Learn o-soto-gari, the major outer reap in judo: kuzushi, the reaping leg and milestones. Honest and safe, with breakfalls and a supervised partner.
The honest answer: o-soto-gari is one of the most important throws in the kyu programme, but also one where breakfalls and throw control make the biggest difference. You lead Uke backward, place your standing leg next to his standing leg and sweep his leg away from behind. The German Judo Federation names o-soto-gari as an example for the first throws already at 7th kyu and makes it a mandatory technique at 4th kyu [1].
At the same time the DJB itself points out: with younger children the throwing dynamics of o-soto-gari must be reduced, for example by setting the sweeping leg down [1]. That is a clear signal that this throw is learned with respect. Expect several weeks until the reap fits together with upper body and standing leg. That is a practical rule of thumb, not a study.
One hand on the sleeve, the other on the lapel. You stand upright, knees slightly bent, eyes forward. Milestone: you can lead Uke backward evenly without tipping backward yourself.
With a pull on the sleeve and a push on the lapel you bring Uke back and sideways until his weight rests on one leg and the heel carries. Without this balance break the reap is brute work. A 3D analysis showed that in hip and leg throws such as o-soto-gari the largest impulse already occurs in the kuzushi and fit-in phases [4]. Milestone: Uke stands on his heel and has to give way.
You place your standing leg close next to Uke's standing leg, knee slightly bent, upper body tilted forward, head up. Milestone: you reach the position in one step without Uke moving away.
The reaping leg does not swing as a rigid rod but as a chain: trunk, hip, knee, foot. In a kinematic study black belts differed from novices mainly in a higher trunk angular velocity and a stronger foot extension shortly before contact [3]. Milestone: in uchi-komi you notice that your leg accelerates from the hip and not just from the knee.
With your coach's approval you throw for the first time, at first with reduced swing. The pulling hand stays on the sleeve and accompanies Uke to the landing. In competition an ippon requires speed, force, landing on the back and controlled landing [2]. Milestone: Uke lands on his back, chin to chest, hands slapping, you stay standing.
With o-soto-gari, research looks especially closely at the head and knee of the person thrown. In a dummy trial with one experienced thrower, the rotational head acceleration in o-soto-gari was around 4,300 to 5,500 rad/s² when the back of the head hit the mat [7]. With correct breakfall technique, the acceleration in two judo experts was clearly lower when the head did not hit the mat [8]. Novices show a greater neck extension momentum in the backward fall after o-soto-gari than experienced judoka [6]. The knee of the person thrown is also affected: among 43 cruciate ligament cases in judo, an attack with o-soto-gari was the most frequent single cause (16.8%) [9], and in a Japanese database of 2,142 adolescents, a low-impact pattern with o-soto-gari appeared in 42.6% of cases [10]. So: controlled reap, never to the knee, only with a partner who can do ukemi securely, on tatami and under supervision. This guide explains and motivates, it does not replace supervised training.
Practise the throw "silently" first: place your standing leg, bring Uke into position and stop the reap. Then ask your coach: "Would Uke fall now?" If the position is right, the throw falls by itself in the real attempt. The goal is not the swing, but the place where the swing acts.
And do not do your uchi-komi only standing still. In a study with children, static uchi-komi, uchi-komi with a walking tori and drills with a preceding kuzushi were compared; the groups did not differ significantly, but the kuzushi lead-in helped with the pulling and pushing actions [5]. An argument to practise in motion as well.
Yes, as a learning throw. The German Judo Federation names it already for 7th kyu [1]. Because Uke falls backwards, you need secured ushiro-ukemi and an attentive coach.
Because the throwing dynamics are large. The DJB names setting the sweeping leg down as an example [1]. In a study with children around age 9, none of four practice types brought a clear advantage, but uchi-komi with kuzushi preparation helped with the pulling and pushing actions [5].
Yes. The rules explicitly score o-soto-gari even when your foot or leg becomes entwined with your partner's during the throw. Kawazu-gake, on the other hand, is forbidden [2].
That is individual. The basic movement sits after a few weeks, in randori you need clearly longer. That is a practical rule of thumb, not a study.
Ko-soto-gari: The Minor Outer Reap That Sets the Beat in Randori