Ryukyu Kobudo contains a variety of weapons ranging from the more common such as the bo, sai and tonfa, to the more exotic such as the rochin and tinbe, and suruchin. All of these weapons require years of study with a competent teacher to gain mastery. This requires not only the detailed study of the solo kata, but naturally the two-person fighting sets handed down for each weapon.
Most (but not all) of the Taira lineages of Kobudo teach a two-person set for the Sai defending against the Bo that was created by Taira Shinken. It contains ten sequences encompassing a variety of techniques found within the traditional Sai kata passed down by Taira. To give you an idea of what it looks like, here is an example of the Bo against Sai two-person set as demonstrated by myself and Yoshimura sensei back in 2007.
All ten sequences are challenging but sequences four and eight can be particularly daunting. These two sequences are especially difficult as they require the student to jump and duck in order to perform the sequences properly. In sequence four, the defender must parry the bo and then quickly dodge a second strike to the legs by leaping up and then countering with a strike to the head. While in sequence eight, the defender has to immediately avoid rapid strikes by the bo, first to the leg and then to the head, by jumping up and then immediately crouching down.
For some students these two sequences can be a real challenge either because they do not possess adequate leg strength to perform the jumps or they lack the flexibility in the lower body (waist, thighs, ankles) to squat down, or both. As a teacher I’ve encountered students that could not initially perform these techniques at all. Some for legitimate reasons such as previous injury or limitations due to age, while others lacked the attributes I mentioned before. For students with legitimate physical limitations then an appropriate substitute set of techniques need to be used. However, for students without limitations but who are unable to do the techniques, what are they supposed to do?
Some teachers may feel that specific practice of the sequences themselves is adequate training to learn how to do them correctly, but this assumes that the student has the proper attributes in the first place. I feel this approach is setting the student up for failure as it may result in a number of issues: potential injury for the student, inability to perform the sequence, loss of confidence, etc. A better approach would be to train the attributes necessary to perform the technique.
For improving leg strength I would start with some general lower body exercises such as the squat. This could progress from low residence to high residence such as body weight squat to goblet squat to barbell squat. The nice thing about weight training is that it there is ample research that shows that it also improve flexibility at the same time. Once there has been an improvement in strength the student can move on to some single-leg exercises such as lunges, split squats, or Cossack squats again going from no load to a heavier load. The reason I would include single leg exercises is because in sequences four the right leg bears more of your body weight when you jump. As for the amount of weight you should be lifting, it is better to be on the conservative side with students and have them lift 50% to 60% of their 1-rep maximum for 3 sets of 8 to 10 repetitions.
Plyometric exercises can also be useful but are risky as they have a higher chance of injury when done improperly. Plyometric exercises include: squat jumps, split squat jump, single-leg push-off, and box jumps. Some good illustrations of progressively more difficult plyomentric exercises can be found here. If you do decide to include ploymetric exercises then it is best to start with easier exercises and do a lower number of sets and repetitions, perhaps 2 sets of 6 to 8 repetitions.
After a 4 to 6 weeks of strength training twice a week then the student may be ready to perform sequences 4 and 8 in their entirety. Before then, then can modify the sequence as necessary or continue to modify it after the strength training is completed until they have the mechanics down properly. For example, avoiding the low strike to the leg with the bo by aiming it in front of the leg instead of at the leg. This way the student can practice jumping without fear of being hit if they miss time the jump or do not jump high enough.