The recent review by James Fisher, James Steele, Stewart Bruce-Low and Dave Smith should be on the “must read” list for everyone with an interest in exercise. In fact, you should download and read it before you read the rest of this post which is the fourth of several commentaries I will be writing on their review.
Over forty years ago in Nautilus Training Principles Bulletin Number 1, Chapter 15, Arthur Jones wrote,
There is no slightest evidence which indicates a difference between strength and endurance; accurately measuring one of these factors clearly indicates the existing level of the other. That is to say; if you know how much endurance a man has, then you should also know how strong he is – or vice versa. But such a relationship between strength and endurance is only meaningful in individual cases; it does not hold true for the purpose of comparing one individual to that of another – thus you cannot fairly compare one man’s endurance to another man’s strength. Secondly, I am using the term “endurance” only in the sense of “muscular endurance”, the ability of a muscle to perform repeatedly under a particular load – I am not momentarily concerned with cardiovascular endurance, which is an entirely different matter.
By training for endurance, increases in strength are produced in direct proportion to increases in endurance – and vice versa.
The majority of studies on muscular endurance performed since confirms this. Muscular strength and endurance are related. If you become stronger in an exercise you will be able to perform more repetitions with a specific amount of weight or perform some task involving the same muscles longer or more times because it requires a smaller percentage of your strength to do so.
For example, you can complete many more repetitions of an exercise with a quarter of your one repetition maximum (1RM) than with half. If you double your strength the amount of weight that used to be half of your 1RM will now be only a quarter and you will be able to complete many more repetitions with it than you could previously.
Relative muscular endurance – the ratio of muscular strength to endurance – appears to be largely genetically dictated and not trainable. If you become stronger you will be able to perform more repetitions with some absolute weight, but all else being equal the number of repetitions you can perform with an amount of weight relative your 1RM will be around the same.
Suppose when you started doing an exercise your 1RM was one hundred pounds. At the time you could perform a certain number of repetitions with seventy pounds, or seventy percent of your 1RM (which is a measure of relative load, and not intensity, as discussed in part 1). If you increase your 1RM for that exercise to two hundred pounds you will be able to perform many more repetitions with seventy pounds because it is now only thirty five percent of your 1RM (absolute muscular endurance), but you will be able to perform around the same number of repetitions with seventy percent of your 1RM as when you started (relative muscular endurance).
I wrote “all else being equal” above because there are other factors which can affect this, neural adaptations and skill being big ones. This is part of the reason muscular strength increases may not always appear to translate into muscular endurance increases when comparing workout performance with other activities. As you practice and improve a particular athletic or vocational skill your movement will become more economical, resulting in less wasted energy and reduced rate of fatigue. This will result in a greater improvement in endurance than would come from muscular strength increases alone.
What does this mean in practical terms? If you want to improve your general muscular endurance you should focus on becoming stronger and use a repetition range that works well for you for that purpose. You could also do this by performing more repetitions with less weight, however it is not necessary to do so. If you want to improve your muscular endurance in a specific activity focus on both becoming stronger and improving your skill in that activity.