Intensity = Inroad / Time

Q&A Muscle Fatigue, Inroad, and Intensity

Question: Is there a difference between inroad and fatigue? Some HIT trainers seem to use them interchangeably. What does inroad have to do with exercise intensity?

Answer: Fatigue and inroad are related concepts, but not the same thing. Fatigue is the reduction a muscle’s ability to produce force. Inroad is a measure of fatigue, usually expressed as the percent difference between your starting and momentary strength.  For example, if your strength at the start of an exercise is one hundred units of force and fatigue reduces it to eighty units of force your inroad would be twenty percent.

Inroad is also a measure of the average intensity of an exercise when compared with total exercise time. The higher your intensity of effort during an exercise, the greater your rate of inroad will be. You have to work harder to achieve an inroad of twenty percent within sixty seconds than an inroad of only ten percent, or to achieve an inroad of twenty percent within sixty seconds than within ninety seconds.

Intensity = Inroad / Time

For example, the deadlift is much harder when performed continuously, without setting the weight down between repetitions. When done this way momentary muscular failure will be achieved more quickly than if you rest between repetitions (and some studies suggest this will stimulate greater increases in strength and size).  In either case, if the exercise is performed to failure you will have reached maximum intensity, but if you get there faster your average intensity during the exercise was higher.

You could compare the difference in the average intensity between the two approaches by comparing the time to failure using the same approximate percentage of your one repetition maximum (for the sake of example we will ignore all of the practical problems with doing this accurately). For example, if you achieve a twenty percent inroad within forty seconds your average intensity would be twice as high as if you achieved the same inroad in eighty seconds (.5 percent inroad per second versus .25 percent). This would only be true for a single individual though; because different people with different percentages of slow, intermediate, and fast twitch muscle fiber types fatigue at different rates with the same percentage of 1RM.

Inroad is not directly proportional to exercise effectiveness, however. While some fatigue is necessary too much fatigue can be counterproductive and achieving momentary muscular failure appears to be far more important if your goal is to build bigger, stronger muscles. Studies comparing the effects of training with lighter loads and higher repetitions (higher inroad) with heavier loads and lower repetitions (lower inroad) have shown no significant differences in muscular strength and size increases, on average. If there was a direct relationship between inroad and growth stimulation we would expect to see greater increases in muscular strength and size with lighter weights and longer sets, but this is not what happens.

Instead, there appears to be a broad range of inroad that is effective, however like most things what is optimal would vary between individuals and having either too little or too much for an individual will compromise results. This is why people who have a fast fatigue response/higher percentage of fast twitch fibers tend not to respond as well to longer set durations and people who have a slow fatigue response/higher percentage of slow twitch fibers tend not to respond as well to shorter set durations.

 

References:

Gie?sing J, Fisher J, Steele J, Rothe F, Raubold K, Eichmann B. The effects of low-volume resistance training with and without advanced techniques in trained subjects. J Sports Med Phys Fitness. 2016;56(3):249-58.

Morton RW, Oikawa SY, Wavell CG, et al. Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men. J Appl Physiol. 2016;121(1):129-38.

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