Proper strings for MIYATA bows are Asa- zuru (hemp strings) or synthetic strings which are not too strong and durable ( Senbon-tsuru is good for MIYATA bow).
Generally, brace height needs at least 15cm or more at the top of Nigiri, to stable bow and get strings strained enough to have better performance and avoid damaging a bow.
During the drawing process, potential energy is increasing. The maximized potential energy in Kai turns into Kinetic energy to fly an arrow by Hanare.
Following conservation law of energy in physics, the total energy is conserved in this process. However, problem is that all the energy is not turned into kinetic emery to fly an arrow, but some t of energy lost in impact on a bow, depending on how to shoot. This impact damages the bow, which might cause the bow to be broken. If the string is too tough to be durable with this shock such as one for western archery, the impact is much stronger than hemp string and soft type synthetic strings. Hemp string and soft type synthetic strings will be cut with the impact, instead of damaging the bow.
Western archery solved this problem of impact to strengthen bow equipment. For instance, Western archery bow has adopted wider limb, which is dynamically effective than traditional British long bow. Also, western archery equipment is more durable than Japanese Kyudo equipment: metal handle and hard synthetic strings. Stabilizer and damper relive the shock by shooting.
However, traditional Kyudo bow equipment is much more fragile than Western Archery equipment. Long and thin bows made by bamboo and wood, arrow made by bamboo, and thin hemp strings.
Kyudo sloved this problem to evolve Asian Khatra technique to the Japanese original technique, to minimize energy loss in shooting. Specifically, the string is locked by the Tsuru-Makura (groove) in Yugake of Kata-Boushi (Kyudo glove) when Kyudo archer in drawing (Hiki-Wake). Kyudo archers gradually twist the bow in drawing process, and finally in Kai the string is released from the groove of Yugake by the force of twisted bow. In this way, most of the elastic potential energy turns into kinetic energy to fly arrow straight. In other words, this shooting way realizes minimizing energy loss to damage a bow.
Twisting the bow by Yunde (left hand) is very difficult, and Teno-Uchi (how to grip the bow) is most important, and Kyudo archers need to practice for a long time in a right way. Therefore, using hemp strings or soft type synthetic strings, and keeping 15cm or more in brace height are important, to have better performance and avoid damaging a bow with a wrong shot.