Halakhah su Salmi 146:8

You must bend your knees and bow four times during the Shemoneh Esrei, at the beginning of the first berachah and at its conclusion,³⁵The berachah which ends with the words Magein Avrohom is the conclusion of the first berachah. and at the berachah of Modim, at its beginning and end.³⁶The berachah ending with the words Hatov shimecha ulecha naeh lehodos is the conclusion of Modim. When saying Baruch, bend your knees; and when you say atah bow until the vertebrae of the spinal column protrude, and you must also bow your head. Before pronouncing the Name [Hashem],³⁷The Shulchan Aruch states that one should straighten up as he says Hashem. straighten up slowly (as it is said;) "Hashem straightens the bent."³⁸Psalms 146:8. Similarly, when saying Modim, bend your knees and bow,³⁹Our custom, however, is not to bend the knees when saying Modim. and before pronouncing the Name [Hashem], you should straighten up. Do not bend too low, so that your mouth is opposite your belt, because this is an act of ostentation. For an old person or an invalid to whom bending the knees is painful, just nodding the head is sufficient. It is forbidden to bow additionally at the other berachos, either at their beginning or their end.