Peak extension moments at the knee were not significantly different between habitual RFS and habitual FFS runners. However, when habitual FFS runners switched to an imposed RFS, the knee extension moment increased by 29% (P = 0.016). Knee abduction moments were significantly smaller in habitual FFS versus habitual RFS runners (−105%, P = 0.010), but when either group changed to their imposed foot strike technique, they did not alter their abduction moments (Table 2 and Fig. 1). Peak instantaneous power absorption at the knee was greater in all RFS conditions compared with FFS. There were no significant differences in either peak moments or powers at the hip joint between habitual RFS versus habitual FFS or when runners switched to an imposed technique (Table 2 and Fig. 1).
The effect of habitual and imposed foot strike techniques on work, average power, and average moment rate at the ankle, knee, and hip joints during stance.
No significant main effect of foot strike or condition was present in work, average power, average moment rate, or percentage contribution of each joint to total work and average power. However, there were significant interaction effects in all multivariate analyses (Tables 3 and 4). Significantly greater negative ankle average power (49.5%, P = 0.003; Table 3) was observed in habitual FFS versus habitual RFS runners. Post hoc tests also revealed a significant increase in ankle negative average power during stance in imposed FFS versus habitual RFS running (63%, P = 0.001) and habitual FFS versus imposed RFS (49%, P = 0.004) (Table 3). Positive average ankle power was not different between habitual RFS versus habitual FFS runners; however, when habitual RFS runners switched to an imposed FFS, it increased by 19.3% (P = 0.012). Similar differences in ankle work were observed to those of ankle average power (Table 3). The percentage contribution of the ankle joint to total negative lower limb work and average power during stance was significantly lower in an RFS technique compared with an FFS technique both when comparing habitual and imposed conditions (Fig. 2). The average plantarflexion moment rate was significantly greater in habitual FFS versus habitual RFS (31%, P = 0.001) and versus an imposed RFS (24%, P = 0.008) (Table 4). The average ankle internal rotation moment rate was significantly greater when habitual RFS runners switched to an imposed FFS (34%, P = 0.011; Table 4).
Negative knee average power during stance was significantly different across all conditions. Post hoc tests revealed that habitual RFS runners had 49% (P = 0.003) greater negative average power at the knee than habitual FFS runners and 45% (P < 0.001) greater negative average power than when they performed an imposed FFS technique. When habitual FFS runners switched to an imposed RFS, they had 40% (P = 0.001) greater negative average power at the knee (Table 3). The percentage contribution of the knee joint to total negative lower limb work and average power during stance was significantly greater in an RFS technique compared with an FFS technique both when comparing habitual and imposed conditions (Fig. 2). Similar to peak knee moments, the average knee extension moment rate was significantly greater when habitual FFS runners switched to an imposed RFS (16%, P = 0.002; Table 4). Despite not being statistically significant after a Bonferroni adjustment (P = 0.028), the knee abduction moment rate was 87% greater in habitual RFS versus habitual FFS runners, whereas there was only a 3% and 16% difference (nonsignificant) in habitual RFS versus imposed FFS and habitual FFS versus imposed RFS, respectively (Table 4).
After post hoc tests, there were no statistically significant differences in work, average power, or average moment rate at the hip joint between foot strike techniques or conditions (Tables 3 and 4). Although habitual RFS and habitual FFS runners produced a similar amount of positive average power during stance, a trend was present for greater (82%; P = 0.035) average power when habitual RFS runners switched to an imposed FFS. On the contrary, when habitual FFS runners switched to an imposed RFS, they produced 43% less average power (P = 0.091; Table 3). The hip contributed approximately the same percentage of total positive and negative work and average power (Fig. 2) in both RFS and FFS techniques.
The effect of habitual and imposed foot strike techniques on total lower limb work and average power.
