Effects of altered stride frequency and contact time on leg-spring behavior in human running

Kinematic alterations after two high-intensity intermittent training protocols in endurance runners

PURPOSE

This study aimed to evaluate running kinematic characteristics during the early and late stages of 2 high-intensity intermittent training (HIIT) protocols with similar external load but different average running pace, as well as to compare the fatigue-induced changes during both HIIT protocols at a kinematic level.

METHODS

Eighteen endurance runners were tested on a track on 2 occasions: 10 runs of 400 m with 90-120 s recovery between running bouts (10 × 400 m), and 40 runs of 100 m with 25-30 s recovery between running bouts (40 × 100 m). Heart rate was monitored during both protocols; blood lactate accumulation and rate of perceived exertion were recorded after both exercises. A high-speed camera was used to measure sagittal-plane kinematics at the first and last runs during both HIIT protocols. The dependent variables were spatial-temporal parameters (step length and contact and flight time), joint angles during support (relative angles of the hip, knee, and ankle), and foot strike pattern.

RESULTS

High levels of exhaustion were reached by the athletes during both workouts (blood lactate accumulation >12 mmol/L, rate of perceived exertion >15; peak heart rate (HR_peak) > 176 bpm). A within-protocol paired t test (first vs. last run) revealed no significant changes (p ≥ 0.05) in kinematic variables during any of the HIIT sessions. A between-protocol comparison with the first run of each protocol revealed the effect of running speed on kinematics: +2.44 km/h during the 40 × 100 m: shorter contact and flight time (p  ≤  0.01) and longer step length (p = 0.001); greater hip flexion (p = 0.031) and ankle extension (p = 0.001) at initial contact; smaller knee and ankle flexion (p < 0.001) at midstance; and greater hip extension at toe-off (p < 0.001).

CONCLUSION

HIIT sessions including runs for 15-90 s and performed at intensity above the velocity associated with maximal oxygen uptake did not consistently perturb the running kinematics of trained endurance runners.

The Effect of Prosthetic Alignment on Prosthetic and Total Leg Stiffness While Running With Simulated Running-Specific Prostheses

Running-specific prostheses (RSP) are designed to replicate the spring-like behavior of the biological leg in people with a lower limb amputation. Running performance strongly depends on stiffness of the RSP. The aim of this study was to investigate the effects of angle of alignment of the RSP on its stiffness, and how this affects total leg stiffness and the gait pattern during running. Ten able-bodied athletes performed eight trials on a treadmill with running-specific prosthetic simulators, while the alignment of the blades relative to the socket was set in four different angles (0, 5, 10, and 15°) during two different step frequency conditions (free and imposed). RSP stiffness, total leg stiffness, residual leg stiffness, and spatiotemporal parameters were measured. In both step frequency conditions, the RSP stiffness decreased linearly with increasing angle of alignment. Able bodied athletes were able to compensate for the decreased RSP stiffness, and keep total leg stiffness almost invariant, by increasing residual leg stiffness through a more straight the knee at initial contact. This study confirms that alignment is an important factor to take into account when optimizing the RSP. Whether the observed compensations are feasible in amputee athletes needs further investigation.

Influence of excess weight on lower-extremity vertical stiffness and metabolic cost of walking

The purpose was to test whether lower-extremity vertical stiffness and gait mechanics explain differences in energy cost of walking (Cw) between individuals with normal weight (NW) and obesity (OB). Ten OB (33.1 ± 2.0 kg m^-2) and 10 NW (24.2 ± 1.3 kg m^-2) walked for six minutes on an instrumented treadmill at 1.25 m s^-1 while Cw, lower-extremity kinematics, and vertical stiffness (K _vert) were measured. NW completed another trial with a loaded vest (NWL) to simulate the BMI of the obese group. Cw was 24% greater in OB (277.5 ± 45.3 J m^-1) and 23% greater in NWL (272.7 ± 35.7 J m^-1) than NW (211.0 ± 27.0 J m^-1, P < 0.005). Mass-specific Cw (Cw_kg) wasn't different between conditions (P = 0.085). Lower-extremity K _vert was 40% higher in OB (32.7 ± 5.2 kN m^-1) than NW (23.3 ± 4.7 kN m^-1, P < 0.001), but neither was different from NWL (27.5 ± 3.4 kN m^-1, P > 0.05). Mass-specific K _vert (P = 0.081) was similar across conditions. K _vert was related to Cw (r = 0.55, P = 0.001). Cw_kg wasn't different between NW or OB, but there was a negative correlation between BMI and Cw_kg driven by lower Cw_kg in NWL. Cw and K _vert covaried in proportion to body mass, but mass-specific K _vert was unrelated to Cw_kg. Mass-specific K _vert was lower in NWL than OB due to NWL's greater angle of attack, center of mass displacement, and joint range of motion.

