Photo credit: CrossFit Huntsville
I stumbled across a question on Quora that I found pretty interesting:
I felt that while many of the answers were examples of things that build strength, (“Suffering”, “Silence”, “My mother”,) I wanted to contribute a more comprehensive way to think about building strength. Here’s my answer:
I would define strength as the ability to exert or resist force.
This ability has three components – maximum load, endurance and recovery. The more you can handle (or dish out), the longer you can handle it and the faster you recover is what makes you strong in any dimension.
Since the original definition of strong comes from physical strength, I think it makes a lot of sense to start by looking at how physical strength works and then drawing parallels to other versions of it, including emotional, mental or professional strength.
Defining Physical Strength
What contributes to a person’s strength?
Of course your muscle are a primary factor – specifically the cross-sectional area of the muscles recruited.  The more fibers you activate, the more force you can produce. It makes sense that the bigger your muscles are, the more cross sectional area you have to recruit from.
But there’s also a neurological component – understanding how to best activate your muscles to fire both a higher percentage of your total fibers, and also the intensity of your recruitment affects the total force you can exert.
This is most surprisingly shown via mental training – where people who imagine doing ankle exercises for several weeks can produce more force in a before and after trial that was significantly higher than people who did no training, and close to people who did actual physical training. 
Finally you have to consider the wider environment in which the act of strength takes place. If you are well rested, hydrated and have done a solid active warmup, you are going to be a lot stronger than if you just woke up from a late night of drinking and partying.
How Strength Is Built
So how do you actually strengthen muscles?
You progressively overload muscles with increasingly more challenging exercises in volume, intensity, frequency or time, then allow the body to rest and recover, while making sure to feed it enough protein and other nutrients. 
Biologically, progressive overload causes tiny tears in your muscle fibers, which your body reacts to by healing with new tissue growth along with neurological reinforcement of recruiting those fibers. You don’t get strong by lifting the same weight over and over again. You have to do more, push yourself harder and constantly struggle and strain to continue seeing new strength gains.
Of course, if you try to squat 2x your body weight with no strength training of any kind, you may hurt yourself. That’s going to set you back and reduce your strength. You want to overload without injury. The key is understanding your limits and pushing hard without going too hard.
Finally, you have to do a range of exercises to strengthen different muscle groups and different types of motion. A gymnast has more explosive power but less endurance compared to a marathoner; a shot-putter will have stronger upper bodies while speed skaters will have stronger lower bodies, etc. They are all strong in different ways.
Understanding Other Dimensions of Strength
Now that we understand how strength works in the physical dimension, we can draw parallels to understand how we get stronger in other dimensions:
- Challenge yourself with progressively more difficult exercises.
- Force yourself to solve problems that are outside your comfort zone. Once you’ve mastered algebra, move to trigonometry and then calculus.
- Allow yourself adequate time to rest – people who nap after lessons learn faster than those who stay awake. 
- Remember that like the different types of physical strength (upper body, lower body, explosive, endurance) there are also multiple types of mental strength (verbal, quantitative, strategic, interpersonal, etc)
- It seems weird to force emotional challenges upon yourself, but I do think the way you react to emotional struggles that you encounter would determine whether you grow stronger from them.
- Avoiding, ignoring or reactively dealing with emotional problems would likely result in little to no growth in strength.
- Leaning into difficulties, embracing the struggle and finding ways to handle the situation maturely and with grace and dignity is more likely to result in greater emotional strength.
- I’m personally not very spiritual, but I would imagine the analogy holds.
- Feed yourself with the learnings from scripture, self-reflection and spiritual teachers.
- Embrace the challenges you face along your spiritual journey
- Constantly seek to deepen your spiritual practices of prayer, meditation, right thinking and loving kindness.
The Professional Dimension
Finally, I want to point out a dimension that wasn’t mentioned in the question – the professional dimension.
The more skill, experience and network you have, the stronger you’ll be professionally. You’ll be able to weather shocks like losing your job, and also use your strength to get projects you want, earn promotions, etc.
The progressive overload works here too: I remember listening to a talk by Drew Houston where he explained that when he started Dropbox, he was totally unprepared to run a billon dollar business. But he didn’t have to. First he just had to build a prototype, find a cofounder, get distribution, etc.
As he mastered each challenge he faced, he got better – and was faced with a bigger challenge to overcome. 5 grueling years later, he’s grown into incredibly strong business leader.
 “Peak force production is related to the physiological cross sectional area (PCSA), which estimates the sum of the cross sectional area of all the fibers.” Muscle Physiology – Introduction to Muscle
 “Differences in raw torque production after training in the 2 practice groups resulted in significant percentages of improvement for the physical practice group (25.28%) and the mental practice group (17.13%), but not for the control group (−1.77%).” Can Mental Practice Increase Ankle Dorsiflexor Torque?
 “In order to achieve more strength as opposed to maintaining the current strength capacity, the muscles (see skeletal muscles) need to be overloaded which stimulates the natural, adaptive processes of the body which develops to cope with the new demands placed on it.” Progressive overload
 “Those who remained awake throughout the day became worse at learning. In contrast, those who napped did markedly better and actually improved in their capacity to learn.” http://www.sciencedaily.com/releases/2010/02/100221110338.htm