Significant interaction effects were found for both total lower limb positive and negative average power (P < 0.001 and P = 0.002, respectively; Table 3). Post hoc tests revealed a significant increase in the total limb positive average power when habitual RFS runners switched to an imposed FFS (17%, P = 0.002; Table 3) and a significant decrease in the total lower limb positive average power when habitual FFS runners switched to an imposed RFS (−10.5%, P = 0.014; Table 3). There was a significant increase in the average total negative power when habitual RFS runners switched to an imposed FFS (8.9%, P = 0.007; Table 3). Differences in total lower limb work between habitual and imposed techniques were similar to those for average power and outlined in Table 3.
The higher prevalence of FFS running among elite athletes and the claims by some of reduced injury rates (7) have led to a tendency for both high-level and recreational athletes to adopt an FFS technique despite naturally preferring RFS running. In this investigation, we assessed joint- and limb-level mechanical differences both between habitual RFS and habitual FFS runners, as well as the effect of changing between foot strike techniques.
The difference between habitual running technique joint mechanics.
Our finding that habitual RFS and FFS runners did not differ in the amount of total lower limb mechanical work or average power when running at 4.5 m·s−1 indicates that one technique does not offer a clear-cut mechanical advantage over the other. This corroborates the recent finding by Gruber et al. (12) that no difference in metabolic cost exists between habitual RFS and FFS runners. It may instead be the altered loading profile and distribution of work and average power between lower limb joints, resulting in varying mechanical demands on the musculoskeletal system, that explains the functional effect of foot strike technique and injury risk.
The current study reinforces the notion that habitual RFS runners place more demand on the knee joint in both the sagittal and frontal planes, whereas habitual FFS runners placed more demand on the ankle joint in the sagittal plane (13,25,35). In a systematic review by van Gent (31), it is reported that 19.4%–79.3% of runners experience lower limb injuries each year with the knee being the predominant site of injury (7.2%–50%). Given that 75% of runners adopt an RFS technique (14), the high occurrence of knee injuries compared with other joints may be associated with the large mechanical demand that occurs at the knee during stance in RFS running. Although the lower mechanical demand at the knee might place habitual FFS runners at a lower risk of knee injury, the higher mechanical demand at the ankle might, on the other hand, place FFS runners at a greater risk of ankle-related injuries such as Achilles tendinopathy and/or rupture or eccentric loading injuries in the triceps surae group.
Although the mechanics of the stance phase itself dictates most of the differences between foot strike techniques, our study also shows, for the first time, that the rate of mechanical loading and thus the cumulative work and moment production also plays an important role. Indeed, we found a more pronounced difference in the average ankle moment rate and average negative ankle power in FFS versus RFS runners compared with differences in peak ankle moments and negative ankle work. In contrast, differences in the average knee moment rates and average negative knee power between foot strike techniques were smaller than those observed for peak knee moments and negative work. These findings suggests that the ankle in FFS running might be especially susceptible to cumulative overload injury, although the relative importance of peak and cumulative joint loading to musculoskeletal injury remains unclear.
By extending our measurements to include positive work and average power, we found that the ankle contributed equal amounts of positive work in habitual RFS and FFS running (Fig. 2A and E) even though there is greater negative work at the joint in FFS running. It is well known that the Achilles tendon is capable of storing energy absorbed at the ankle (negative work) as elastic strain energy and returning this energy to provide positive ankle joint work in the second half of stance (3,10,15,20). It is possible, therefore, that elastic recoil provides a greater contribution to positive work at the ankle in FFS runners. However, if this is the case, it does not translate to a benefit in the total metabolic cost of running (12).
The effect of switching technique: RFS to FFS.
For switching techniques to be effective as an injury prevention/management strategy, the aim is for the imposed technique to replicate the habitual mechanics. When habitual RFS runners were instructed to switch to an FFS technique, they were able to replicate the sagittal plane mechanics observed during habitual FFS running. However, despite there being no difference in ankle internal rotation moments or average moment rate between habitual RFS and habitual FFS runners, an imposed FFS increased these variables by 33% and 34%, respectively, (P = 0.012 and P = 0.011). In addition, the lower abduction moments about the knee joint in habitual FFS versus RFS runners reported here and in the study of Kulmala et al. (19) were not replicated in imposed FFS running. Considering that knee abduction moments possibly have a stronger link to knee injuries (29,34), as well as the increase in ankle transverse plane loading in imposed FFS running, calls to question the advice some coaches are giving to their athletes to change from an RFS to an FFS to reduce their injury risk.