Regular changes in foot strike pattern during prolonged downhill running do not influence neuromuscular, energetics, or biomechanical parameters

Research has suggested that a high variability in foot strike pattern during downhill running is associated with lower neuromuscular fatigue of the plantar flexors (PF). Given the popularity of trail running, we designed an intervention study to investigate whether a strategy with regular changes in foot strike pattern during downhill running could reduce the extent of fatigue on neuromuscular, energetics and biomechanical parameters as well as increase an uphill time-to-exhaustion trial (TTE) performance. Fourteen experienced trail runners completed two interventional conditions (separated by 15 days) in a pseudo-randomised and counter-balanced order that consisted of 2.5-h of treadmill graded running with (switch condition) or without (control condition) a change between fore- and rear-foot strike pattern every 30 s during the downhill sections. Pre and Post, neuromuscular tests were performed to assess PF central and peripheral fatigue. Energy cost of running was assessed using an indirect calorimetry system and biomechanical gait parameters were acquired with an instrumented treadmill. TTE was performed after both the graded running conditions. There were not significant condition × time interactions (p ≥ .085) for any of the variables considered, and TTE was not different between the two conditions (p = .755). A deliberate strategy to alternate between foot strike patterns did not reduce the extent of fatigue during prolonged graded running. We suggest that it is not the ability to switch between foot strike patterns that minimises fatigue; rather the ability to adapt foot strike pattern to the terrain and therefore a better running technique.

Landing-Takeoff Asymmetries Applied to Running Mechanics: A New Perspective for Performance

BACKGROUND

Elastic bouncing is a physio-mechanical model that can elucidate running behavior in different situations, including landing and takeoff patterns and the characteristics of the muscle-tendon units during stretch and recoil in running. An increase in running speed improves the body's elastic mechanisms. Although some measures of elastic bouncing are usually carried out, a general description of the elastic mechanism has not been explored in running performance. This study aimed to compare elastic bouncing parameters between the higher- and lower-performing athletes in a 3000 m test.

METHODS

Thirty-eight endurance runners (men) were divided into two groups based on 3000 m performance: the high-performance group (Phigh; n = 19; age: 29 ± 5 years; mass: 72.9 ± 10 kg; stature: 177 ± 8 cm; 3000time: 656 ± 32 s) and the low-performance group (Plow; n = 19; age: 32 ± 6 years; mass: 73.9 ± 7 kg; stature: 175 ± 5 cm; 3000time: 751 ± 29 s). They performed three tests on different days: (i) 3000 m on a track; (ii) incremental running test; and (iii) a running biomechanical test on a treadmill at 13 different speeds from 8 to 20 km h-1. Performance was evaluated using the race time of the 3000 m test. The biomechanics variables included effective contact time (t ce), aerial time (t ae), positive work time (t push), negative work time (t break), step frequency (f step), and elastic system frequency (f sist), vertical displacement (S v) in t ce and t ae (S ce and S ae), vertical force, and vertical stiffness were evaluated in a biomechanical submaximal test on treadmill.

RESULTS

The t ae, f sist, vertical force and stiffness were higher (p < 0.05) and t ce and f step were lower (p < 0.05) in Phigh, with no differences between groups in t push and t break.

CONCLUSION

The elastic bouncing was optimized in runners of the best performance level, demonstrating a better use of elastic components.

How does the slope gradient affect spatiotemporal parameters during running? Influence of athletic level and vertical and leg stiffness

BACKGROUND

The current evidence leaves certain questions unanswered, including whether well-trained athletes adapt to different slope gradients in the same way as amateurs, and whether stiffness influences spatiotemporal adaptations during uphill running.

RESEARCH QUESTION

This study aimed to determine the effect of different slope gradients (0%-11%) on spatiotemporal gait characteristics during running, taking into account the influence of athletic level, vertical and leg stiffness.

METHODS

Male endurance runners (12 amateurs, 10 highly-trained) performed a running test on a motorized treadmill. The running velocity was set at 12 km/h, and participants completed six different running conditions (0, 3, 5, 7, 9 and 11% gradients). Spatiotemporal parameters were measured using the OptoGait system. Vertical (Kvert) and leg (Kleg) stiffness were calculated according to the sine-wave method.

RESULTS

A 2 (amateur; highly-trained) × 6 (running conditions) ANOVA found no significant between-group differences in spatiotemporal parameters at any gradient (P ≥ 0.05); however, significant Kvert and Kleg differences (P < 0.05) were found within both groups with increasing gradients. Stepwise linear regression analysis showed that Kleg was strongly associated with contact time (R²  = 0.797, P < 0.001), whereas Kvert was associated with spatiotemporal adaptations to different slope gradients (R²  = 0.547, P = 0.002).

