Pectoralis Minor Index: Does Ethnicity Hold Relevance? Estimation of Pectoralis Minor Length in the Indian Population and Its Correlation with Hand Length

Abstract

Objective

Pectoralis Minor Index (PMI) is a proposed parameter to evaluate the pectoralis minor length (PML), eliminating the effect of subject’s variability of height. Neither a PMI standard value nor any cutoff value to label a shortened pectoralis minor (PM) has been accepted yet, which can be applied to every individual. Moreover, the length of the PM has never been correlated to any fixed reference in the body. Hence, we estimated the PML in the Indian population and investigated its correlation to the individual’s hand length.

Methodology

A cross-sectional study was conducted including 100 adult subjects without any shoulder pathology. Subjects with history of fracture/treatment involving upper limb/spine were excluded. Two assessors evaluated the height, PM length and hand length of subjects. PMI and hand correlation was evaluated using their mean values.

Results

Mean PML and PMI for dominant and non-dominant shoulder were calculated to be 18.11/18.21 cm and 10.53/10.59, respectively. Mean hand length of dominant and non-dominant hand was found to be 18.27 cm and 18.31 cm, respectively. Pearson correlation coefficient between right/left PML with right/left hand length was 0.67 and 0.63, respectively, suggesting a good correlation (p < 0.01).

Conclusions

PMI varies in different ethnic groups, which makes PMI a less reliable indicator for managing shoulder pain in ethnic groups where reference values are yet not available. Contralateral PMI can be used as a reference value in unilateral shoulder pathologies with short PML. Hand length can become an important parameter in evaluating painful shoulders even in bilateral pathologies. Hand length can be used as an easy and quick technique to compare the PML and effect of physiotherapy in patients with diagnosis of short PML, attending follow-up OPD. Though, a study comparing PML of normal subjects and patients with shoulder pain will be further required in different ethnic groups for further validation of this study.

Introduction

Shoulder pain is a common problem in athletes and even in normal individuals [1], amongst whom around two-fifths of the patients suffer from shoulder impingement symptoms (SIS) [2, 3]. In absence of any bony abnormality, faulty posture which includes forward head posture, scapular protraction, humeral internal rotation and increased thoracic kyphosis have been proposed as few possible predisposing factors leading to SIS [4]. Scapular dyskinesia occurring secondary to imbalance of periscapular muscles has also been considered as a common reason for shoulder pathologies [5]. Pectoralis minor muscle shortness has been postulated as an important muscle causing scapular dyskinesia. A direct measurement of the pectoralis minor muscle (PM) length has been proposed to be a good indicator of scapular protraction [6, 7]. The PM originates on ribs 3–5 and inserts on the medial inferior border of the coracoid process. Because of this anatomical position of the PM, shortening can lead to an increase in scapular anterior tilting and internal rotation and a decrease in scapular upward rotation, which can be a predisposed condition for SIS [8, 9]. The association between these deviations and shoulder pain is based on the theory that prolonged positional changes lead to soft tissue elongation on one side of the joint and shortening on the opposite side [4]. In addition, it is speculated that the PM length is potentially shortened due to repetitive use of that muscle, often seen on the dominant side in overhead athletes [10–12]. A measurement technique of the PM length using measuring tape and caliper is validated on human cadavers and by both in vitro and in vivo testing [4, 8]. Borstad et al. developed a normalization index known as Pectoralis Minor Index (PMI) to eliminate the variability arising due to height and muscle length variability among subjects [8]. Several researchers did study pectoralis minor, measuring and comparing its various parameters [8, 13–16]. On reviewing the results of PMI of several studies, a conclusion has been drawn by us that different studies have different values of PMI even in normal individuals. This implies that no universal standard value of PMI is there, which can be applied to every individual. Thereby, no cutoff value of PMI can be agreed upon to label a shortened pectoralis minor (PM), reducing the absolute relevance of PMI. To use PMI as a valid and reliable parameter for diagnosing cause of shoulder pain, every ethnic group should have a reference value of PMI. However, it may not be possible for every ethnic group to have its own reference value. Hence, there is a strong need to find any fixed reference in the body correlating with PM length. In individuals with primary bilateral shortened PM, if a reference can be found which correlates with PM length it would be of immense use.

No study has been done in the Indian population till now involving normal subjects. This study was designed to estimate the PM length in the Indian population, develop a reference value of PMI in an asymptomatic population and to investigate PM length correlation to the individual’s hand length which may be useful to formulate an exercise protocol in patients with bilateral shortened PM.

