When we think of Spiritual Disciplines in relationship to the practice of martial arts, many people automatically think of Shaolin Monks or Zen temples. Many others have very little interest in anything remotely "spiritual" or religious. Some will say " I don't want any of that mystical CRAP!".
It is not hard to think of austere practices and disciplines in those settings. Extended periods of Zazen for hours on end during sesshin. Purification rituals under ice cold waterfalls. The restricted diet. Not to mention being whacked with the Kyosaku if you happen to dose off during meditation. Then there are the mind bending Koan.
One book I often recommend is "Mystics and Zen Masters". It does a beautiful job of finding contrasts and commonalities between Eastern and Western Monastic practices.
Richard Foster in his 1978 book "Celebration of Discipline" examines what are considered the historic disciplines of the Christian church. He divides them into three categories.
First, the inward disciplines of prayer, fasting, meditation, and study in the Christian life.
Second, the outward disciplines of simplicity, solitude, submission, and service.
Third, the corporate disciplines of confession, worship, guidance, and celebration.
In The Road Less Traveled, M. Scott Peck talked of the importance of discipline. He described four aspects of discipline:
Delaying gratification: Sacrificing present comfort for future gains.
Acceptance of responsibility: Accepting responsibility for one's own decisions.
Dedication to truth: Honesty, both in word and deed.
Balancing: Handling conflicting requirements. Peck talks of an important skill to prioritize between different requirements.
Many practitioners and teachers of martial arts today talk about spirit and discipline. Others lead by example and never speak about the matter. Taking the time to look into these methods and practices gives us and idea of how participating in a systematic method of disciplined practices can help us grow even when it is not addressed directly. We can also see how those same disciplines can benefit us purely approached with a secular and psychological mindset. Like Merton I enjoy digging into the contrasts and commonalities. I can enjoy both.