SIGNIFICANCE

An increased slope gradient (0-11%) at a given running velocity (12 km.h^-1) caused spatiotemporal adaptations (i.e., increased CT and SF and decreased FT, SL and SA) regardless of the athletic level of the runner, although a non-significant trend differentiated the adaptations between the amateur and highly-trained groups. The results also indicated that leg stiffness plays a key role in the characteristics of spatiotemporal gait during level running, whereas vertical stiffness is strongly associated with spatiotemporal adaptations when running uphill.

Step frequency patterns of elite ultramarathon runners during a 100-km road race

Step frequency (SF) in running has received substantial interest from researchers, coaches, therapists, and runners. It has been widely studied in controlled settings, but no published study has measured it continuously in elite-level competition. The present study used wrist-based accelerometers in consumer-grade watches to monitor SF and SF variability of competitors in the 2016 100-km World Championship road race. Using linear mixed-model regression, SF and SF variability were assessed across the race. The average SF (steps-per-minute) of competitors ( n = 20) was 182.0 spm (range: 155.4-203.1 spm). Race fluctuations in SF were influenced only by the speed the competitors were running, with faster speeds being associated with greater SF (5.6 spm/m·s^-1, P < 0.001). Independently of this speed relation, SF did not significantly change over the course of the race. SF was further linked to the runner's stature (-123.1 spm/m, P = 0.01) but not significantly related to sex, weight, age, or years of experience. The SF coefficient-of-variation was inversely associated with running speed and distance covered, with runners demonstrating decreasing variability both at faster speeds and as the race progressed. Together, these results add ecological evidence to observations of a speed dependency of SF in a highly trained, elite population of runners and suggest that in road race conditions, SF changes only with speed and not fatigue. Furthermore, it presents evidence that the variability of an elite runner's SF is linked to both speed and fatigue but not to any other characteristics of the runner. The current findings are important for runners, clinicians, and coaches as they seek to monitor or manipulate SF. NEW & NOTEWORTHY Stride frequency (SF; or the synonymous "cadence") has become a popular point of monitoring and manipulation in runners. Advances in wearable technology have enabled continuous monitoring of SF. This study is the first to examine SF and SF variability patterns throughout an entire road race in elite ultramarathon runners. This adds ecological, normative data to the field's understanding of SF and demonstrates how it relates to running speed, fatigue, and individual characteristics.

The implications of time on the ground on running economy: less is not always better

A lower duty factor (DF) reflects a greater relative contribution of leg swing to ground contact time during the running step. Increasing time on the ground has been reported in the scientific literature to both increase and decrease the energy cost (EC) of running, with DF reported to be highly variable in runners. As increasing running speed aligns running kinematics more closely with spring-mass model behaviors and re-use of elastic energy, we compared the centre of mass (COM) displacement and EC between runners with a low (DFlow) and high (DFhigh) duty factor at typical endurance running speeds. Forty well-trained runners were divided in two groups based on their mean DF measured across a range of speeds. EC was measured from 4-min treadmill runs at 10, 12, and 14 km·h−1 using indirect calorimetry. Temporal characteristics and COM displacement data of the running step were recorded from 30-s treadmill runs at 10, 12, 14, 16, and 18 km·h−1. Across speeds, DFlow exhibited more symmetrical patterns between braking and propulsion phases in terms of time and vertical COM displacement than DFhigh. DFhigh limited global vertical COM displacements in favor of horizontal progression during ground contact. Despite these running kinematics differences, no significant difference in EC was observed between groups. Therefore, both DF strategies seem energetically efficient at endurance running speeds.

Using stiffness to assess injury risk: comparison of methods for quantifying stiffness and their reliability in triathletes

Background

A review of the literature has indicated that lower body stiffness, defined as the extent to which the lower extremity joints resists deformation upon contact with the ground, may be a useful measure for assessing Achilles injury risk in triathletes. The nature of overuse injuries suggests that a variety of different movement patterns could conceivably contribute to the final injury outcome, any number and combination of which might be observed in a single individual. Measurements which incorporate both kinetics and kinematics (such as stiffness) of a movement may be better able to shed light on individuals at risk of injury, with further analysis then providing the exact mechanism of injury for the individual. Stiffness can be measured as vertical, leg or joint stiffness to model how the individual interacts with the environment upon landing. However, several issues with stiffness assessments limit the effectiveness of these measures to monitor athletes’ performance and/or injury risk. This may reflect the variety of common biomechanical stiffness calculations (dynamic, time, true leg and joint) that have been used to examine these three stiffness levels (vertical, leg and joint) across a variety of human movements (i.e. running or hopping) as well as potential issues with the reliability of these measures, especially joint stiffness. Therefore, the aims of this study were to provide a comparison of the various methods for measuring stiffness during two forms of human bouncing locomotion (running and hopping) along with the measurement reliability to determine the best methods to assess links with injury risk in triathletes.