Methodology

Hundred subjects without shoulder symptoms were recruited through the orthopedic and physical therapy outpatient department in the tertiary care teaching hospital where the study was conducted. Duly signed witnessed informed consent was taken from every subject and subjects were made aware of all their rights including the right to withdraw from the study at any stage of the investigation.

Inclusion criteria:

1. Age over 18,

2. Absence of shoulder pain during the last year and

3. Absence of a history of fractures, treatment or surgery of the shoulder girdle and hands.

Exclusion criteria: a history of fractures, treatment or surgery to the lumbar, thoracic, cervical spine and upper limbs.

All subjects were informed about the purpose and design of the study. Two assessors evaluated all the subjects. After questioning of any known shoulder and/or cervical region problem, one of our researchers performed further tests. Pain or limitation in cervical spine range of motion was checked. Rotator cuff disease evaluation was done by impingement tests (Neer’s test, Hawkins–Kennedy test), evaluation of painful arc during active arm elevation, pain or weakness with resisted isometric external rotation, internal rotation or scapular plane abduction with the humeral internal rotation (Jobe’s empty can test). Drop arm test and lag sign test were also done to reveal if the rotator cuff is ruptured or intact. All the 100 subjects were found to be normal in all the tests and hence were included in the study. There were no dropouts in the study.

First, the subject’s body height was measured. Then, both shoulders were measured three times by each rater (assessor). The order of testing (between all 6 measurements and for the choice of assessor) was randomized by coin toss. First, the order of rater was tossed, followed by the order of sides. Successively, both shoulders were tested independently. Both raters were blinded for hand dominance, and to the outcome of each other’s findings. To assure blinding, each assessor left the examining room when the other assessor was performing an assessment.

To detect the exact location of the coracoid process and rib 3, 4 and 5, palpation was used.

PM length measurement procedure: For the anthropometric measurement of the PM length, we had adapted the protocol of Borstad et al. [16], as previously described by Cools et al. [10]. The instructions were to palpate two anatomical reference points which in line represent the PML:

1. The inferomedial aspect of the coracoid process

2. The caudal edge of the fourth rib at the sternum.

The distance between these two bony reference points was measured with a vernier caliper and a flexible tape. To neutralize variations of muscle length resulting from respiration, subjects were asked to exhale before the measurement and to inhale only after the measurement. The subjects were in a supine position, with the elbows extended alongside the body with the palm placed on the examining table to minimize postural influences of the thoracic spine and to optimize muscle relaxation of the surrounding musculature as described by Cools et al. [10] (Fig. 1a, b).

Fig. 1.

a Photograph illustrating the measuring technique of pectoralis minor length using flexible measuring tape. b Photograph illustrating the measuring technique of pectoralis minor length using vernier caliper

Then, the Pectoralis Minor Index was calculated using formula PMI = [PM length (cm)/subject’s height (cm)] × 100, as described by Borstad et al. [8].

The hand length of the subjects was measured with a measuring tape on the volar side from the base of the thenar eminence to the tip of the middle finger (Fig. 2).

Fig. 2.

Photograph illustrating the measuring technique of hand length using flexible measuring tape

Statistical Analysis

The Statistical Package for the Social Sciences was used for analyzing the collected data (version 22.0 for Windows; SPSS Inc. Chicago, IL). Normality of the variables was visually tested for a Gaussian distribution. Pectoralis Minor Index was calculated for dominant and non-dominant shoulder. Correlation of hand length and pectoralis minor length was calculated using Pearson’s correlation coefficient.

As both raters tested all the subjects, the reliability of the technique and the reliability of the measurements, i.e., interrater reliability were analyzed using intraclass correlation coefficient (ICC) and standard error of measurement (SEM). Two-way random-effects ICC model was used (absolute agreement). Here, a reliability coefficient less than 0.50 was an indication of poor reliability; reliability coefficients between 0.50 and 0.75, an indication of moderate reliability; reliability coefficients between 0.76 and 0.90, an indication of good reliability; reliability coefficients over 0.90, an excellent reliability.

Results

Mean age of the study group was 24 years and all the subjects were males. The youngest subject of the study was of 19 years and eldest of 35 years. Ninety-three subjects were right hand dominant and seven were left hand dominant. Mean height of the group was 171.9 cm (range 160–184 cm).

