Adaptations To Short Interval Sprint Training: Part 2!

In the first article we talked about interval training and the metabolic system. Here I am going to focus on morphological adaptations that muscles undergo in response to short term sprint training.
Part 1 | Part 2

In the first part of this article, we looked at how interval training in the 10 second or less phase can impact the metabolic system of the exerciser. This is an intense type of training that many people would have trouble performing, but if you complete it the benefits are numerous. You will be able to see similar benefits with intervals at a slightly lower intensity performed within the 10-30 second range, however the adaptations will be slightly different.

Morphological Adaptations

Muscle Fiber Types

    Now we'll look at the morphological adaptations that muscles undergo in response to short term sprint training. The common classification system used to distinguish between muscle fibers is slow twitch or type I, fast twitch oxidative-glycolytic or type IIa, and fast twitch glycolytic or type IIb.

    This classification system is based on the pH sensitivities of the muscle tissues ATPase activity. This property helps determine the expression of the myosin heavy chain isoforms of the muscle fibers, which is responsible for the contractile characteristics (speed and force output).

    Type IIb has the highest power outputs, type IIa is intermediate, and type I shows the lowest contractile speed and force. It shows that type IIb has ten times the maximum unloading shortening velocity of type I (Leveritt & Ross, 2001). So naturally, the higher the percentage of type IIb muscle fibers an athlete has, the better sprinter they will be.

Muscle Fibers: How Do They Differ?
Our body is composed of many different groups of muscles and each person has a unique composition of muscle tissue in their body. Read here learn about the 3 different muscle fibers.
[ Click here to learn more. ]

Switching Muscle Fibers

    When an individual undergoes a sprint training program, assuming they are following a proper protocol, their muscle fibers will gradually begin to switch over to favor the fast twitch over the slow twitch.

    Further, studies demonstrate that type IIb muscle fibers are the default gene that the body goes to when there is a lack of stimulation (rest). Also important, are intensity of stimulation and recovery given to the muscles.

    Animal Tissue Study

      In a study done on animal tissue, three scenarios were used.

      • In the first scenario, the muscle tissue was stimulated at a low frequency every 15 seconds.

      • In the second, it was stimulated at a high frequency every 15 seconds.

      • Finally, in the third scenario, the tissue was stimulated with a high frequency every 15 minutes (Ausoni S., et al., 1990).

      After two months of stimulation, the low-frequency 15-second group demonstrated 79% type I fibers, the high-frequency 15-second group demonstrated 90% type I, and the high-frequency, 15-minute group demonstrated 52% type I fibers.

    Interpreting The Study

      This clearly shows that it is not only the type of stimulation the muscles receive, but also how much recovery time they are given between bouts of stimulation that determines their contraction characteristics.

      A possible reason for this is because when sprints are done with shorter rest periods, the aerobic system is called into play more and this system is more often characterized by type I fibers.

      So even though you are still sprinting, it is beneficial for the muscles to keep an oxidative capability to perform with short rest intervals.

      Further, the frequency of sessions will also play a factor. Performing sprint training 3 times a week will produce a far greater shift to type IIa fibers than performing sprint training twice daily will (Leveritt & Ross, 2001). Finally, genetic predisposition to certain types of muscle tissue fiber types will also influence how an individual reacts to a particular training protocol.

Muscle Size

    Next to consider are changes to the actual muscle size. During the first 6-to-7 weeks there is not much noticeable difference in muscle size with response to sprint training, despite a significant increase in performance level. As one continues to train with sprint sessions though, from 8 weeks onward, there is a noticeable increase in the cross sectional area of both type I and II muscle fibers (Leveritt & Ross, 2001).

    Type II / I Ratio

    It should be noted however, that it is not so much the size of the muscle fibers that determine how successful a sprint athlete is but more the type II/type I ratio of the muscle fibers.

    Therefore, the greater the percentage of type II muscle fibers the athlete has, the most likely greater performance level they will have.

    Hypertrophy Not Desirable

      Most sprint athletes would be better off without a great deal of muscular hypertrophy since it would limit them if they perform weight-bearing activities (running for example). This would not be as much of a consideration if they were involved in other activities such as cycling.

      In either case, due to the nature of the energy systems involved, you will usually see more muscle mass on sprint training athletes than you will on endurance trained athletes, the extent however might not be as great as if you had been comparing an endurance athlete to a bodybuilder though.

The Sarcoplasmic Reticulum

    Adaptations to the sarcoplasmic reticulum also take place when sprint training is performed. The SR (sacroplasmic reticulum) is responsible for muscle contraction in that it releases Ca+ which then enables the crossbridge cycle to take place. The re-uptake of the Ca+ occurs after the contraction is completed and the calcium is pumped from the cytosol back to the SR.

    What Is Ca+?
    Ca+ stands for a positively charged Calcium ion. An ion is an atom or a group of atoms that has acquired a net electric charge by gaining or losing one or more electrons.

    The Sarcoplasmic Reticulum (shown to the right in light blue) is responsible for releasing Ca+ in muscles.

    The more developed the SR is, the faster this process can take place, and therefore, the faster the muscular contraction and relaxation rate will be. Generally, the faster the fiber twitch, the greater the volume of the SR. Also, along with the greater volume, they will also have a greater density of Ca+-ATPase enzymes which allow the muscle fiber to vary the force level it produces (Leveritt & Ross, 2001).

Neuron To Fiber Action Potentials

    A final adaptation that is not backed by numerous research studies, but has shown some evidence of occurrence is that of muscle conduction velocity. This is the rate of action potentials that can be sent from the motor neuron to the muscle fibre.

    It appears as though this conduction velocity is largely correlated with muscle fiber type and fiber cross-sectional area (Leveritt & Ross, 2001). This measurement can be used to distinguish between endurance trained and sprint trained athletes accurately, as the slowest sprint trained athletes usually have much faster conduction velocities than even the fastest trained endurance athletes.

    As studies are limited in this area though, it is hard to determine how much sprint training will increase muscle conduction velocity. However, the studies done so far have shown a consistent trend toward higher conduction rates after undergoing sprint training, particularly among those who were slow to begin with (Bianchi, S., Rossi, B. et al, 1997).


While reading all of these adaptations, keep in mind that this happens in response to intense training. Most people don't reach the intensity level discussed in many of these studies to elicit these exact effects. While you definitely will experience some similar results with less intense sprint sessions, ones that many people are capable of performing, they most likely would not be of the same magnitude of those here.


  1. Ausoni, S., Gorzna, L., Schiaffino, S., et al. Expression of myosin heavy chain isoforms in stimulated fast and slow rat muscles. J Neurosci 1990; 10(1):153-60.
  2. Bianchi, S., Rossi, B., Siciliano, G. et al. Quantitative evaluations of systemic and neuromuscular modifications induced by specific training in sedentary subjects. Med Sport: 50 (1).
  3. M. Leveritt & A. Ross. (2001). Long-term Metabolic and Skeletal Muscle Adaptations To Short-Sprint Training. Sports Medicine, 31(15).

Part 1 | Part 2