Methods

Vertical, leg and joint stiffness were estimated in 12 healthy male competitive triathletes on two occasions, 7 days apart, using both running at 5.0 ms⁻¹ and hopping (2.2 Hz) tasks.

Results

Inter-day reliability was good for vertical (ICC = 0.85) and leg (ICC = 0.98) stiffness using the time method. Joint stiffness reliability was poor when assessed individually. Reliability was improved when taken as the sum of the hip, knee and ankle (ICC = 0.86). The knee and ankle combination provided the best correlation with leg stiffness during running (Pearson’s Correlation = 0.82).

Discussion

The dynamic and time methods of calculating leg stiffness had better reliability than the “true” method. The time and dynamic methods had the best correlation with the different combinations of joint stiffness, which suggests that they should be considered for biomechanical screening of triathletes. The knee and ankle combination had the best correlation with leg stiffness and is therefore proposed to provide the most information regarding lower limb mechanics during gait in triathletes.

Effects of upper and lower body wearable resistance on spatio-temporal and kinetic parameters during running

Wearable resistance training involves added load attached directly to the body during sporting movements. The effects of load position during running are not yet fully established. Therefore, the purpose of this research was to determine spatio-temporal and kinetic characteristics during submaximal running using upper, lower and whole-body wearable resistance (1-10% body mass (BM)). Twelve trained male runners completed eight 2-min treadmill running bouts at 3.9 m/s with and without wearable resistance. The first and last bouts were unloaded, while the middle 6 were randomised wearable resistance conditions: upper body (UB) 5% BM, lower body (LB) 1%, 3%, 5% BM and whole body (WB) 5%, 10% BM. Wearable resistance of 1-10% BM resulted in a significant increase in heart rate (5.40-8.84%), but minimal impact on spatio-temporal variables. Loads of 5% BM and greater caused changes in vertical stiffness, vertical and horizontal force, and impulse. Functional and effective propulsive force (2.95%, 2.88%) and impulse (3.40%, 3.38%) were significantly (p < 0.05) greater with LB5% than UB5%. Wearable resistance may be used to increase muscular kinetics during running without negatively impacting spatio-temporal variables. The application of these findings will vary depending on athlete goals. Future longitudinal studies are required to validate training contentions.

Lack of Influence of Muscular Performance Parameters on Spatiotemporal Adaptations With Increased Running Velocity

Roche-Seruendo, LE, García-Pinillos, F, Haicaguerre, J, Bataller-Cervero, AV, Soto-Hermoso, VM, and Latorre-Román, PÁ. Lack of influence of muscular performance parameters on spatiotemporal adaptations with increased running velocity. J Strength Cond Res 32(2): 409-415, 2018-This study aimed to analyze the influence of muscular performance parameters on spatiotemporal gait characteristics during running when gradually increasing speed. Fifty-one recreationally trained male endurance runners (age, 28 ± 8 years) voluntarily participated in this study. Subjects performed a battery of jumping tests (squat jump, countermovement jump, and 20-cm drop jump), and after that, the subjects performed an incremental running test (10-20 km·h) on a motorized treadmill. Spatiotemporal parameters were measured using the OptoGait system. Cluster k-means analysis grouped subjects according to the jumping test performance, by obtaining a group of good jumpers (n = 19) and a group of bad jumpers (n = 32). With increased running velocity, contact time was shorter and flight time and step length were longer, whereas cadence and stride angle were greater (p < 0.001). No significant differences between groups (p ≥ 0.05) were found at any running speed. The results obtained indicate that increased running velocity produced no differences in spatiotemporal adaptations between those runners with good jumping ability and those with poor jumping ability. Based on that, it seems that muscular performance parameters do not play a key role in spatiotemporal adaptations experienced by recreational endurance runners with increased velocity. However, taken into consideration the well-known relationship between running performance and neuromuscular performance, the authors suggest that muscular performance parameters would be much more determinant in the presence of fatigue (exhausted condition) or in the case of considering other variables such as running economy or kinetic.

