High intensity training - how long should each exercise be?

Take two people who can just barely lift 100 for one repetition. Now lighten the weight to 80 pounds and see how many reps each person gets. You might find that there will be a wide range of variability. One person might get eight reps, while the other will get 12. Why is that?

Those who have primarily fast-twitch muscle fiber will be able to lift a greater weight for their size. Those who are more effective at recruitment of muscle fibers will be able will lift more weight as well. Someone who is primarily fast-twitch and very effective at recruiting a lot of muscle fiber for the task at hand will be very strong and very fast. Dara Torres is an example.

The flip side of that is those who are primarily fast-twitch and very effective muscle fiber recruiters will fatigue much quicker. Those that fatigue quicker will be better suited to exercises of shorter duration to avoid over-training. A slow twitch person who is less effective at muscle fiber recruitment will require a longer duration of an exercise to reach a level of fatigue that is necessary to stimulate a change in the body. Some will do well with an exercise duration of less than one minute on a particular muscle group, while others might benefit more with times over two minutes for the same muscle group.

The composition of fast- and slow-twitch muscle fibers varies among individuals, and it varies for the various muscle groups within each individual. As trainers we do not know the muscle fiber composition of each client. We cannot do biopsies to determine that composition. Noticing the different rates of fatigue among individuals we can infer that a client might possibly be more fast-twitch than slow twitch and make necessary changes in the duration of certain exercises.

That can be problematic though. Sticking to a scheme of tailor-made set times may work well at first, but eventually the improvement will stop as the body adapts to the same stimulus week after week. At Austin Personal Trainers and New Orleans Fitness Training we might determine that a set time is better for a certain individual on a certain exercise and perform that protocol more often, but we feel variety in the length of times is important as well. We take our clients through a variety of different exercises, changing sequences, differing amount of sets for each muscle group, and exercises of different durations, so that they are constantly presented with a new challenge to keep the positive adaptations ongoing.

If a client stalls at certain time for a given weight we'll make changes by avoiding that particular exercise for awhile and substituting different exercises addressing the same muscle group. Those exercises will have different durations. When we go back to that exercise that the person had stalled on we very often find they outperform their previous best time by a surprising margin.