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Published in final edited form as: Gait Posture. 2024 Feb 10;109:220–225. doi: 10.1016/j.gaitpost.2024.02.008

Cues to Land Softly and Quietly Result in Acute Reductions in Ground Reaction Force Loading Rates in Runners

Lauren K Sara a,b,c,*, Logan W Gaudette a, José Roberto de Souza Júnior a,b,d, Adam S Tenforde a,b, Lindsay Wasserman a, Caleb D Johnson a,b,e
PMCID: PMC10939780  NIHMSID: NIHMS1968871  PMID: 38364508

Abstract

Background:

A common gait retraining goal for runners is reducing vertical ground reaction force (GRF) loading rates (LRs), which have been associated with injury. Many gait retraining programs prioritize an internal focus of attention, despite evidence supporting an external focus of attention when a specific outcome is desired (e.g., LR reduction).

Research Question:

Does an external focus of attention (using cues for quiet, soft landings) result in comparable reductions in LRs to those achieved using a common internal focus (forefoot striking while barefoot)?

Methods:

This observational study included 37 injured runners (18 male; mean age 36 (14) years) at the OMITTED Running Center. Runners wore inertial measurement units over the distal-medial tibia while running on an instrumented treadmill at a self-selected speed. Data were collected for three conditions: 1) Shod-Control (wearing shoes, without cues); 2) Shod-Quiet (wearing shoes, cues for quiet, soft landings); and 3) Barefoot-FFS (barefoot, cues for forefoot strike (FFS)). Within-subject variables were compared across conditions: vertical instantaneous loading rate (LR, primary outcome); vertical stiffness during initial loading; peak vertical GRF; peak vertical tibial acceleration (TA); and cadence.

Results:

Vertical LR, stiffness, and TA were lower in the Shod-Quiet compared to Shod-Control p<0.001). Peak vertical GRF and cadence were not different between Shod-Quiet and Shod-Control. Reductions in stiffness and LR were similar between Shod-Quiet and Barefoot-FFS, and GRF in Barefoot-FFS remained similar to both shod conditions. However, runners demonstrated additional reductions in TA and increased cadence when transitioning from Shod-Quiet to the Barefoot-FFS condition (p<0.05).

Significance:

These results suggests that a focus on quiet, soft landings may be an effective gait retraining method for future research.

Keywords: tibial acceleration, biomechanics, running, ground reaction forces, kinetics

1. INTRODUCTION

Vertical ground reaction force (GRF) loading rates (LRs) have been associated with running-related musculoskeletal injuries, including in a prospective study on female runners [1,2]. As such, LR reduction is often a goal of gait retraining, an intervention utilized for the prevention and treatment of running-related injuries [3,4]. Many gait retraining programs indirectly address LR reduction by targeting changes in kinematic or spatiotemporal variables, such as foot strike pattern, trunk lean, or cadence [3,5]. However, the literature is limited and mixed on which of these gait modifications is most effective, both in terms of reducing LRs and limiting injury risk [6]. A promising alternative may be a gait retraining strategy that centers on an external focus of attention, allowing a runner to select the combination of gait modifications that feel most natural for them to change.

An external focus of attention refers to a focus on the effects of movement, such as how the movement feels or how loud it is, without emphasizing a specific (internal) movement strategy. In contrast, an internal focus refers to an attentional focus on body movements, such as where to land on the foot, how far to lean the trunk forward, or how to position the hips. To achieve reductions in LRs, many current gait retraining approaches utilize an internal focus of attention, despite evidence supporting use of an external focus to promote motor performance and learning [7,8]. The most common gait modification with an internal focus, forefoot striking, often includes a transition to minimalist or no (barefoot) footwear [6]. However, transitioning to minimalist or barefoot running has been associated with an increased risk of bone stress injuries [9,10].

Auditory feedback is commonly utilized when promoting an external focus during gait retraining. Examples include accelerometry and smartphone apps to monitor and provide biofeedback scaled to the magnitude of impact [11,12,13]. Additional studies have evaluated the use of music to alter running cadence [14,15,16,17]. A few studies have evaluated the efficacy of simple cues to land softly and/or quietly on force of impact [18,19,20], an intervention that would provide an inexpensive and clinically feasible approach to LR reduction. However, these efforts have largely focused on healthy, pain-free runners [11,12,18,19,21,22,23]. Among those evaluating clinical populations, the studies have relied on a combination of internal and external focus, such as when employing auditory cues to modify from a rearfoot to forefoot strike (an internal, kinematic strategy) [13]. The efficacy of such strategies performed in isolation (i.e., not combined with an internal focus) in clinical populations is unclear.

