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Why long duration murmer is seen in dyanamic ascultation

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"Heart Murmurs"[MeSH Terms] AND auscultation

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dynamic auscultation long duration murmur hypertrophic cardiomyopathy mechanism

If you mean “why does a murmur become louder during dynamic auscultation?”:
Dynamic maneuvers change preload, afterload, and contractility, which changes blood flow velocity and pressure gradients across valves or the left ventricular outflow tract. More turbulent flow or a larger gradient produces a louder murmur.
Key examples:
  • Standing or Valsalva strain decreases venous return (preload), making the left ventricle smaller.
    • In hypertrophic obstructive cardiomyopathy (HOCM), a smaller LV worsens dynamic LV outflow obstruction, so the systolic murmur becomes louder.
    • In mitral valve prolapse, reduced LV volume makes prolapse occur earlier, so the click and murmur occur earlier and the murmur may last longer.
  • Squatting or passive leg raise increases venous return and systemic vascular resistance.
    • This usually reduces the HOCM murmur because the LV cavity becomes larger and obstruction decreases.
    • It generally makes fixed-flow murmurs such as aortic stenosis louder.
  • Handgrip increases afterload.
    • It increases murmurs of mitral regurgitation, aortic regurgitation, and ventricular septal defect because more blood is driven backward or across the defect.
    • It decreases HOCM and usually aortic stenosis murmurs.
So, the murmur does not become loud simply because of the maneuver itself. It changes because the maneuver alters cardiac chamber size, flow, and the pressure gradient that causes turbulence. Dynamic-auscultation reference
If by “long duration” you specifically mean an MVP murmur becoming longer, it is because reduced LV volume causes the valve to prolapse earlier in systole, leaving more of systole for mitral regurgitation.
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