Neuromechanical adaptations to slippery sport shoes

Although shoe friction has been widely studied in occupational ergonomics, information was lacking about friction in sport shoes. The purpose of the study was to examine the neuromechanical adaptations to different shoe-surface interface in an aerobic-gym specific movement. Sixteen females performed 10 change of direction movements in two shoe conditions differing by their outsoles (ethyl-vinyl-acetate: EVA and rubber: RB) to ensure significant differences in mechanical coefficients of friction (EVA = 0.73 ± 0.07 and RB = 1.46 ± 0.15). The kinematics, kinetics and muscle activities of the right lower-limb were analysed. Statistical parametric mapping was used to investigate the kinematics and kinetics adaptation to the different shoe-surface coefficients of friction. The participants had a longer stance duration in the EVA compared to the RB condition (526 ± 160 ms vs. 430 ± 151 ms, p < .001). The ankle and knee joints powers and works were lower during both the braking and the push-off phases in the EVA as compared to the RB condition. Preactivation of the agonist muscles (soleus, gastrocnemius medialis and vastus medialis) decreased in the EVA compared to the RB condition (-28.5%, -26.5% and -49.0%, respectively). Performing a change of direction movement with slippery shoes reduced the ankle and knee joints loadings, but impaired the stretch-shortening cycle performance. Participants demonstrated thus a different neuromechanical strategy to control their movement which was associated with a reduced performance.

Stiffness as a Risk Factor for Achilles Tendon Injury in Running Athletes

BACKGROUND

Overuse injuries are multifactorial resulting from cumulative loading. Therefore, clear differences between normal and at-risk individuals may not be present for individual risk factors. Using a holistic measure that incorporates many of the identified risk factors, focusing on multiple joint movement patterns may give better insight into overuse injuries. Lower body stiffness may provide such a measure.

OBJECTIVE

To identify how risk factors for Achilles tendon injuries influence measures of lower body stiffness.

METHODS

SPORTDiscus, Web of Science, CINAHL and PubMed were searched for Achilles tendon injury risk factors related to vertical, leg and joint stiffness in running athletes.

RESULTS

Increased braking force and low surface stiffness, which were clearly associated with increased risk of Achilles tendon injuries, were also found to be associated with increased lower body stiffness. High arches and increased vertical and propulsive forces were protective for Achilles tendon injuries and were also associated with increased lower body stiffness. Risk factors for Achilles tendon injuries that had unclear associations were also investigated with the evidence trending towards an increase in leg stiffness and a decrease in ankle stiffness being detrimental to Achilles tendon health.

CONCLUSION

Few studies have investigated the link between lower body stiffness and Achilles injury. High stiffness is potentially associated with risk factors for Achilles tendon injuries although some of the evidence is controversial. Prospective injury studies are needed to confirm this relationship. Large amounts of high-intensity or high-speed work or running on soft surfaces such as sand may increase Achilles injury risk. Coaches and clinicians working with athletes with new or reoccurring injuries should consider training practices of the athlete and recommend reducing speed or sand running if loading is deemed to be excessive.

Positive Effects of Augmented Feedback to Reduce Time on Ground in Well-Trained Runners

CONTEXT

Successful elite sprint to long-distance runners are known to have shorter ground-contact time (GCT) than their less successful counterparts.

PURPOSE

To investigate whether augmented feedback (aF) about GCT can reduce the time on ground (TOG) per minute in long-distance runners and, if so, whether this reduction improves running performance.

METHODS

Thirty well-trained runners were allocated to 3 groups. The intervention group (IG) received visual aF about their GCT during 8 high-intensity interval sessions in the 4-wk training period and were instructed to minimize GCT. The 1st control group (CG1) trained with the IG but was not given any feedback. The 2nd control group (CG2) followed their own training routine. Data were obtained pre- and postintervention for all 3 groups. The dependent variable was TOG per minute, computed from step frequency and GCT.

RESULTS

The IG significantly reduced TOG (P = .043, -1.7%, 90%CL -3.1;-0.3) and improved their mean 10 × 400-m performance time (P < .001, -1.5%, 90%CL -1.9;-1.1). In contrast, the 2 control groups revealed unchanged values, indicating that normal high-intensity training and an individualized routine without aF were not able to reduce TOG. The fact that CG1 received the same instructions and participated in the same training sessions as the IG underlined that aF was crucial to reduce TOG.

CONCLUSIONS

The provision of aF about GCT seems to be a promising approach that should be considered during training practice of well-trained runners.

Effects of different percentages of body weight support on spatiotemporal step characteristics during running

This study aimed to determine the effect of different percentages of body weight support (BWS) on spatiotemporal step characteristics during running. 26 endurance runners (age: 37 ± 9 years) completed a running treadmill protocol consisting of 6 different conditions (BWS combinations: 0-50%), with velocity maintained at 12 km/h. Each condition lasted 1 minute. Step angle, ground contact time (CT), flight time (FT), step length (SL) and frequency (SF), and duration of phases during stance time (phase1: initial contact; phase2: midstance; phase3: propulsion) were measured for every step during the test using a photoelectric cell system. Compared with the baseline condition (100% BW), FT was longer, CT was shorter, SL was longer, SF was lower, and the step angle was higher with each increase in BWS (p < 0.05). Also, some changes were observed in the duration of phases during stance time: phase1 did not experience changes across experimental conditions (p = 0.096), phase2 decreased and phase3 increased as BW was supported (p < 0.05). These results indicate that as BW was supported, runners showed longer FT and SL, shorter CT, lower SF, and greater step angle as well as some changes in the phases during the ground contact. Therefore, this study highlights the effect of different percentages of BWS on spatiotemporal parameters.

