Gentleness requires Strength. This seemingly paradoxical relationship is illustrated in verse 42:19:

There are many Islamic insights into this but before I look at the Quran and hadith I would like to look at a Christian source.

There is a very inspirational Catholic Saint called Francis de Sales who was born to an Aristocratic French family in the 16th century. "He became noted for his deep faith and his gentle approach to the religious divisions in his land resulting from the Protestant Reformation." Such was his conviction in the Catholic Church that he roamed through the countryside for years and eventually was able to bring back 60,000 Protestants to Catholicism after an initial lengthy period of bitter rejection and harsh conditions. 

His statements are wisdom for all, such as: 

Any way to go back to the Quran and the root word lām yā nūn (ل ي ن) it is used in Surah Saba  (34th Chapter of the Qur'an) appearing in another verbal form as (أَلَ) ala. This is in a verse about Prophet David/ Dawood where the Quran says وَأَلَنَّا لَهُ الْحَدِيدَ "We softened iron for him". A claim that David was the first to become proficient in manufacturing chain mail. It is said part of iron's strength is its pliability, that even under great pressure it will bend rather than break. 

Reflecting a Taoist principle of Lao-Tzu:

 “If a branch is too rigid, it will break. Resist, and you will perish. Know how to yield, and you will survive.” 

This is illustrated in the hadith, 'the strong one is not who overcomes the people by his strength, but the strong one is who controls himself while in anger.'

‏ لَيْسَ الشَّدِيدُ بِالصُّرَعَةِ، إِنَّمَا الشَّدِيدُ الَّذِي يَمْلِكُ نَفْسَهُ عِنْدَ الْغَضَبِ

These principles need to be incorporated into our salaat (prayer, supplication, turning to God, remembering God, celebrating praise of God, practicing gratitude and praise, glorifying God), if we fail to grasp them will either be sluggish and fidgety or stiff and rigid in salaat. We should be able to remain still but not rigid. 

Beautifully illustrated by a saying Imam as Sadiq about his father, Ali ibn al Husayn, that when he "stood for Salaat he was like the trunk of a tree, nothing of which would move unless the wind would move it.”

So we need to have a strong but gentle stance, but more importantly, we need this gentle strength for Bowing and prostration, ruku (bowing) and sajud (prostration). A considerable amount of strength is required to ensure we are entirely controlled in these, especially coming down to sajud/prostration. We should at no point be in a state of free fall, retaining enough strength that if at any point we were required to, we could simply reverse the manoeuvre without any difficulty. 

So we are not actually ever letting gravity pull us down, This requires considerable core strength. Likewise coming out of ruku/bending or sajud/ prostration should always be gentle. It should be so gentle that potentially if we wanted to return back to Ruku or Sajood it could be done with great ease. Say for instance we thought the imam had come out of ruku or sajood but hadn't we could return back with complete calmness.

With this strength, it becomes easy to retain a quality that for the Maliki fiqh at least is obligatory throughout the salaat and that is اطمأن itminan, tranquillity.

In an important hadith Sunan Abu Dawud 857 which is one of the few hadith that the prophet explicitly speak about how the prayer should be performed stress the one should bend until your joints are relaxed يَرْكَعُ حَتَّى تَطْمَئِنَّ مَفَاصِلُهُ. This is I think the reason why it is not considered correct to lock the knees in the Maliki Fiqh. This instruction is repeated twice for each of the prostrations specifically: يَسْجُدُ حَتَّى تَطْمَئِنَّ مَفَاصِلُهُ.

We should become aware of this physical strength that is required in our worship and how this strength is essential to retaining a quality of gentleness and tranquillity throughout. The practice of salaat in its self will build up that necessary core strength but becoming aware of the muscles that are needed for this will help to more effectively build up that required strength thereby radically improving the quality of our salaat.