An Investigation of Leg and Trunk Strength and Reaction Times in Hard-Style Martial Arts Practitioners

There’s a lot of useful and interesting information in this paper, but one of the conclusions stand out:

This study has revealed (rather surprisingly) that the trunk strength is not significantly increased in practitioners of hard-style marital arts. Given that this physically demanding sport is associated large and random movements of the trunk, it would be wise for training regimes to increase focus on trunk stability exercises to increase strength of the abdominal and back muscles. This may lead to an increase in performance and possibly lessen the likelihood of injury or incidence of low back pain; this however, would require further research.

For some strategies to strengthen the musculature involved and avoid muscular imbalances, look here and here.