It is well recognised that any aid you give your horse should be clear and brief. Sustaining an aid, whether it is with your hand, leg or seat is likely to create a resisting or blocking response from your horse, not to mention a degree of tension.
Can you imagine blocking your horse from running forward by tightening your hands, arms, seat and tummy muscles (all part of a half-halt) for an extended period without getting tense? Just try holding your breathe for a minute off your horse and see if you remain relaxed. I see riders do this all the time, not breathing and then blaming the horse for getting tense. The easiest solution is to keep breathing but better still, align your half-halt with your breathing. Let me explain.
One of the best ways for a rider to release any tension is to release the aid they are applying, then if necessary, repeat it. Don’t just keep applying an aid and keep getting stronger if your horse is not responding. Release it for a moment so as not to build tension (in yourself or your horse) then repeat it more firmly.
I find the best way to do this is to time the aid with your breathing; not just small puffs of air that only reach the top of the lungs but deeper breaths that get right down to your diaphragm. Deeper breathing oxygenates the blood better supplying the muscles with more fuel that can help prevent them from fatiguing (ie. becoming tense).
What most riders don’t realise about the breathing process is that the contraction of the diaphragm pulls the ribs upwards and outwards lifting the chest and abdominal contents with it. This has the effect of lightening the rider’s depth of seat. Conversely, when you breathe out, your seat deepens. So if you apply a half-halt (or any aid for that matter) as you breathe out, the aid will be more effective. If your horse doesn’t respond sufficiently, breathing in again not only lightens your seat but it also releases tension you and your horse are most likely building before you breath out again and reapply your aid.
So when you ride next, focus on breathing correctly as an exercise. Notice how much more relaxed you and you horse are. Then start timing you aids, particularly your half-halts with your breaths out. I believe you will not only find your aids more effective but that you and your horse will remain more relaxed. If things do go pear shaped for a moment, try correcting your breathing before you correct you horse.