As Alex starts off with defining strength. Strength is a term that gets used a lot in an imprecise way.
To a exercise professional, strength is maximal force production. In addition to the sources that Alex provides we see the same from the NSCA and ACSM research. It's the same in my Doctorate in Physical Therapy program.
Force = mass*acceleration. So increasing force production means increasing acceleration of the body and weapon.
On the other hand, endurance is defined as the amount of time that an amount of force can be produced. In this case increased endurance means more repetitions, not more force in each repetition. There is a clear inverse relationship between power and duration of force production. Therefore strength and endurance are at opposite ends of a spectrum of force production.
In exercise science intensity is defined as a percentage of maximal force production. This is different from the colloquial and dictionary usage of intensity. So a Crossfit workout that leaves you puking may have been intense, but not in the technical use of the word, since it was high rep training. A 100m sprint may not feel as draining as a marathon, but the sprint was more intense.
So, a single exercise intensity cannot increase both endurance and strength.
Does that mean that any claim that an exercise program increases both endurance and strength bogus? Well, only mostly. If you take a sedentary person and increase their physical activity - by any means - then that person will gain both strength and endurance. But only for the first 8-12 weeks. After that, one of the characteristics is going to plateau based on the program's intensity.
And this is where we get the personal testimonials of a program that does everything. And the research backs up this fact that any exercise will improve most things in a sedentary person. So, a personal trainer, or a person selling a book, can even claim to be supported by the science. But only by cherry-picking the research instead of looking at the entire body of research.
How Intense is Strength?
I defined above that strength gains are made at a given intensity. That intensity is 67% and higher (1, 2).
And this is where a clarification of something Alex said is really necessary. The repetition range he quotes is 20RM and lower. While the sources above give 12RM and lower.
Note though that the IOC source that Alex provides also specifies the duration of those sets - 30 seconds or less. I can do 20 reps in 30 seconds only by moving the weight faster, which requires more force. Or I can do reps that take 3-4 seconds each and do only 8-10 reps. That 3-4 seconds is 1-2 seconds down, pause at the bottom, 1 second up and then pause at the top.
Twenty slow controlled reps is not strength training, it's muscular endurance. But 20 medicine ball throws in 30 seconds is strength and power.
Most strength training is done at intensities of 12RM and lower. Significant strength gains - and significant power gains - occur at intensities higher than 8RM. As such strength training should focus on those numbers.
Strength Isn't the Only Thing
Of course it isn't. And I'm certainly not saying that you should win a fight simply by being stronger than the other guy.
But more relevantly, strength is not the only characteristic relevant to training. Increasing endurance allows a person put in more training time. And that's good for us. Therefore a balance should be found between strength and endurance training. We can do that three ways:
- Remember that strength increases will also increase muscular and cardiovascular endurance. Not as effectively as dedicated endurance and cardio work, but sometimes it's all that's needed. Note though that this is not a two way street. Endurance and cardio do very little for strength.
- Include lower intensity exercises. Perform the accessory exercises of the program at a lower intensity to help cover muscular endurance needs. These are the single leg or arm exercises or the trunk/core specific exercises.
- Periodization. Start the program at lower intensities and gradually increase the intensity. Don't just increase the weight as you get stronger. Increase the weight enough to decrease the number of reps possible. Aim to have the highest strength and power portions in time for a particular important event, date or competition. However, periodization is it's own topic for a much longer post.
Train smart and know what strength is. And how it is different from endurance and cardiovascular conditioning.