Translated by Ralph Anzarouth and an anonymous friend
The philosophers wrote about strength in three contexts: the first is the physical force and power which has been naturally endowed and which in itself is not a virtue; the second is courage and total fearlessness; the third is the endurance and stamina to bear with a stout heart all the events and troubles which befall him. The latter two are indeed a proof of strength.
However, the Sages of blessed memory clarified in the Ethics of the Fathers (Avot 4, 1): "Who is the strong man? One who dominates his instinct, as it is said 'a patient man is preferable to a strong man’."
Rabbi Shlomo Ibn Gvirol wrote that it is proper to apply this trait in the service of G-d in the same way as did Moses and Pinchas etc., and the judicious man should behave accordingly, in a balanced manner, in order to avoid being labeled a madman, and thus conduct himself in the most reasonable and upright manner.
The trait of strength can be seen in the lion which always sleeps with its eyes open and if the hunter approaches it, as soon as it senses his approach, it will hide its track with its tail and its hair and will keep itself hidden from the hunters. And when it sees that it has no alternative, like a brave warrior it will turn and fight to the end without fear or dread.
We also find that the Holy One blessed be He is designated by the term 'Mighty', as it is written (Psalms 24, 8): "Hashem is brave and mighty, Hashem is a valiant warrior". The sun is termed 'warrior', as it is written (ibid. 19, 6): "It will rejoice to follow its path like a warrior" and it is said (ibid. 45, 4): "Gird your sword on your thigh, brave man, as it represents your glory and splendor". About the verse (Proverbs 8, 4): "I will call you 'men', Our Sages of blessed memory said (Talmud Bavli, tractate Yoma 71a): "These are the erudite scholars who seem [modest and weak] like women and act with strength like men".
Tullius1 said that one should be strong in battle and brave in adversity. Socrates said that fleeing when necessary is wiser and braver than dying. He further said that patience the gateway to mercy. Ptolemy said that one who wants to fight adversities should arm himself with patience.
In the [Holy] Land there was a hero who was greater and stronger than anyone else: Samson the son of Manoach, as it appears in the Book of Judges (chapters 13, 14, 15, 16). Our Sages of blessed memory said (Talmud Bavli, tractate Sotah 9b): "Rav Asi said: 'Tzara and Eshtaol were two high mountains and Samson uprooted them and ground them one against the other". It is also taught there (Talmud Bavli, tractate Sotah 10a) that Rabbi Shimon said: "The breadth of Samson's shoulders was sixty cubits, as it says (Judges 16, 3) 'Samson lay down […] he took hold of the gates of the city and placed them on his shoulders." And it is taught that the gates of Gaza were at least sixty cubits wide. And it is said about him that he was created in a heavenly form2.
Notes of the translators:
 i.e. Tully.
 Our Sages of blessed memory explain that this description, given to Samson and to four other biblical figures, means that they received exceptional gifts at birth, see Talmud Bavli, Sotah 10a.
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 26 about strength is found at pages 54-55.