Singing Mahler"s "Symphony for a Thousand" is an example of High Romantic period styling. In other words, we singers were allowed to use vibrato and great gushes of emotional nuances. Handel's "Messiah" must be sung with completely different techniques and very different styling. No vibrato, lots of attention to breathing and articulation, and a lot of very fast athletic singing. We have had a few extremely tiring practice sessions where we came home exhausted from getting those melismas back into our muscle memory.
One of the ways we have been practicing the long 16th note passages is to do permutations. That means to practice the groups of 4-note patterns by singing them in different rhythms to carefully memorize the sequences. This helps the singer master not only the musical patterns but also the breath control, vowel placement, and tricky twists and turns of pitches. Those abdominal wall muscles really get a work out!
We worked on practicing the melismas that were normally in 4/4 time in triple meter, varied the lengths of the notes, and varied the order of the slow and quick notes. We never changed the actual shape of the pattern, just the length and speed of the notes in the sequence. It seemed to help. We were able to clean up the unification within the sections and line up the beats better.
Permutations are used primarily with keyboard players who need to train their finger muscles to negotiate the terrifically fast passages. Through isolating the most difficult section, working it with all different permutations, the most difficult section becomes the most secure section. As performers, we know we must practice and practice to make the hardest part the easiest. In the meantime, we must train as if we must run a marathon!