Optimal stride frequencies in running at different speeds

During running at a constant speed, the optimal stride frequency (SF) can be derived from the u-shaped relationship between SF and heart rate (HR). Changing SF towards the optimum of this relationship is beneficial for energy expenditure and may positively change biomechanics of running. In the current study, the effects of speed on the optimal SF and the nature of the u-shaped relation were empirically tested using Generalized Estimating Equations. To this end, HR was recorded from twelve healthy (4 males, 8 females) inexperienced runners, who completed runs at three speeds. The three speeds were 90%, 100% and 110% of self-selected speed. A self-selected SF (SFself) was determined for each of the speeds prior to the speed series. The speed series started with a free-chosen SF condition, followed by five imposed SF conditions (SFself, 70, 80, 90, 100 strides·min-1) assigned in random order. The conditions lasted 3 minutes with 2.5 minutes of walking in between. SFself increased significantly (p<0.05) with speed with averages of 77, 79, 80 strides·min-1 at 2.4, 2.6, 2.9 m·s-1, respectively). As expected, the relation between SF and HR could be described by a parabolic curve for all speeds. Speed did not significantly affect the curvature, nor did it affect optimal SF. We conclude that over the speed range tested, inexperienced runners may not need to adapt their SF to running speed. However, since SFself were lower than the SFopt of 83 strides·min-1, the runners could reduce HR by increasing their SFself.

Preventing running-related injuries using evidence-based online advice: the design of a randomised-controlled trial

Introduction

Running-related injuries (RRIs) are frequent and can lead to cessation of health promoting activities. Several risk factors for RRIs have been identified. However, no successful injury prevention programme has been developed so far. Therefore, the aim of the present study is to investigate the effect of an evidence-based online injury prevention programme on the number of RRIs.

Methods and analysis

The INSPIRE trial is a randomised-controlled trial with a 3-month follow-up. Both novice and more experienced runners, aged 18 years and older, who register for a running event (distances 5 km up to 42.195 km) will be asked to participate in this study. After completing the baseline questionnaire, participants will be randomised into either the intervention group or control group. Participants in the intervention group will get access to the online injury prevention programme. This prevention programme consists of information on evidence-based risk factors and advices to reduce the injury risk. The primary outcome measure is the number of self-reported RRIs in the time frame between registration for a running event and 1 month after the running event. Secondary outcome measures include the running days missed due to injuries, absence of work or school due to injuries, and the injury location.

Ethics and dissemination

An exemption for a comprehensive application is obtained by the Medical Ethical Committee of the Erasmus University Medical Centre Rotterdam, Netherlands. The results of the study will be published in peer-reviewed journals and presented on international congresses.

Trial registration number

NTR5998. Pre-results

Is There an Economical Running Technique? A Review of Modifiable Biomechanical Factors Affecting Running Economy

Running economy (RE) has a strong relationship with running performance, and modifiable running biomechanics are a determining factor of RE. The purposes of this review were to (1) examine the intrinsic and extrinsic modifiable biomechanical factors affecting RE; (2) assess training-induced changes in RE and running biomechanics; (3) evaluate whether an economical running technique can be recommended and; (4) discuss potential areas for future research. Based on current evidence, the intrinsic factors that appeared beneficial for RE were using a preferred stride length range, which allows for stride length deviations up to 3 % shorter than preferred stride length; lower vertical oscillation; greater leg stiffness; low lower limb moment of inertia; less leg extension at toe-off; larger stride angles; alignment of the ground reaction force and leg axis during propulsion; maintaining arm swing; low thigh antagonist–agonist muscular coactivation; and low activation of lower limb muscles during propulsion. Extrinsic factors associated with a better RE were a firm, compliant shoe–surface interaction and being barefoot or wearing lightweight shoes. Several other modifiable biomechanical factors presented inconsistent relationships with RE. Running biomechanics during ground contact appeared to play an important role, specifically those during propulsion. Therefore, this phase has the strongest direct links with RE. Recurring methodological problems exist within the literature, such as cross-comparisons, assessing variables in isolation, and acute to short-term interventions. Therefore, recommending a general economical running technique should be approached with caution. Future work should focus on interdisciplinary longitudinal investigations combining RE, kinematics, kinetics, and neuromuscular and anatomical aspects, as well as applying a synergistic approach to understanding the role of kinetics.

