The Extensor Paradox in running

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The original article below on the topic at hand, was published in Biomechanics of Distance Running in 1990. As you read the data and conclusions from this research you’ll see that the running  scientific community was not ready to accept the idea of the role of gravity as a leading force in running. A classical vision of gravity strictly as a vertical force was predominant in the scientists’ minds and didn’t allow them to look at the facts from a different perspective. The most important thing there, a relationship between extensor muscles and gravity as one non-conflicting system with reciprocal coordination between them, was overlooked.

The data in this article clearly demonstrates Nature’s wisdom of coexistence, when one force yields to the other to allow them both be used to their fullest. In the Pose Method the concept of gravity as a leading force in forward movement is the most fundamental one, and the data from the extensor’s paradox article confirm this.

I would like to specifically point out for you the data showing when the quad muscles cease any electrical activity. According to the research data, it happens immediately after the midstance, when the so-called “push off” efforts are supposed to be exerted. This conflicting logic brought the authors to this particular name of the article. The commonly accepted understanding of the leg extension as a forward propulsive force in running is not supported by the data of the electrical activity of the muscles responsible for the knee extension in this research. But, at the same time, with this data available, the researchers did not come to any conclusion about the role of gravity.

If we accept gravity as a leading force in horizontal movement of the body in running, then it is only logical that such muscle behavior does not interfere with the work of gravity. It is easy to say that this logic has been established from the beginning of biological life on Earth. From this point of view, our conscious efforts to produce the forward propulsion were “ignored” by Nature. However, some perception of muscle efforts on support, which we have during the support time before a midstance, gives us an illusion of this “push off” happening. Most runners sincerely believe in ‘push off efficiency’ and its necessity in order to run, because of their perception and deceptive visual appearance.

Our common sense is based on and is limited by our understanding of the subject, and hence is a very deceptive thing that often doesn’t coincide with abstract logic, which we have to use in order to see the hidden reality of functioning of systems. For this matter we have to use the system of reference of Nature, applying scientific terminology, according to which Gravity is a predominant force by all accounts. Then we’ll be able to see how the forces are interacting in the hierarchically structured system, each with its own space and time of involvement in the action of running.


Human Kinetics Books, 1990

Chapter 6. Muscle Activity in Running. The Extensor Paradox Experiment
by Irene S. McClay, Mark J. Lake, Peter R. Cavanagh

It is well known that knee flexion occurs just before and immediately after footstrike during running to cushion the impact of landing (Milliron & Cavanagh, this volume). Once the downward movement of the center of gravity associated with this cushioning phase has finished, knee extension begins and the propulsive phase of the cycle continues.

There is evidence from Brandell (1973) and Mann and Hagy (1980b) that the quadriceps are generally silent during the phase of knee extension following the cushioning. Few experiments have focused on this puzzling aspect of knee joint action during running. The purpose of the experiment described in this section was to examine the activity of the three heads of the quadriceps that are amenable to surface recording during distance running and to simultaneously measure the angle of the knee joint.

Subjects and Speed
Six male recreational runners, ages 19 to 26, experienced in treadmill running with no history of recent injury, volunteered for the study. Each subject ran at a constant speed of 4.0 m · S-¹ on a motorized treadmill. This speed was chosen as it was in the middle of the range used by previous workers.

Equipment and Method of Analysis
To investigate knee extensor muscle activity during the stance phase of running, EMG of the vastus medialis, vastus lateralis, and rectus femoris muscles of one leg were recorded using a battery-powered GCS 67 Electromyographic Processor. Silver-silver chloride electrodes with on-site preamplifiers were placed in the middle of the muscle belly after thorough preparation of the skin. An inertia switch attached to the heel was used to define the cycle endpoints and knee angle was recorded simultaneously with a self-aligning ULGN-67 Electrogoniometer. This design compensates for errors in placement and does not assume a fixed center of rotation for the joint. The electrogoniometer was calibrated for knee angle by comparing voltage output against knee angle measured by a protractor.

The EMG processor, together with the goniometer and footswitch signals, were interfaced with an SMS 1000 computer, which sampled at a rate of 500 Hz per channel. The raw EMG signal was prefiltered using a high pass filter of 75 Hz cut-off frequency. Custom software allowed for storage, processing, and display of the data. An example of the raw data for the complete 5-second sampling period is shown in Figure 6.6a, and the region surrounding footstrike is shown with greater resolution in Figure 6.6b.

