Most Japanese kendojo have a suburi bokken hidden somewhere in a dark corner. These come in various shapes and sizes including oversized shinai, implements that look like overweight bokken or the massive hexagonal clubs used in some kenjutsu styles. What they have in common is that you see very few people using them.
The few times I have seen people working with heavy suburi bokken, it has been in unstructured sessions without the supervision of an instructor. Based on this limited evidence, I had the feeling that they were doing more harm than good. By this I mean that they had to make adjustments to their posture and cutting action to support the extra weight.
In trying to control a heavy bokken, the grip tends to tighten, making the angle of the wrists more acute and causing the biceps to take the strain. This in turn brings more shoulder strength into play and as a result the exponent may find himself leaning forward, which is at odds with the correct upright posture that we aim to develop.
Suburi bokken have been used by for many years and by many great kendo masters past and present, obviously this means that in the right hands they are an aid to developing good kendo. Where they cause problems, is when they are used incorrectly. In the hands of a kenshi who has good posture, cutting action and tenouchi, or under the supervision of a good instructor, they should help strengthen good technique.
The same good be said about katate, or singlehanded suburi , particularly if done for a high number of continuous repetitions. Without guidance a natural reaction is to adjust the position of arm and shoulder to take the strain. This will have a negative effect on cutting technique.
Whereas an adult male’s shinai should weigh around 520 gm, suburi bokken can be three times that weight or more. An iaito or shinken is approximately double the weight of a shinai, ranging from 900 gm to 1.2 kg and Iaido practitioners are taught to do a good job of cutting correctly with these. Correct technique is the answer regardless of the weight of the weapon.
In kendo we need to keep an upright posture with our weight distributed evenly between our feet. Our tanden should be braced and our arms hands and shoulders relaxed as we make the swing and we should finish with sharp tenouchi at the point or just beyond the point of impact. If we can do this, then the heaviest weapon and the largest number of reps should help rather than damage our kendo.