Current evidence regarding attentional focus in gait retraining supports the use of an internal focus when a specific movement or position is desired, but an external focus when a particular outcome (such as decreased LRs) is desired [24]. Promising evidence exists that an external focus of attention during gait retraining would reduce LRs [22,23]. However, it has only been evaluated in combination with minimalist and barefoot conditions, in male-only cohorts, or in healthy runners. No studies have directly compared gait modifications that incorporate an external-only focus of attention to either internal-only or combined internal and external foci in male and female runners with a musculoskeletal injury.

Additionally, no previous work has focused on the effects of an external focus of attention on measures of lower extremity stiffness. Stiffness is a measure of how compliant the lower extremities are, which serves to absorb external forces during running [25]. Several studies have demonstrated that a runner’s leg and knee stiffness are closely associated with their vertical force characteristics [26,27,28]. Further, it has been shown that runners adjust their leg stiffness almost immediately when transitioning from stiffer to softer surfaces, maintaining their natural vertical force pattern [29]. While some degree of stiffness in the lower extremities is necessary to facilitate efficient running through kinetic energy return, excessive stiffness has been linked with musculoskeletal injuries [30,31]. Given this, stiffness may be an important measure to target with gait retraining programs, in addition to vertical LRs.

Thus, our primary aim was to investigate whether externally focused gait modifications in people with running-related injuries produce comparable reductions in LRs as are seen in one of the most common, internally focused modification: forefoot striking while barefoot. We hypothesized that an external-only focus, through cues for quiet and soft landings, would lead to acute reductions in LRs comparable to those seen in barefoot running with a forefoot strike.

2. METHODS

2.1. Participants

Runners were evaluated at the OMITTED Running Clinic, seeking treatment for a running-related musculoskeletal injury. As part of their standard of care, runners received a gait assessment on an instrumented treadmill. Runners included in the current analyses were those evaluated in the clinic between December 2021–October 2022. Runners were excluded who were habitual non-rearfoot strikers, defined as contacting the ground with a portion of their foot other than the heel first, or who did not complete all testing conditions due to the nature of their injury. A waiver of informed consent for Secondary Data Use was obtained from the hospital Institutional Review Board (Protocol #: 2017P000481). Herein, runners included in the analyses will be referred to as “participants”, for simplicity.

2.2. Procedures

After basic demographic and anthropometric data (including age, height, and weight) were collected, inertial measurement units (IMUs) were fastened over the distal-medial tibia, just above the superior border of the calcaneus, on both legs of every participant (Blue Thunder IMU, IMeasureU, Auckland, NZ). Participants ran on an AMTI instrumented treadmill (AMTI, Watertown, MA), with two force plates embedded beneath the belts to measure GRFs. Participants ran at a self-selected, comfortable speed, defined as a speed they would run at for an easy run where they could maintain conversation. Participants were allowed to warmup for 3 minutes, followed by three conditions for data collection: 1) Shod-Control (wearing shoes, without cues); 2) Shod-Quiet (wearing shoes, with cues for quiet and soft landings); and 3) Barefoot-FFS (barefoot, with cues for forefoot strike and to land softly and quietly (FFS)). For each of the gait modification conditions (2 & 3), participants were given instruction on the modification and then allowed to run for at least one minute to acclimate to the new condition before data were collected.

For the Shod-Control condition, participants ran in their normal footwear, without any adjustments to their form. For this condition, participants were also asked to perform a modified-Stroop test, a visual processing cognitive task [32]. As described previously [33], this was used as a distraction method, to keep participants from consciously focusing on their running form. For the Shod-Quiet condition, participants also wore their normal footwear and were provided instructions to land as softly and quietly as possible. Participants were given no specific instruction on how to accomplish this. For the Barefoot-FFS condition, participants ran barefoot and were cued to land on the balls of their feet (i.e., a forefoot strike pattern), as well as to land as softly and quietly as possible. During all three conditions, GRF and IMU data were collected at 1500Hz and 1000Hz respectively, and sagittal-plane video was recorded with a Basler camera (PiA640-210GC, Basler AG, Ahrensburg, Germany) at 125Hz.