Results were evaluated and comparison was sorted according to dominant and non-dominant side. Table 1 presents the mean PM length results, together with PMI and hand length. Interrater reliability analysis is presented in Table 2.

Table 1.

Results

Parameters	Dominant side (SD)	Dominant side (range)	Non-dominant side (SD)	Non-dominant side (range)
Pec. minor length (in cm)	18.11 (1.51)	14.8–22.5	18.21 (1.36)	15.5–22.5
Pectoralis Minor Index	10.53 (0.77)	8.64–12.54	10.59 (0.70)	9.05–12.54
Hand length (in cm)	18.27 (0.93)	16.3–21.5	18.31 (0.94)	16.7–21.3

Table 2.

Interrater variability in our study

Parameters	Interclass correlation coefficient		Standard error measurement
	Single measure	Average measure
Pec. minor length (dominant side)	0.96	0.98	0.43
Pec. minor length (non-dominant side)	0.95	0.97	0.45
Hand length (dominant side)	0.94	0.97	0.32
Hand length (non-dominant side)	0.94	0.96	0.33

Using Pearson correlation statistical test, a strong correlation was found between right/left shoulder PM length with right/left hand length with Pearson correlation coefficient, 0.67 and 0.63 with significance value < 0.01 for right and left sides, respectively (Fig. 3a, b). Stronger correlation was also seen between contralateral hand length and PM length on comparing using Pearson correlation test. Correlation between right hand length and left PML was calculated to be 0.712 and correlation between left hand length and right PML was 0.58 with statistically significant results (p value < 0.01).

Fig. 3.

a Photograph illustrating the graph representing the correlation between the left hand length and ipsilateral pectoralis minor length. b Photograph illustrating the graph representing the correlation between the right hand length and ipsilateral pectoralis minor length

Discussion

Pectoralis minor length is under continuous scrutiny as a cause of shoulder pain and scapular dyskinesia. Lewis et al. proposed an alternate method of measuring pectoralis minor shortness, which involves measurement of distance from the posterolateral angle of the scapula (arcomion) to the examination table of a supine patient. This parameter is known as acromion table (AT) distance. AT distance > 2.54 cm is suggestive of pectoralis minor tightness [16]. Intrarater reliability of this parameter has an ICC > 0.90, but still this test has not been advised to be used for clinical assessment as it lacks diagnostic accuracy [6]. Moreover, this technique is poorly correlated with a normalized measure of pectoralis minor length [16]. In our study, we measured PML as described by Borstad et al. using measuring tape and vernier caliper [16]. Reliability of this technique of measuring PML has been tested by various researchers. In our study, interrater reliability was analyzed and we found ICC > 0.90 (ranging 0.94–0.98) for PML and hand length with SEM ranging from 0.32 to 0.45, suggesting excellent reliability of the technique used for measuring PML and hand length. Our results corroborated with earlier studies which also concluded “good to excellent” intra and interrater reliability with ICC > 0.75 at least [13, 15, 17, 18] (Table 3).

Table 3.

Interrater variability of PML measuring technique in previous studies

Parameters	ICC	Standard error measurement
Struyf et al. [13]	0.76–0.87	0.21–0.32
Lee et al. [15]	0.82–0.87	–
Jain et al. [17]	0.96	–
Rosa et al. [18]	0.86–0.97	0.70–0.84
Our study	0.94–0.98	0.32–0.45

Length of muscles and even bony parameters can vary according to different weight and height of individual, belonging to different ethnic groups. Pectoralis minor length may also follow similar variation. To normalize the variability arising due to height and muscle length variability among subjects, the Pectoralis Minor Index has been postulated by Borstad et al. [8]. But, on reviewing various studies measuring PMI, it has been observed that PMI also varies among groups [8, 13–16]. This concludes that a single value of PMI cannot be accepted for every individual, which raises concerns on the reliability of PMI as a parameter to diagnose shoulder pathologies if a reference value for the analyzed ethnic group is not available.

Our hypothesis of “different ethnic groups have different PMI” has been substantiated by results of our study. We found a major difference between PMI in previous studies and our study (Table 4) [8, 13–16]. This indicates that ethnic variations do exist in PMI and further research is required to calculate the PMI of different ethnic groups. Mean PMI in our study is 10.53 (SD = 0.77) and 10.59 (SD = 0.70) for the dominant and non-dominant sides. Borstad defined pectoralis minor muscle tightness as any individual having PMI < mean PMI of group—1 SD [16]. Applying the same formula to Indian population in our sample size, the cutoff reference value of PMI to label a “short pectoralis minor” came out to be 9.76 and 9.89 for dominant and non-dominant side, respectively. We have analyzed a major difference in the cutoff reference value of PMI for “short PM” in our study and previous studies (Table 4).

