Funny you should ask, Paul. I recently had a “discussion” online about this very topic with another forum member. His comments are in blue.
...high reps (8-15) combined with lower weight will increase your endurance strength!
If your aim is to build strength-endurance why stop at 15 repetitions? What not do 25? 50? 100? Hell, since the moto lasts 45 minutes, why not do squats for 45 straight minutes? That's the fallacy in that line of thinking. Where do you draw the line? When does resistance training lose it's utility to enhance sport performance and when would you be better off using another mode of exercise (i.e. cycling, mountain biking, running, etc.) to build the requisite endurance? When do you just ride? .......
...... Who cares if you are strong enough to bench press your bike 5 times? It has no purpose in this sport.
Bullshit! Increasing maximal strength enhances your ability to resist fatigue at submaximal loads - especially as those submaximal loads approach maximum. In other words, the stronger you are, the more repetitions (work) you can do with lighter weights and/or the less effort you have to put out to move heavier loads.
Strength training affects all fiber types. If type I (oxidative/fatigue-resistant) fibers increase their contractile capabilities, the need to recruit type II (glycolytic/fatigable) fibers is greatly reduced; lessening the build-up of lactate and delaying fatigue. Additionally strength training has been shown to reduce the amount of muscle activated for a given load which would further lower the metabolic demand for the same force output.
Let's use the squat as an example. If the goal is to do as many repetitions as possible at a fixed load of 135 pounds, who do you think will win (i.e. have greater strength-endurance)? Athlete A whose max squat is 150 pounds or Athlete B whose max squat is 350 pounds? The answer is obviously Athlete B since 135 pounds represents only 39% of his one-repetition maximum (1RM). He could probably knock out 20+ reps at that weight. Athlete A, on the other hand, would be lucky to do 2 repetitions since 135 pounds is 90% of his best effort.
With a fixed load of 200+ pounds traveling at high speed over variable terrain, MX demands the ability to produce/resist a significant amount of force. How fatiguing that effort will be will depend greatly on the strength level of the individual. A weaker rider will have a very difficult time handling the stress, will get tired very quickly, and will run the risk of falling back in the pack or, worse, crashing badly. This individual will definitely benefit from getting stronger (i.e. lifting heavy).
A stronger rider will fair much better and, therefore, might benefit from another mode of exercise or a variation of resistance training to develop other capabilities. They might choose to engage in interval training or circuit training to develop the ability to withstand metabolic (vs. neuromuscular) fatigue. The only time I could see an MXer lifting in the 8-15 rep range would be to induce hypertrophy (i.e. build muscle). Very few riders, however, are looking to add any appreciable amount of weight.
Obviously, MX demands a lot more than maximal strength but if you are going to use resistance training to supplement your riding (which I think you should) you'd be better off lifting heavy. I have yet to see an athlete too strong for his sport.
Reprinted with permission from Steven Bubel. For more great Q&A's, visit Steven's website at http://www.mxconditioning.com