Both GRF and IMU data were processed using custom MATLAB scripts (v.2020a, Mathworks, Natick, MA). Force data were low-pass filtered using a 4th order Butterworth filter (cutoff= 50Hz). The primary variable of interest was vertical instantaneous loading rates (LR), calculated as the peak change in vertical forces during early stance [34]. Secondary GRF variables of interest were vertical stiffness during initial loading (“stiffness”) and peak vertical force. Vertical stiffness was calculated as described previously [25,30], as the change in vertical force divided by the vertical change in center of mass position. Additionally, from the accelerometer sensor in the IMU, peak vertical tibial accelerations (TAs) were extracted as described previously [35]. Variables were averaged across all available strides (n≈ 20-25). Foot strike pattern was assessed visually from sagittal plane videos, using criteria previously described by Futrell et al. [34]. In brief, 5 consecutive foot strikes were used to classify patterns. The first frame with visible compression of the shoe sole was identified for each foot strike. The pattern was classified as rearfoot if the rear half of the foot was contacting the ground, forefoot if it was the fore half, and midfoot if both contacted simultaneously. Cadence was calculated in steps per minute.

2.3. Data analysis

Data were analyzed using IBM SPSS (v.26.0, Armonk, NY). Normality was tested using Shapiro-Wilk tests, while sphericity was tested using Mauchly's tests. To assess differences between gait modification conditions, 1x3 repeated measures analysis of variance (ANOVAs) were used for parametric data and Friedman’s tests for non-parametric. Post-hoc comparisons were performed with Wilcoxon signed-rank tests, applying Bonferroni adjustments. A subgroup analysis was performed to explore the strategies used by participants who were able to successfully reduce their vertical LRs by at least 15% in the Shod-Quiet condition. The 15% threshold was selected for two reasons: (1) to produce similarly sized subgroups for comparison; and (2) to approximate the magnitude of acute LR reductions reported in a prior gait retraining study [36]. Independent t-tests were used to compare mean cadence between the groups who were and were not successful in reducing their LRs. Chi-square tests were used to compare the groups for the proportions of participants that changed to a FFS pattern in the Shod-Quiet condition. A priori significance was set to α < 0.05. Effect sizes were determined using Cohen’s d (d). Values of d = 0.2-0.5 were defined as small, d = 0.51-0.8 as medium, and d > 0.8 as large [37].

3. RESULTS

Forty shod, rearfoot-strike runners, none of whom were habitual barefoot runners, were assessed. Three participants were excluded due to missing data, resulting from them being unable to complete the gait assessment due to the nature of their injury or severe pain that inhibited their running. Therefore, 37 runners (18M/19F) were analyzed. Participants presented a mean age of 36.14 (14.41) years, weight of 69.36 (14.57) kg, height of 1.71 (0.09) m, and body mass index of 23.28 (3.03) kg/m2. Self-selected mean speed was 2.51 (0.32) m/s.

There were significant main effects (Figure 1) of gait condition on LR (F1.31,47.31= 12.82, p< 0.001, η2 = 0.26), stiffness (F1.43,51.75= 12.96, p< 0.001, η2 = 0.27), TA (F1.53,55.12= 27.20, p< 0.001, η2 = 0.43), and cadence (F1.70,61.36= 20.58, p< 0.001, η2 = 0.36). Pairwise comparisons demonstrated significantly lower mean LR (mean difference = −14.71, 95%CI = −20.67 to −8.75, d= 0.83), stiffness (mean difference = −11.81, 95%CI = −19.19 to −4.42, d=1.16), and TA (mean difference = −1.71, 95%CI = −2.41 to −1.00, d = 0.86) for Shod-Quiet compared to Shod-Control. Significantly lower LR (mean difference = −21.97, 95%CI = −35.39 to −8.55, d = 0.83), stiffness (mean difference = −24.19, 95%CI = −38.30 to −10.08, d = 0.47), TA (mean difference = −2.87, 95%CI = −4.06 to −1.67, d = 1.16), and higher cadence (mean difference = 6.05, 95%CI = 3.55 to 8.56, d = 0.78) were found for Barefoot-FFS vs Shod-Control. Finally, TA was significantly lower (mean difference = −1.17, 95%CI = −2.16 to −0.18, d = 0.49), and cadence higher (mean difference = 7.29, 95%CI = 3.69 to 10.89, d = 0.65), in Barefoot-FFS compared to Shod-Quiet.