Reduced prosthetic stiffness lowers the metabolic cost of running for athletes with bilateral transtibial amputations

Inspired by the springlike action of biological legs, running-specific prostheses are designed to enable athletes with lower-limb amputations to run. However, manufacturer’s recommendations for prosthetic stiffness and height may not optimize running performance. Therefore, we investigated the effects of using different prosthetic configurations on the metabolic cost and biomechanics of running. Five athletes with bilateral transtibial amputations each performed 15 trials on a force-measuring treadmill at 2.5 or 3.0 m/s. Athletes ran using each of 3 different prosthetic models (Freedom Innovations Catapult FX6, Össur Flex-Run, and Ottobock 1E90 Sprinter) with 5 combinations of stiffness categories (manufacturer’s recommended and ± 1) and heights (International Paralympic Committee’s maximum competition height and ± 2 cm) while we measured metabolic rates and ground reaction forces. Overall, prosthetic stiffness [fixed effect (β) = 0.036; P = 0.008] but not height ( P ≥ 0.089) affected the net metabolic cost of transport; less stiff prostheses reduced metabolic cost. While controlling for prosthetic stiffness (in kilonewtons per meter), using the Flex-Run (β = −0.139; P = 0.044) and 1E90 Sprinter prostheses (β = −0.176; P = 0.009) reduced net metabolic costs by 4.3–4.9% compared with using the Catapult prostheses. The metabolic cost of running improved when athletes used prosthetic configurations that decreased peak horizontal braking ground reaction forces (β = 2.786; P = 0.001), stride frequencies (β = 0.911; P < 0.001), and leg stiffness values (β = 0.053; P = 0.009). Remarkably, athletes did not maintain overall leg stiffness across prosthetic stiffness conditions. Rather, the in-series prosthetic stiffness governed overall leg stiffness. The metabolic cost of running in athletes with bilateral transtibial amputations is influenced by prosthetic model and stiffness but not height.

NEW & NOTEWORTHY We measured the metabolic rates and biomechanics of five athletes with bilateral transtibial amputations while running with different prosthetic configurations. The metabolic cost of running for these athletes is minimized by using an optimal prosthetic model and reducing prosthetic stiffness. The metabolic cost of running was independent of prosthetic height, suggesting that longer legs are not advantageous for distance running. Moreover, the in-series prosthetic stiffness governs the leg stiffness of athletes with bilateral leg amputations.

The effect of unilateral arm swing motion on lower extremity running mechanics associated with injury risk

Many field sports involve equipment that restricts one or both arms from moving while running. Arm swing during running has been examined from a biomechanical and physiologic perspective but not from an injury perspective. Moreover, only bilateral arm swing suppression has been studied with respect to running. The purpose of this study was to determine the influence of running with one arm restrained on lower extremity mechanics associated with running or sport-related injury. Fifteen healthy participants ran at a self-selected speed with typical arm swing, with one arm restrained and with both arms restrained. Lower extremity kinematics and spatiotemporal measures were analysed for all arm swing conditions. Running with one arm restrained resulted in increased frontal plane knee and hip angles, decreased foot strike angle, and decreased centre of mass vertical displacement compared to typical arm swing or bilateral arm swing restriction. Stride length was decreased and step frequency increased when running with one or both arms restrained. Unilateral arm swing restriction induces changes in lower extremity kinematics that are not similar to running with bilateral arm swing restriction or typical arm swing motion. Running with one arm restrained increases frontal plane mechanics associated with risk of knee injury.

Regulation of step frequency in transtibial amputee endurance athletes using a running-specific prosthesis

Running specific prostheses (RSP) are designed to replicate the spring-like behaviour of the human leg during running, by incorporating a real physical spring in the prosthesis. Leg stiffness is an important parameter in running as it is strongly related to step frequency and running economy. To be able to select a prosthesis that contributes to the required leg stiffness of the athlete, it needs to be known to what extent the behaviour of the prosthetic leg during running is dominated by the stiffness of the prosthesis or whether it can be regulated by adaptations of the residual joints. The aim of this study was to investigate whether and how athletes with an RSP could regulate leg stiffness during distance running at different step frequencies. Seven endurance runners with an unilateral transtibial amputation performed five running trials on a treadmill at a fixed speed, while different step frequencies were imposed (preferred step frequency (PSF) and -15%, -7.5%, +7.5% and +15% of PSF). Among others, step time, ground contact time, flight time, leg stiffness and joint kinetics were measured for both legs. In the intact leg, increasing step frequency was accompanied by a decrease in both contact and flight time, while in the prosthetic leg contact time remained constant and only flight time decreased. In accordance, leg stiffness increased in the intact leg, but not in the prosthetic leg. Although a substantial contribution of the residual leg to total leg stiffness was observed, this contribution did not change considerably with changing step frequency. Amputee athletes do not seem to be able to alter prosthetic leg stiffness to regulate step frequency during running. This invariant behaviour indicates that RSP stiffness has a large effect on total leg stiffness and therefore can have an important influence on running performance. Nevertheless, since prosthetic leg stiffness was considerably lower than stiffness of the RSP, compliance of the residual leg should not be ignored when selecting RSP stiffness.

