The major finding of the present study was that the ankle joint stiffness remained the same, whereas the knee joint stiffness increased with the running speed. Stefanyshyn and Nigg (30) reported ankle joint stiffness values of 5.68 N·m·deg−1 in running (4 m·s−1) and 7.38 N·m·deg−1 in sprinting (7.1–8.4 m·s−1), respectively. The present ankle joint stiffness values are very much in line with the results of sprinters in the study of Stefanyshyn and Nigg. However, in that study the sprinters were still accelerating their speed during the measurements, whereas the runners maintained constant speed. The mechanics of running during acceleration is different from the constant speed phase (18), but the actual influence of acceleration on the ankle joint stiffness is unknown. In the present study the knee joint stiffness increased from 17 to 24 N·m·deg−1, and this increase was most marked from 90% to maximal speed. This change was accompanied by decreased flexion of the knee joint (19° at 70% vs 13° at maximum running speed, NS). The joint stiffness patterns of this study (constant ankle joint stiffness and increase in knee joint stiffness with increasing running speed) followed well the tendency observed by Arampatzis et al. (2) at low running velocities. As the purpose of the study was to examine the joint stiffness regulation (via neural control of muscles), the relative running speeds (same effort/input levels) were chosen instead of absolute running speeds (same output level) because the neuromuscular requirements of each individual to reach or to maintain certain absolute running speed are closely related to effort level which can be totally different between the subjects.
The anatomical structure of the quadriceps femoris muscle group is clearly different from the triceps surae muscle group with shorter tendinous part and larger amount of muscle tissue (e.g., 34). These anatomical differences make the quadriceps muscle group able to control the musculotendinous stiffness better via the muscular activation. Large standard deviations in the knee joint stiffness reveal the large interindividual variation that is most remarkable at higher speeds. In fact, in some subjects the stiffness of the knee joint was almost constant throughout increasing speeds. Whether this is because of real individual differences in joint stiffness patterns/regulation or just intra-individual variation between contacts remains to be explored further.
In the present study, the vertical stiffness increased linearly with the running speed. It is consistent with the previous results in slow running (2,12). Increase in the vertical stiffness was accompanied by decreased vertical displacement of the COM during the eccentric phase (−37.9%, P < 0.01). The absolute values of the present study for vertical stiffness as well as for vertical displacement of the COM shows a linear continuum to the results of Arampatzis et al. (2) measured at lower running speeds (ranged from 2.5 to 6.5 m·s−1).
The observed co-contraction between the plantarflexor and dorsiflexor muscles, as well as between the knee extensors and knee flexor muscles, will stiffen the joints and the whole leg for the forthcoming impact to the ground (14). Muscle activation of plantarflexors and knee extensors increased during the preactivation phase (Fig. 2) as speed increases. The preactivation of these muscles will increase the stiffness of those muscle-tendon units to tolerate and absorb high impact loads at the beginning of the ground contact (10,23). Additionally, the preactivation of the triceps surae muscle together with stretch reflex activity will ensure the high muscular (and ankle joint) stiffness to support and push the body off the ground as has been pointed out by Komi and Gollhofer (21). This enhanced activation by the stretch reflex is also evident in the present study in the SOL muscle where the activation peaks at 50 ms after the beginning of the contact phase (Fig. 2). Taking into account the electromechanical delay of 13–15 ms reported by Nicol and Komi (28), the mechanical response of stretch reflex would then occur about 60–70 ms after the beginning of the contact. The average contact times in our study were 130 ms at 70% and 94 ms at maximal running speed. Hence, the mechanical effect of stretch reflex response appears at the end of the braking phase or early push-off phase of sprint running.
According to the results of the present study, it seems that in sprint running the spring-like behavior of the leg is adjusted by changing the knee joint stiffness while the stiffness of the ankle joint remains constant. This theory is further reinforced by the joint moment-angle relationships presented in Figures 3 and 4. The ankle joint moments were lower in the concentric phase, which implies that the subjects were not able to tolerate the applied load and utilize the stored elastic energy as efficiently as in the knee joint where the joint moment-angle curves followed the same path in the eccentric and concentric phases. However, we have to be cautious in concluding the role of the knee joint stiffness in sprint running because of the large variation in knee joint stiffness values especially at higher running speeds (Fig. 5).
In the present study, both the ankle joint stiffness and joint moment were constant at different running speeds. Arampatzis et al. (2) reported an increase in the ankle joint moment while the stiffness of the ankle joint showed a curvilinear pattern (without any significant difference) with the increasing running speed from 2.5 to 6.5 m·s−1. In the studies of Stefanyshyn and Nigg (30) and Farley and Morgenroth (8), the ankle joint stiffness increased with the running speed and hopping height. However, contrary to the present study, the ankle joint moment was also increased correspondingly. It is possible that the different results are simply due to task differences. The submaximal levels of tasks in the other studies were probably not high enough to fully load the triceps surae muscle-tendon unit, as subjects were still able to increase their ankle joint stiffness. Furthermore, the different nature of the tasks will make the comparison difficult between sprinting and hopping in place (8) as well as between constant speed sprinting and acceleration (30). However, we believe that the constant ankle joint stiffness observed in this study might be due to dominating role of the (constant) tendon stiffness already discussed earlier.
In the whole subject group, the ankle joint stiffness showed higher values with shorter contact times at all measured relative running speeds (Fig. 6). Unfortunately, no correlation was found between ankle or knee joint stiffness and running speed. It could be explained by increased output of hip extensor muscles because the increase in running speed is achieved by increasing the work and power produced by the hip extensors (23). Still one would assume that stiffer ankle and knee joint will transmit the work done by hip extensors better and thus propelling the body forward more effectively as in world-class level sprinters (17). According to the results of this study, it seems that ankle or knee joint stiffness may not be a limiting factor in increasing the running speed. However, high ankle joint stiffness may shorten the ground contact time and thus enhance the mechanical efficiency of locomotion.
The authors would like to acknowledge the assistance of Mr. Markku Ruuskanen and Ms. Sirpa Roivas (technical preparations) as well as Ms. Pirkko Puttonen and Ms. Marja-Liisa Romppanen (data analysis). This study was supported in part by a grant (119/722/99) from the Ministry of Education (Finland).
Address for correspondence: Sami Kuitunen, Neuromuscular Research Center, Department of Biology of Physical Activity, University of Jyväskylä, P.O. Box 35, FIN-40351 Jyväskylä, Finland; E-mail: firstname.lastname@example.org.
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