Table 4.

Comparison with previous studies

Various studies	PMI (SD)	Cutoff reference for short PM (PMI—1 SD)
Borstad et al. [8]	8.1 (0.50)	7.65
Borstad et al. [16]	8.24 (0.80)	7.44
Struyf et al. [13]	9.01 (0.59) dominant side 9.22 (0.60) non-dominant side	8.42 dominant side 8.62 non-dominant side
Yeşilyaprak et al. [14]	8.58 (0.75)	7.83
Lee et al. [15]	9.15 (0.45)	8.70
Our study (2016)	10.53 (0.77) dominant side 10.59 (0.70) non-dominant side	9.76 dominant side 9.89 non-dominant side

Considering the limitation of PMI as a valid and reliable indicator, we tried to find out a new fixed reference parameter which can be individual specific. We hypothesized that hand length of an individual subject can act as a good clinical fixed reference parameter for comparing the PML. In our study, we evaluated the correlation of hand length with ipsilateral PML using Pearson’s correlation test. A good correlation with ratio 1:1 was seen between the two parameters with Pearson’s correlation coefficient of 0.67 and 0.63 for right and left side, respectively, with statistically significant result (p value < 0.01) (Fig. 3a, b). No study has ever compared PML with any fixed reference in body. Hence, positive correlation of PML and hand length holds its clinical significance.

Pectoralis minor muscle stretching exercise has been found to alleviate the shoulder pain and other symptoms, suggesting physiotherapy as the major treatment modality for short pectoralis minor muscle [4, 19]. But, data are still lacking in literature which can decide the end point of physiotherapy. In our study, dominant and non-dominant shoulder PMI did not show much variation, suggesting that contralateral PMI values can be used as reference value for unilateral shoulder pathologies leading to short PM length. Moreover, hand length of an individual patient can act as end point of PM length to be achieved with physiotherapy to get rid off of shoulder pain occurring secondary to “short pectoralis minor muscle”, especially in bilateral pathologies. A good correlation between contralateral hand length and PML concludes that hand length can be used to measure the effect of physiotherapy in follow-up of patients with short PML (Fig. 4). This simple maneuver is an easy way to compare the PML with a reference value in a quick manner especially helpful in OPD where measuring PML with measuring tape and caliper is time taking and cumbersome. Moreover, it can be used as a screening test, as it is easy and quick to measure and can be done in OPD.

Fig. 4.

Photograph illustrating the use of hand length as a tool for comparing pectoralis minor length in OPD patients

Conclusions

PMI varies in different ethnic groups, which makes PMI a less reliable indicator for managing shoulder pain in ethnic groups where reference values are yet not available. Contralateral PMI can be used as a reference value in unilateral shoulder pathologies with short PML. A positive correlation between hand length and PM length concludes that hand length can become an important parameter in evaluating painful shoulders especially when the problem is bilateral. Hand length can be used as an easy and quick technique to compare the PML and effect of physiotherapy in patients with diagnosis of short PML, attending follow-up OPD. The Pectoralis Minor Index done in the Indian population can now be a useful tool in screening sportspersons/military recruits and athletes/swimmers at the time of their recruitment and accordingly guide them in training programs. Though, a study comparing PM length of normal subjects and patients with shoulder pain will be further required in different ethnic groups for further validation of both old and new parameters. Also, a multicentre study involving different regions of India involving a larger sample size will further increase the validity of our study.

Limitations of the Study

As the study was conducted in normal subjects without any shoulder pathologies, it lacks comparative data with the patients of should pain. Moreover, correlation of hand length with PM length is totally a new concept which requires further validation by doing research on different ethnic regions and on patients with shoulder pain too. Also, due to cultural reasons and lack of consent, we could not include healthy female subjects in the study.

Compliance with Ethical Standards

Conflict of interest

The authors declare that there is no conflict of interest.

Ethical approval

This cross-sectional study had no ethical issues. Anthropometric measurements were taken after informed consent.

Informed consent

Informed consent for submission of data for publication was taken from each participant. All the subjects were healthy individuals. No intervention was done.

Authorship declaration

All authors listed meet the authorship criteria according to the latest guidelines of the International Committee of Medical Journal Editors and all authors are in agreement with the manuscript.