Severe back pain after Kicks

[Jimbo]

One year ago, I intensified karate training for a tournament and after performing many roundhouse and back roundhouse kicks to thigh and stomach height, I had severe musclular pain on the left side of my lower back, just above the pelvis. It seemed to re-occur in the left lower back after a roundhouse kick with the right leg. This injury had me bent over like an old man for several days.

I stopped Karate, did not partake in the tournament, and rested my back for 6 weeks. Since then I have weight trained my legs, lower back ( back extensions to 3 x 15 reps holding 10kg, I'm 107 kg male), and good mornings (3x 15 reps of 25kg) worked on crunches sit-ups and various press-ups.

I have just attempted to intensive my karate training again, and my lower back feels very tender on both sides just above the pelvis. My osteopath said that leg raises ( I was doing the intensively am/pm) to the front would put strain on my lower back because my hamstrings were short.

I understand that the lower back needs to be strong to perform martial arts kicks.

My questions are..

-Does the strength needed in the lower back for kicks come from the muscles that feel worked after back extensions on a bench with weights? If so should I maintain or increase the endurance or general strength?

-Should I attempt to isometrically stretch the hamstrings to (quickly) prevent what feels like a pull on the lower back, before my back goes again, and I miss another tournament?

Hope someone can help, got about 6 weeks before the tournament.

Jimbo