Figure 1.

Figure 1.

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Peak instantaneous loading rate (LR) of the vertical GRF was lower in the Shod-Quiet and Barefoot-FFS conditions compared to the Shod-Control condition. Error bars represent standard error of the mean; **p<0.001.

Twenty-five (67.6%) participants were able to reduce their LR by ≥15% in the Shod-Quiet condition. However, the subgroup analyses showed that there were no significant differences in mean cadence (mean difference= 3.89 steps per minute, p= 0.34, d= 0.33) or the proportion of participants who changed to a FFS pattern (24% vs 8.3%, p= 0.389) between participants who were or were not successful in reducing their LRs. Strictly for comparison, 29 (78.4%) of participants were able to meet the 15% LR reduction threshold in the Barefoot-FFS condition.

4. DISCUSSION

To our knowledge, this is the first study to investigate in injured runners the acute effects of landing softly and quietly without additionally providing specific kinematic or spatiotemporal cues. When cued to run quietly and softly, runners significantly reduced their vertical LRs, without a significant difference in reductions compared to those achieved in the Barefoot-FFS condition. Peak instantaneous vertical LRs were lower in the Shod-Quiet and Barefoot-FFS conditions compared to the Shod-Control condition. The mean reductions in LR when transitioning from the Shod-Control condition to the Shod-Quiet (14%) and Barefoot-FFS (30.5%) conditions in this study were similar to those reported in previous studies that used a faded two-week design (18-34%) [36,38,39]. Further, these reductions are similar to those reported in a previous study (19.1%) demonstrating that runners who were cued to lower their vertical LRs had a significantly reduced risk of prospective injury [40].

We also found reductions in secondary variables of interest during the Shod-Quiet condition. Vertical stiffness at initial loading was decreased compared to the Shod-Control condition, and the reduction was to a similar degree in the Barefoot-FFS condition. Previous research evaluating vertical stiffness at initial loading has found that magnitudes of over 73 kN/m are associated with running-related injuries, including patellofemoral pain, plantar fasciitis, and Achilles tendinopathy [30]. In this study, the mean vertical stiffness in the Shod-Control group was over 73 kN/m, but it was reduced below this threshold in both the Shod-Quiet and Barefoot-FFS conditions. Vertical TA was also reduced in the Shod-Quiet condition compared to the Shod-Control condition. Although the Shod-Quiet condition showed higher mean TA compared to the Barefoot-FFS condition, the effect size was small. Further, vertical LRs were similar between the conditions, and there is considerably more support for the association of injury to vertical LRs compared to TA.

Cadence was not different between the Shod-Control and Shod-Quiet conditions. Interestingly, within the Shod-Quiet condition, there also was no difference in cadence between those who were and were not successful at reducing their LR by at least 15%. There was similarly no difference in the proportion of runners who changed to a FFS pattern among those who did and did not reduce their LR by at least 15%. However, these subgroup analyses were perhaps limited by insufficient power, with 25 runners who met the LR threshold and 12 who did not. Regardless of potential sample limitations, only a small proportion of runners who successfully reduced their LRs transitioned to a FFS pattern (24%).

While reductions in LRs were similar between the Shod-Quiet and Barefoot-FFS conditions, the results of our secondary analyses showed that reductions in the Shod-Quiet condition were not the result of a consistent change in foot strike pattern or cadence. Targeting these two variables has been shown to be an effective method of reducing LRs [28,39]. However, our results suggest that runners likely used a variety of strategies to achieve the reductions, instead of one dominant strategy. Of note, our study was unable to capture either precise foot strike angle or sagittal plane knee stiffness, variables previously shown to be relevant for vertical LRs and LR reduction [26,27,41].

