In erb's palsy, the superior and inferior subscapular nerve are affect

In erb's palsy, the superior and inferior subscapular nerve are affected or not

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Short answer: No — the upper and lower subscapular nerves are not clinically affected in Erb's (Erb-Duchenne) palsy. They are typically spared, and this sparing is actually what produces the classic clinical posture.

Why:

Erb's palsy results from traction injury to the C5-C6 roots / upper trunk of the brachial plexus (classically from a difficult vaginal delivery with shoulder dystocia, or trauma with wide separation of the head and shoulder) - Schwartz's Principles of Surgery, p. 1804; Miller's Review of Orthopaedics.

The nerves that arise from the upper trunk (and are therefore affected) include:

Suprascapular nerve (C5, C6) - supraspinatus and infraspinatus
Nerve to subclavius (C5, C6)
Axillary nerve (via posterior cord, heavy C5-C6 contribution) - deltoid, teres minor
Musculocutaneous nerve (via lateral cord, C5-C6 fibers) - biceps, brachialis, coracobrachialis

The upper and lower subscapular nerves, which innervate subscapularis (and teres major, for the lower one), are generally not clinically involved. This is clinically important: because the external rotators/abductors of the shoulder (supraspinatus, infraspinatus, deltoid via suprascapular/axillary nerves) are paralyzed while the internal rotator subscapularis (and pectoralis major) remain functional and unopposed, the arm falls into the characteristic "waiter's tip" (policeman's tip) posture - shoulder adducted and internally rotated, elbow extended, forearm pronated, wrist flexed (Schwartz's Principles of Surgery, p. 1804; Miller's Review of Orthopaedics, "Nerves of the upper extremity").

So the internal rotation seen in Erb's palsy is not because the subscapular nerves are damaged - it's the opposite: they're intact and unopposed, driving the arm into internal rotation once the opposing external rotators/abductors are knocked out.

Summary table:

Nerve	Origin	Affected in Erb's palsy?
Suprascapular	Upper trunk (C5-C6)	Yes
Nerve to subclavius	Upper trunk (C5-C6)	Yes
Axillary	Posterior cord (C5-C6 dominant)	Yes
Musculocutaneous	Lateral cord (C5-C6 dominant)	Yes
Upper/lower subscapular	Posterior cord	Generally spared - clinically not affected

This is distinct from Klumpke's palsy (lower plexus, C8-T1), which affects the ulnar and median nerve territory (intrinsic hand muscles) and spares the shoulder entirely.

But lower and upper subscapularis have c5 and c6 as roots. So they should be affected, right ?

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Good catch, and you're right to push on this. The full picture is more nuanced than "subscapular nerves are simply spared." Here's the anatomical reasoning:

Yes, upper and lower subscapular nerves do carry C5-C6 fibers - so in a severe or complete Erb's-type lesion, subscapularis and teres major can show some weakness. My earlier framing overstated the "sparing" as absolute. The more accurate explanation has two parts:

1. Where they sit anatomically matters

The suprascapular nerve and nerve to subclavius branch directly off the upper trunk itself, right at the point of injury (Erb's point, where C5 and C6 unite). They take the full brunt of a traction injury there.
The upper and lower subscapular nerves, by contrast, arise from the posterior cord - a structure formed by the posterior divisions of all three trunks (upper C5-C6, middle C7, lower C8-T1) merging together (Gray's Anatomy for Students, "Branches of the posterior cord"; Imaging Anatomy Text and Atlas Vol 3, "Subscapular Nerve"). Even though the upper subscapular nerve's fascicles are "predominantly C5 and C6" with occasional C7 contribution, they are physically downstream of a confluence point that a proximal upper-trunk traction injury doesn't always damage as severely or as consistently as the branches that come directly off the trunk itself.

2. Functional compensation masks any weakness that does occur

Subscapularis (internal rotation) has a synergist in pectoralis major, which is supplied by lateral and medial pectoral nerves with substantial C7-T1 contribution - largely unaffected in an upper-trunk lesion. So even if subscapularis is somewhat weak, pec major alone can still drive strong internal rotation.
Teres major (also internal rotation/adduction) is backed up by latissimus dorsi (thoracodorsal nerve, C6-C8), so again there's redundancy.
This is why the "waiter's tip" posture (adduction, internal rotation) still dominates clinically - the net effect is unopposed internal rotation, whether subscapularis is 100% intact or just partially weak, because the antagonists (supraspinatus/infraspinatus via suprascapular nerve, deltoid via axillary nerve) are knocked out much more completely and consistently.

3. Textbook framing distinguishes lesion level

Localization in Clinical Neurology notes explicitly: "Subscapular nerve palsies usually occur with posterior cord brachial plexus lesions" - a different lesion location than the root/upper-trunk lesion that defines Erb's palsy. So isolated subscapular nerve involvement is more characteristic of a posterior cord injury (e.g., from crutch palsy, humeral neck fracture, or shoulder dislocation) than of a classic C5-C6 root/upper trunk traction injury.

Bottom line: Root value alone doesn't fully predict clinical involvement - lesion location along the plexus and the anatomical point at which a nerve branches off matter just as much. In practice, subscapular nerve involvement in Erb's palsy is inconsistent and usually mild-to-absent, which is exactly why the internally-rotated "waiter's tip" posture is preserved as the hallmark sign. In more severe/complete C5-C6 avulsions, some subscapularis/teres major weakness may be demonstrable on exam, but it's rarely emphasized because it doesn't change the net clinical picture.

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