There’s a brief video which addresses this question nicely. It is entitled “A Talk on Why Right-Handed Hits are Not Good” (なぜ剣道では右手打ちがダメなのかというお話し) [Sasakawa RightHandNG] by Gekidan Sasakawa-sama.* Sasakawa-sama presents two main reasons for using more of the left hand instead of the right – in Japanese and with body movements. I provide a brief summary of his comments and their approximate time-marks. As mentioned earlier, the chudan kamae is assumed with the left hand at the base of the shinai (tsukagashira) .
First, the left hand at the shinai base as the pivot point (fulcrum) maximizes the strength of a strike (at the 0:48 min mark) vis-a-vis the right hand positioned further away from the base. (For a given angular speed (degrees/sec) of a shinai tracing the arc of a circle, the shinai tip speed (meters/sec) increases with a larger radius – according to physics [Neal]). If, on the other hand, the right hand served as the pivot point, the tip speed would diminish (given the same angular speed, since the radius of the arc traced by the tip would decrease) (at 1:10, 2:03 and 2:45).
Second, having the left hand as the power hand increases the reach and also the power of the strike (datotsu with fumikomi). The base of the shinai is propelled forward with the left hand, arm and shoulder along with the left side of the body and the hips facing forward (1:36). If, instead, the right hand is the power hand, the body may tend to rotate anti-clockwise where the right shoulder and right half of the body go forward and the left shoulder and left half remain behind (1:44 and 2:29). This would reduce the reach of the shinai (and would likely reduce the amount of hip power inserted into the strike.) This may also manifest itself as a stuttering of the foot work in fumikomi (3:05). (I suspect that this could also be associated with a slower kicking of the left foot towards the right foot in fumikomi.)
Tangent: As to the reason for having the left instead of right hand at the shinai base, I can only speculate with some side notes:
- The reply to “Why is the left hand more dominant in kendo?” in [Kendo-Guide] provides some insights and potential historical reasons.
- It could follow from the positioning of the scabbard on the left side of the hip for which drawing with the right hand closer to the tsuba would follow.
- But then why have the scabbard on the left side? Potentially greater control and speed in drawing the sword with the right hand.
- Other possibilities:
- Perhaps, it’s the same reason right-handed golfers are taught to place the left hand at the end of the club. For example, “Proponents of the left-sided swing (for right-handed people) claim that allowing the left side to dominate better keeps the club on plane and promotes greater accuracy [Smith Golf].”
- The positioning of the feet with the left in the rear and right in front which could anatomically favor the left arm and hand in the rear and the right arm and hand in front (or vice-versa).
- Perhaps it relates to how the left arm swings forward in the natural human gait as the right leg swings forward in the air while pushing off the left leg as shown in this “Animation Reference – Athletic Male Standard Walk”. Or,
- Perhaps, the reaction time may be faster. Researchers “think that the left hand should be faster at reaction times involving spatial relationships (such as pointing at a target) [CTI ReactionTimes, p.4].
* It was difficult for me to hear the presenter’s name at the beginning of the video and it doesn’t seem to be listed in the written description. In case I have mis-spelled it, please let me know.
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