(we have had computer problems at work so I could not post recently)
One purpose of weight training is body building. There is no such thing as a weak body builder, but it is also the case that biggest muscles do not equate exactly to greatest strength. There are ways to stimulate muscle growth. It is also the case that some people get stronger and stronger but do not pack on the size. Everyone has been in the gym and seen the skinny guy who is lifting more weight than the muscular guy.
The goal of body building is to look good. There is nothing wrong with that, most of us would prefer to look good. However, in the spiritual life "looking good" is a problem. On Ash Wednesday we always read the Gospel message from Matthew 6, "Beware of practicing your piety before others in order to be seen by them..." Jesus conclusion, if you do then you have already got your reward. The reward is the praise of others who think highly of you because of your spiritual practices (he identifies prayer, fasting and almsgiving). In a body building contest one parades before the judges who measure your body against criteria (like symmetry, size, shape). If your body looks better than the other one, it does not matter if they are in better shape, stronger or faster. It does not look for objective measures of strength.
In real life this can produce problems. The use of drugs and some practices can abuse the body. On occassion, it may produce serious harm, even death. In the spiritual life the same thing happens, only here the negative and deadly consequences are more frequent. "He isn't as strong as he looks" may be an ego bruiser, but he isn't as holy as he looks is a soul destroyer. So it is key to our growth and development that our soul-building is based on objective criteria and not an "eye test" for others.
However, doing any type of "exercise" can lead one to an outcome focus. If I am seeking a personal record for lifting the most weight I ever lifted, or running the fastest time for a 10K, I can be consumed. Once again, if the goal of my work out is setting records, paradoxically, I may do long term injury in the pursuit of those goals. There is a range of of exercise which provides optimum health, then there is a point where there are diminishing returns. The benefits are outweighed by wear and tear when the purpose is on competition rather than health. Once again, this is a choice a person makes, but the world is littered with 'crippled up' former athletes who were 'specimens' at twenty-five and 'medical cases' at fifty. In the same way, engaging in spiritual practices as a competition (trying to be "more holy" than someone else) can produce its own toxic consequences. If the goal of the spiritual life is union with God (and it is) then whatever takes our focus off God and onto ourselves (like trying to set records) will always end up being a barrier to our relationship, even if we look good or set records in the process.
So a valuable lesson for the soul from the body is the danger of choosing the wrong goal! Hope this helps in your journey. All spiritual disciplines are focused on God and our relationship with the Trinity!