Previously we have shown that increased Achilles tendon length is related to reduced net oxygen uptake (increased economy) while walking on the flat (McCarthy et al. 2006
). In addition, longer Achilles tendon lengths explained greater walking economy of AA women compared to EA women (McCarthy et al. 2006
), suggesting AA women are able to obtain more elastic energy reuse from their longer tendons, walk with less muscle contractile activity and thus use less energy while walking.
These results were only observational so caution must be made in attributing cause and effect between tendon length and reduced walking economy. It can not be ruled out that differences in walking economy between AA and EA women are due to other factors, such as biochemical differences in the muscle. For example, it has previously been shown that Type I muscle fiber type is positively related to muscle metabolic economy, walking economy (Hunter et al. 2001
) and cycling economy (Coyle et al. 1992
). In addition, individuals of African descent may have a lower percentage of Type I muscle fiber (Ama et al. 1986
; Kohn et al. 2007
; Tanner et al. 2002
). However, if the AA women in this study were to have lower Type I muscle fibers, lower not higher walking economy would be expected. So it is unlikely that differences in muscle fiber type explain the ethnic differences walking economy. In this study we also show that there is no difference in muscle metabolic economy of force production in the plantar flexor muscles of AA and EA women (in fact although there are no significant differences, the EA women show a trend toward more economical muscle force production). This would suggest that biomechanic differences may contribute to the ethnic differences in walking economy.
The kinetics of walking were not evaluated in this study, so only speculation can be made concerning potential biomechanic differences. However, it is consistent with the presented data to speculate that tendon length may be involved in any biomechanic differences. We show that the difference between the two ethnic groups is greatly truncated (with no significant difference observed between the two ethnic groups with 48 EA and 48 AA women) when walking up a 2.5% grade, a condition that has been previously shown to increase the proportion of energy from plantar flexor muscle shortening and reduce the proportion of elastic energy used in walking (Sawicki and Ferris 2009
). Finally no significant relationship was observed between grade walking or muscle metabolic economy and tendon length. Taken together these results further support the contention that ethnic differences in Achilles tendon length may be mediating at least some of the increased economy observed in AA women during level walking.
The reduction in the relationship between grade walking economy, rather than flat walking and tendon length would be expected. Sawicki and Ferris (2009)
found that the apparent efficiency acquired during their studies using a robotic ankle exoskeleton decreased as grade increased suggesting that plantar flexor muscle shortening provides a larger fraction of total muscle–tendon positive work. In addition, increases in work at the hip and knee but not the ankle during grade walking decreased the proportion of energy expended at the ankle during grade walking (Sawicki and Ferris 2009
). Both these factors would proportionately decrease the importance of elastic stretch and reuse of energy by the Achilles tendon during grade walking. Consistent with a proportional decreased energy savings from elastic energy storage during grade locomotion, Roberts et al. (1997)
have previously shown that in turkeys, muscle shortening and EMG activity of the lateral gastrocnemius increase while running up an incline as compared to running with no grade. If proportional decrease in elastic energy storage and reuse does occur with grade walking the relationship between walking economy and tendon length should be reduced during grade walking. Further, if tendon length is the main factor mediating differences in walking economy between AAs and EAs the difference in walking economy between AAs and EAs should be reduced when walking up a grade. Our results showing no difference in walking economy between AA and EA women while walking up a 2.5% grade are thus consistent with the hypothesis that a major contributor to differences in walking economy between AA and EA women is increased use of elastic energy storage and reuse with longer Achilles tendons.
Isometric contractions by muscle in the un-stretched position such as used in our 31P muscle metabolic economy model would be expected to obtain little if any stretch shortening cycle elastic energy savings. Therefore, any differences in metabolic economy of force production for these isometric contractions would be thought to be primarily due to biochemical processes within the muscle. The lack of a relationship between muscle metabolic economy and Achilles tendon length for the isometric plantar flexions used in this study would support this supposition. In addition, the lack of a difference between the two ethnic groups also supports the hypothesis that the differences in walking economy between AA and EA women is not due to biochemical or bioenergetic differences between the ethnic groups but is probably biomechanical, with tendon length contributing to these biomechanical differences.
A limitation in this study is that kinetics of walking was not examined. It is possible if not probable that biomechanics of walking were different between the AA and EA women. These potential biomechanical differences may or may not have been influenced by differences in tendon length. However, it is beyond the scope of this study to address those potential differences.
By showing that muscle metabolic economy and grade walking economy are not different but flat walking is different between AA and EA women we support our previous study’s suggestion that longer gastrocnemius tendon length may contribute to greater elastic energy savings in AA women during level walking. Hopefully this work will stimulate further research into the role tendon plays in generating force in human locomotion. It should be apparent that this understanding has important implications for understanding human performance, both during sport and in everyday life.