Figure 6.6a. A 5-s raw experimental record.
Figure 6.6b. A portion of the same experimental record surrounding footstrike shown with greater time resolution.

Five-second samples were collected after each subject had undergone a warm-up period at the test speed. This allowed at least six full cycles of running to be recorded for each individual. For each period of stance, the phasic activity of all three muscles was subjectively determined by comparison with a noise-free baseline. Data from six footstrikes were examined, and mean values were obtained for the time at which rectus femoris, vastus lateralis, and vastus medialis muscle activity ceased. The beginning and end times of the knee extension phase following initial flexion were also determined.

Figure 6.7a illustrates the mean results of six footstrikes for a typical subject. It can be seen that approximately 85 milliseconds before footstrike, muscle activity begins while knee extension is under way. Vastus lateralis is the first to show activity, some 25 milliseconds before vastus medialis and 60 milliseconds before rectus femoris. This period of muscle activity appears to help in stabilizing the leg in preparation for footstrike. All three muscles are active through footstrike while knee flexion occurs, but they cease activity simultaneously approximately 20 milliseconds after peak knee flexion has been achieved. In this subject knee extension continues for a further 150 milliseconds.

Figure 6.7a. Results of phasic quadriceps EMG and knee angle for a typical subject averaged over six footstrikes.
Figure 6.7b Ensemble average results of six subjects of the relationship between phasic quadriceps EMG and knee angle. The values of peak knee extension prior to footstrike, peak knee flexion during stance, and peak knee extension after stance have been joined by straight lines as the mean curve was not determined.

The mean results for the group as a whole are presented in Table 6.1 and shown schematically in Figure 6.7b. The mean time of knee extension that was not accompanied by quadriceps EMG was 133.7 milliseconds (SD = 16.5).

Flexion extension duration Mean all muscle off after peak flexion Mean duration of silence during extension
Mean for

Table 6.1 Mean Values (in Milliseconds) for Critical Phases During Knee Extension During Running at 4 m · s-¹

These results are further illustrated in Figure 6.8, where electrical activity is indicated by the presence of shading over the muscle. The amplitude of the activity is also schematically indicated by the intensity of the shading. The large amount of knee extension that occurs in the absence of muscle activity is readily apparent from this figure.

Figure 6.8. The amplitude of EMG activity throughout the stance phase of running. (The intensity of shading indicates relative amount of activity.)

For the group of runners examined in this study, it is clear that the quadriceps cease their activity shortly after peak stance phase knee flexion has occurred. A phase of knee extension of approximately 130 milliseconds continues without the assistance of the quadriceps. The function of the quadriceps must therefore be described as principally controlling the descent of the body center of gravity after landing. Certainly they help to initiate knee extension, but they rapidly become quiescent when knee extension has been under way for only about 30 milliseconds, a time during which less than 5 degrees of extension has been achieved. The duration of electrical silence in extension is large enough to exclude the possibility that electromechanical delay (EMD) between EMG activity and force production may explain the paradox. EMD time in concentric muscle action has been determined to be 40 to 55 milliseconds (Cavanagh & Komi, 1979; Norman & Komi, 1979), and in rapid movements it may be possible for EMG activity to have terminated before force can be detected.

A reasonable hypothesis may be that hip extensor action during the second half of the stance phase is causing the knee joint to extend. However, if one examines the co-activation of the quadriceps and hamstrings in Figure 6.3, it is apparent that many investigators have found these muscle groups to cease activity at about the same time in the cycle. Neither does there appear to be a prolonged period of gluteus maximus activity that would provide an explanation. Figure 6.4 indicates that the last extensor muscle to cease activity during stance appears to be the gastrocnemius, which is of course also a knee flexor. Because only the quadriceps were measured in the present study, it is not possible to say with certainty what patterns of activity were exhibited in other muscles in these particular subjects. These experiments have, however, shown that the notion of an extensor thrust-with plantar flexors, knee extensors, and hip extensors all being active in late support to generate forward and upward thrust – is in need of modification. They also indicate that the problem is worthy of further investigation using a kinetic approach in addition to multi-channel EMG so that the joint moments can be determined.

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