If one took off his tzitzis while going for a run, playing football or doing exercise, will he need to make a new bracha when putting it on again?
The question revolves around whether or not you were מסיח דעת - a mental interruption or not. This question is a dispute among the poskim as to what is considered a hesech hadaas.
The Shulchan Aruch (סימן ח' סי"ד) says ואם פשט טליתו אפילו היה דעתו לחזור ולהתעטף בו מיד צריך לברך כשיחזור ולהתעטף בו.
If one took his Tallis off, even if he had in mind to put it back on right away, he must make another blessing on the Tallis.
The Rema writes: ויש אומרים שאין מברכין אם היה דעתו לחזור ולהתעטף בו. ויש אומרים דוקא כשנשאר עליו טלית קטן והכי נוהגים.
Some are of the opinion that if one had in mind to put it back on, then another blessing is not required. And some hold that this is only when one already wearing a Tallis katan, and this is the custom.
The Vilna Gaon (ס"ק כז-כח) rules like the Shulchan Aruch that a new blessing is required even when there was intention to put it back on right away.
The book Laws of Daily Living by Rabbi Simcha Bunim Cohen (page 239) says:
A man who removes his Tallis Katan with the intention of putting it back on does not recite a new blessing when he dons it again. Accordingly, a man who undresses for a shower does not recite a new blessing when he dons the same Tallis katan after the shower. Likewise, if a man goes swimming for less then three hours a new blessing is not required if he puts on the same Tallis katan after swimming. However, if he puts on a different Tallis katan after showering or swimming he must recite a new blessing.
If memory serves, the Shulchan Aruch/Rama sound like a new bracha would be required, but contemporay poskim actually set a time limit when one intended to put it back on to begin with - I believe Rav Ovadiah Yosef, z"l, may have said 2hrs; ; Rav MosheFeinstein, z"l: 1 hour; and, yb"l, Rav Chaim Kanievsky, shlit"a: 30 minutes - after which such time has elapsed a new bracha would be necessary.