It could be a couple things. Besides being a postural strength problem, it could be an arm swing problem or an anatomical problem.
It could potentially be a strength issue because the arms are dependent on a the trunk (and vice versa) to transmit their forces to counter those of the legs. Because the trunk isn’t a solid foundation this could potentially cause problems, especially if the trunk is particularly ‘unsolid’ (i.e- low muscle strength and tone).
As far as being a technical arm swing problem, a quick identifier that the arms aren’t doing there job is to see if the magnitude of swing and speed of movement is matching (or almost matching) the movement of the legs.
In some cases, it may appear that armswing is crossing the midline when really the armswing is moving straight (relative to the torso) but the torso is rotating.
An example of an anatomical problem would be if the athlete has very large (mass) legs or wide hips combined with narrow shoulders or skinny arms. Independently these are not so much of a problem but when one is combined with the other, the arms may not be able to control the rotational moment of the legs. The reason for this is that the further a segment is from the midline, the more potential it has to generate angular momentum. The shoulders are generally broader than the hips, which places the arms further from the midline than the legs and allows them to compete favorably with the larger legs in producing vertical angular momentum.
Hopefully something I said made sense and I didn’t just create more questions.