Cues to land quietly or softly are frequently [18,24] – but not always – characterized as external attentional foci. For example, Schuker et al. [42] considered a focus on how the body feels during movement to be an internal focus of attention. A key outcome of their study was that attention to highly automated body processes was disruptive to performance, while attention to how the body feels did not affect movement efficiency. Their findings, despite different characterization of internal versus external foci, support the idea that focusing on quiet and soft landings may be superior to focusing on the movement of individual body segments, improving movement efficiency.

4.1. Limitations & Future Directions

The findings in this study cannot be expanded to infer long-term adaptations that may arise from gait retraining using similar cues. For example, runners who acutely reduced their LRs in a barefoot habituation study ultimately had increased LRs following 8 weeks of gait retraining [43]. However, several studies have shown chronic adaptations following two weeks of gait retraining, with runners cued to land softly demonstrating reduced LRs compared to baseline [40,44]. Both studies required real-time GRF data as visual feedback during retraining. It would be interesting to know whether the magnitude of reduction would be similar if injured runners were simply given cues for quiet and soft landings, particularly when considering that instrumented treadmills may be cost-prohibitive for clinicians and researchers.

The findings in this study similarly cannot be applied to healthy, pain-free runners. However, promising evidence exists that a similar intervention could facilitate LR reduction in healthy runners. Augustine et al. [19] evaluated audio-based biofeedback acutely and after one week of training in healthy, pain-free runners. They used a smartphone app to provide biofeedback in response to decibel level associated with foot strike. They found reductions in peak vertical GRF and LRs in response to this intervention.

This study evaluated injured runners presenting to a running clinic. The sample, being one of convenience, was heterogeneous: running mileage was variable, level of competition ranged from recreational runners to collegiate runners and those training for marathons, and age ranged from 16 to 69. As a result, we are unable to extrapolate findings to dissimilar running populations and settings. For example, it is unclear whether similar responses to cues for quiet and soft landings would be expected among healthy runners engaged in injury prevention programs, in running over varied terrain, or in studies focused on a specific population of interest (e.g., adolescents, older adults, elite athletes).

Future research is needed to evaluate the specific strategies and muscle activation patterns utilized during these conditions, whether and how these conditions impact running economy, and whether the acute responses observed in this study are similar to those resulting from participation in a gait retraining program. As an external focus of attention may result in more natural, participant-specific kinematic strategies, the findings of this and similar research may lead to more effective gait retraining protocols with decreased risk of injury development.

4.2. Conclusions

When runners focused on quiet, soft landings, they were able to reduce their LRs to a similar degree as barefoot running with a FFS pattern. While several specific gait alterations have previously demonstrated efficacy in reducing LRs, the strategies implemented in gait retraining often require an internal focus of attention. The strategies used to achieve reductions in LRs were heterogeneous across our cohort. Thus, it is possible that promoting an external focus of attention may allow a runner to self-select the strategy that is most natural for their body, an outcome that could have implications for improving patient compliance and enhancing motor learning.

Figure 2.

Figure 2.

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Secondary variables of interest across the three study conditions. Vertical GRF was similar across conditions, while peak vertical TA, cadence, and stiffness at initial loading differed between conditions. Error bars represent standard error of the mean; * p < .05, ** p < .001.

HIGHLIGHTS.

  • Cues for quiet, soft landings resulted in acute reductions in loading rates in runners

  • Loading rate reductions were similar to a barefoot, forefoot strike condition Strategies used to achieve quiet, soft landings were varied between runners

Acknowledgements:

For their assistance with and contributions to the development and achievement of this research, the authors would like to thank Josh Posilkin and Molly Bradach.

DECLARATION OF COMPETING INTEREST

Adam Tenforde serves as Senior editor for PM&R Journal. He gives professional talks such as grand rounds and medical conference plenary lectures and receives honoraria from conference organizers. He has participated in research funded by Arnold P. Gold Foundation (physician and patient care disparities), Football Player Health Study at Harvard (health in American-Style Football players), American Medical Society for Sports Medicine (bone density research), Uniform Health Service and Enovis (Achilles tendinopathy) and MTEC/Department of Defense (knee arthritis). He is a paid consultant for State Farm Insurance and Strava. All other authors declare that they have no competing interests.

Footnotes

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6.

COMPETING INTERESTS

The authors declare that they have no competing interests.

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