatus muscle of the dominant limb in men we found that a significantly higher profile of the force–time curves was in men in the fifth decade of life, followed by the individuals of fourth, third and sixth decades of life (p < 0.001) (Table I, Fig. 2a). The same tendency is seen in testing the isometric supraspinatus force-time profiles in the non-dominant upper limbs (p < 0.001, Fig. 2a).

In the group of men in the third decade of life there was no difference in torque–time curves of supraspinatus muscle testing between dominant and non-dominant limb (p = 0.06), but in all the other three groups the dominant upper limb generated significantly higher isometric force by the supraspinatus muscles (all p < 0.001) (Table I, Fig. 2a).

In testing of supraspinatus muscle of the dominant limb in women we found that a significantly higher profile of the force–time curves was in the fourth decade of life, followed by the individuals of the fifth decade (p < 0.001) (Table I, Fig. 2b). The values in the third and sixth decades of life were similar (p = 0.9), and both significantly lower compared with the other two age groups (p < 0.001 for both third- and sixth-decade women in comparison with fourth- and fifth-decade women) (Table I, Fig. 2b).

A similar tendency was seen in testing the isometric supraspinatus force-time profiles in the non-dominant upper limbs, with significantly higher values in the fourth and fifth decade of life (p < 0.001) compared with the third and sixth decades of life, which were similar (p = 0.15) (Table I, Fig. 2b).

In women there was no statistically significant difference between the dominant and non-dominant limb in third and sixth decades of life (p = 0.9), but there was a significant difference comparing the curves of dominant and non- dominant limbs in fourth and fifth decades of life (p < 0.001) (Table I, Fig. 2b).

Infraspinatus muscle testing

Similarly to supraspinatus, we found testing of infraspinatus muscle of the dominant limb in men to show a significantly higher profile of the force–time curves in the fifth decade of life, followed by the fourth, third and sixth decades (p < 0.001 (Table I, Fig. 3a).

Testing of infraspinatus in male non-dominant limbs showed similar high profiles in the fourth and fifth decades of life (p = 0.15), which were significantly higher than the third and sixth decades of life (both p < 0.001) (Table I, Fig. 3a). In all age groups the dominant upper limb generated significantly higher isometric force by the infraspinatus muscles (all p < 0.001, Fig. 3a).

In the dominant arm in women, we found testing of infraspinatus to result in a significantly higher force–time curve profile in the fourth decade of life compared with the other decades (p < 0.001) (Table I, Fig. 3b). There was no significant difference between the dominant limb curve’s profiles in the groups in third and sixth decades of life (p = 0.03) (Fig. 3b). In non-dominant female limbs, the significantly highest profile of the force–time curves was in the fourth decade of life (p < 0.001) in comparison with and followed by the individuals in the third and fifth decades (Fig. 3b). The lowest profile of the force–time curves in women was seen among individuals belonging to the sixth decade of life (p < 0.001) in comparison with all other age groups (Table I, Fig. 3b). In all female age groups above 30 years of age the torque–time curves of infraspinatus muscles were significantly higher in dominant limbs (p < 0.001). Those of the third decade of life showed no significant difference in the profile of the curves of the dominant and non-dominant limbs (p > 0.05, Fig. 3b).

Subscapularis muscle testing

In testing of the subscapularis muscle of the dominant limb in men we found that a significantly higher profile of the force–time curves was in men in fifth decade of life in comparison to and followed by the individuals of fourth, third and sixth decades in descending order (p < 0.001) (Table I, Fig. 4a). In male non-dominant limbs, the profile was highest in those individuals in the fourth and fifth decades, followed by those of the third and sixth decades (p < 0.001) (Table I, Fig. 4a). In all age groups the dominant arm in men generated a significantly higher force than the non-dominant arm (all p < 0.001).

In testing the subscapularis muscle of the dominant limb in women we found a significantly higher profile of the force–time curves in the fourth and fifth decades of life, followed by the individuals of third and sixth decades of life (p < 0.001) (Table I, Fig. 4b). The subscapularis force-time profiles in the non-dominant upper limbs were highest in the female individuals in the third and fourth decades of life, followed by the curves in fifth and sixth decades of life (p < 0.001) (Table I, Fig. 4b). In all the tested groups of women the dominant upper limbs generated a significantly higher isometric force by the subscapularis muscles (p < 0.001) (Table I, Fig. 4b).