In quiet breathing, and in the supine position, where is the apex beat of the hea located?

The apex is the only point of the hea that can be directly identified on the precordium. A cardiac impulse may be visible at the apex, and palpation over it will confirm the presence of the apex beat. The apex is located in the left fifth intercostal space just medial to the midclavicular line. Ref: Ommen S.R., Nishimura R.A., Tajik A. (2011). Chapter 33. Hyperophic Cardiomyopathy. In V. Fuster, R.A. Walsh, R.A. Harrington (Eds), Hurst's The Hea, 13e.