Furthermore, when habitual RFS runners switched to an imposed FFS technique, the total positive average power and the total negative average power increased by 17% and 9%, respectively, (Table 3). This increase in mechanical cost is possibly related to an increase in muscle work and power and thus may be detrimental to running performance. This provides a possible explanation for the recent results of Gruber et al. (12) who found oxygen consumption increased when habitual RFS runners changed to an imposed FFS. The source of the increase in positive average power in imposed FFS running is primarily at the ankle and hip joints (despite a nonsignificant difference at the hip joint after Bonferroni correction), whereas the increase in negative average power is primarily at the ankle joint. It is possible that the elastic mechanisms at the ankle are not as well developed in habitual RFS runners, and therefore, increased work at the hip joint is required to maintain the required work output. The prospect that the calf musculature in habitual RFS runners may limit their ability to effectively adopt an FFS technique was highlighted in a study by Williams et al. (35) where RFS runners who performed a training session using an FFS experienced significant calf fatigue and delayed onset muscle soreness.
The effect of switching technique: FFS to RFS.
In contrast to habitual RFS runners switching technique, when habitual FFS runners adopt RFS running, they are able to replicate all of the joint mechanical characteristics of habitual RFS runners with the exception that they do not increase their frontal plane knee loads (peak abduction moment and average moment rate were 62% and 61% lower than habitual RFS running, respectively). In addition, performing an imposed RFS required 10.5% less positive mechanical average power in the limb to maintain the same running speed. These findings were surprising, suggesting that switching to an imposed RFS could prove a useful strategy for injury rehabilitation while not affecting mechanical performance negatively. More specifically, using an imposed RFS in training may be useful to lower the ankle and Achilles tendon loading in athletes with ankle instability or tendon pathology/injury while at the same time reducing the overall average power, which may help mitigate fatigue.
Contrary to popular claims by some in the running community, this study found no clear mechanical advantage of habitual FFS running over habitual RFS running. Switching between RFS and FFS running techniques may have implications for injury reduction/recovery given the altered distribution in loading between joints but should be weighed against possible overall performance decrements/improvements. Switching from a habitual RFS to an imposed FFS may be detrimental to overall performance because of an increase in both positive and negative average lower limb power, which can help explain the recent finding that imposed FFS running also requires more metabolic energy (12). Furthermore, considering that the high knee abduction moments were not reduced by adopting an FFS technique questions the extent that switching technique will lower knee injury susceptibility. Habitual FFS runners can replicate the joint dynamics of a habitual RFS technique without incurring high knee abduction moments while, surprisingly, also lowering their positive average limb power. In this last regard, FFS runners adopting an imposed RFS technique, rather than the opposite, may prove the most useful training/rehabilitation strategy given the absence of clear mechanical performance decrements. It should be stated, however, that further research is needed to determine whether these findings hold following a training intervention. Nevertheless, this study stands in contrast to the strategy of switching from an RFS to an FFS which is avidly promoted by certain members of the running community (11,23).
The authors would like to thank the participants for taking part in this study, Dr. Ben Jackson from the University of Western Australia for his assistance with the statistics analysis, and two anonymous referees for their valuable comments and constructive suggestions.
S. M. S. was supported through an Australian Postgraduate Award scholarship.
No external funding was received for this work. None of the authors involved in the present study have any conflict of interest, financial, personal, or otherwise, that would influence this research, and the results do not constitute endorsement by the American College of Sports Medicine.
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Keywords:© 2014 American College of Sports Medicine
FOOT STRIKE; JOINT MECHANICS; JOINT WORK; GAIT; COACHING