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Letters and Responses |
Dr Wong also asserts that it was the pectoralis major muscle being measured, not the pectoralis minor muscle. The problem with this assertion is that it fails to consider both anatomical landmarks used in the measurement. By palpating the fourth sternocostal junction and the coracoid process, the vector that is calculated represents the length of the pectoralis minor muscle, but more importantly it is oriented with the longitudinal direction of the muscle fibers. This is critical because the hypothesis of the study was that a change in muscle length will change the number of sarcomeres in series and subsequently the mechanical properties of the muscle. It is the mechanical change, primarily increased passive tension, that ultimately affects posture or movement. Dr Wong asserts that the vector represents the length and line of action of the pectoralis major muscle. However, a vector representing the pectoralis major muscle would require palpation of the bicipital ridge of the humerus and perhaps a different landmark (or landmarks) on the sternum in order to be parallel with the longitudinal fibers. The result of a pectoralis major muscle measurement would be a more horizontal vector than the pectoralis minor muscle vector, as the bicipital ridge is lateral and distal to the coracoid process. In fact, the distance between the coracoid process and the bicipital ridge of the humerus is considerably greater than the distance between the sternocostal junction and the medial end of the fourth rib (assuming this is a more suitable point to represent the pectoralis minor muscle). This greater distance between the coracoid process and the bicipital ridge creates a large source of error when interpreting this measurement to represent the pectoralis major muscle. By failing to consider the proximal landmark of the 2 muscles in question, the argument put forth in Dr Wong's letter is misleading.
The issue of considering both landmarks and the orientation of the vector between them is important with reference to another point made in Dr Wong's letter. Dr Wong states that the correlation of the sternal notch–to–coracoid process distance with the pectoralis minor muscle measurement is understandable because the sternal notch–to–coracoid process distance measurement is "approximately parallel to the clavicular portion of the pectoralis major muscle fibers." Had one landmark been the bicipital ridge and the other landmark been near the sternoclavicular joint to capture the clavicular portion of pectoralis major muscle, perhaps this argument would have some merit. However, by using the appropriate pectoralis minor muscle landmarks, there is no question that the resulting vector is much more vertically oriented than one intended to represent the pectoralis major muscle, particularly the clavicular portion. I stand by my interpretation of this finding that it is the difficulty in capturing 3-dimensional segment orientations with linear measures that results in correlations among static posture variables that may not be intuitive.
It appears from Dr Wong's comments that he questions the preciseness of the pectoralis minor muscle measure. As discussed in my response earlier, the sternocostal junction landmark may be considered a compromise for the actual muscle insertion, and readers will have to determine for themselves whether my response adequately answers the issues raised in Dr Wong's letter. However, the latter part of the letter reveals that perhaps it is the measurement technique itself that is in question. Dr Wong's letter refers to research, including his own, that measures pectoralis minor muscle tightness using a supine measurement technique.3 The fundamental problem is that the supine measure has not been demonstrated to be valid for measuring pectoralis minor muscle length. The findings of my analysis indicate that the supine measure is not appropriate for measuring pectoralis minor muscle length because it is poorly correlated with a valid measure of the pectoralis minor muscle. There are several logical reasons for this: the treatment table affects the pectoralis minor muscle by influencing scapular position, it is not a normalized measurement, and the effect of gravity on the system is altered.
It is perfectly reasonable to conduct research using the best possible methods at your disposal. If the most valid measurement requires instrumentation that is unavailable, then exploring a research question using another technique may be acceptable. The supine measure of pectoralis minor muscle tightness in a within-subject design, such as that used by Dr Wong, appears to be such an example. However, everyone should be cautious about discrediting another researcher's work when trying to justify their own. My kinematic and static posture analyses, as described in the current report and previously,4 remain the only ones to use a validated measure for the length of the pectoralis minor muscle. The concurrent validity5 of the measure was established by using the actual muscle insertions in the gold standard measure, following the anatomical orientation of the muscle fibers, and choosing appropriate anatomical landmarks that represent the true muscle of interest. This validation of the pectoralis minor muscle measurement prior to in vivo analysis reflects a rigorous and thorough scientific process, allowing readers to make their own critical deductions regarding published studies.
Physical Therapy Division
Ohio State University
516 Atwell Hall
453 W Tenth Ave
Columbus, OH 43210-1234 (USA)
borstad.1{at}osu.edu
References
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