Effects of stride frequency and foot position at landing on braking force, hip torque, impact peak force and the metabolic cost of running in humans

Endurance runners are often advised to use 90 strides min(-1), but how optimal is this stride frequency and why? Endurance runners are also often advised to maintain short strides and avoid landing with the feet too far in front of their hips or knees (colloquially termed 'overstriding'), but how do different kinematic strategies for varying stride length at the same stride frequency affect economy and impact peaks? Linear mixed models were used to analyze repeated measures of stride frequency, the anteroposterior position of the foot at landing, V̇O2 , lower extremity kinematics and vertical ground reaction forces in 14 runners who varied substantially in height and body mass and who were asked to run at 75, 80, 85, 90 and 95 strides min(-1) at 3.0 m s(-1). For every increase of 5 strides min(-1), maximum hip flexor moments in the sagittal plane increased by 5.8% (P<0.0001), and the position of the foot at landing relative to the hip decreased by 5.9% (P=0.003). Higher magnitudes of posteriorly directed braking forces were associated with increases in foot landing position relative to the hip (P=0.0005) but not the knee (P=0.54); increases in foot landing position relative to the knee were associated with higher magnitudes (P<0.0001) and rates of loading (P=0.07) of the vertical ground reaction force impact peak. Finally, the mean metabolically optimal stride frequency was 84.8±3.6 strides min(-1), with 50.4% of the variance explained by the trade-off between minimizing braking forces versus maximum hip flexor moments during swing. The results suggest that runners may benefit from a stride frequency of approximately 85 strides min(-1) and by landing at the end of swing phase with a relatively vertical tibia.

Accuracy of PARTwear Inertial Sensor and Optojump Optical Measurement System for Measuring Ground Contact Time During Running

Ammann, R, Taube, W, and Wyss, T. Accuracy of PARTwear inertial sensor and Optojump optical measurement system for measuring ground contact time during running. J Strength Cond Res 30(7): 2057-2063, 2016-The aim of this study was to validate the detection of ground contact time (GCT) during running in 2 differently working systems: a small inertial measurement sensor, PARTwear (PW), worn on the shoe laces, and the optical measurement system, Optojump (OJ), placed on the track. Twelve well-trained subjects performed 12 runs each on an indoor track at speeds ranging from 3.0 to 9.0 m·s. GCT of one step per run (total 144) was simultaneously obtained by the PW, the OJ, and a high-speed video camera (HSC), whereby the latter served as reference system. The sampling rate was 1,000 Hz for all methods. Compared with the HSC, the PW and the OJ systems underestimated GCT by -1.3 ± 6.1% and -16.5 ± 6.7% (p-values ≤ 0.05), respectively. The intraclass correlation coefficients between PW and HSC and between OJ and HSC were 0.984 and 0.853 (p-values < 0.001), respectively. Despite the constant systematic underestimation of GCT, analyses indicated that PW successfully recorded GCT over a wide range of speeds. However, results showed only moderate validity for the OJ system, with increasing errors when speed decreased. In conclusion, the PW proved to be a highly useful and valid application, and its use can be recommended not only for laboratory settings but also for field applications. In contrast, data on GCT obtained by OJ during running must be treated with caution, specifically when running speed changes or when comparisons are made with GCT data collected by other measurement systems.

Is passive metatarsophalangeal joint stiffness related to leg stiffness, vertical stiffness and running economy during sub-maximal running?

This study examined whether passive metatarsophalangeal joints (MPJ) stiffness was associated with leg stiffness (Kleg) vertical stiffness (Kvert) and running economy (RE) during sub-maximal running. Nine male experienced runners underwent passive MPJ stiffness measurements in standing and sitting positions followed by sub-maximal running on an instrumented treadmill. With the individual foot position properly aligned, the MPJ passive stiffness in both sitting (MPJsit) and standing positions (MPJstand) were measured with a computerized dynamometer. Data were collected at a running speed of 2.78m/s, representing a stabilized level of energy expenditure. Pedar pressure insole was used to determine the contact time (tc) and peak reaction force for the calculation of Kleg and Kvert. A respiratory gas analysis system was used to estimate the RE. Bivariate correlation test was performed to examine the correlation among MPJ stiffness, contact time, Kleg, Kvert, and RE. The results showed that MPJsit and MPJstand were inversely correlated with RE (p=0.04, r=-0.68 to -0.69), suggesting that stiffer MPJ improves RE. In addition, MPJsit was correlated positively with Kleg (p<0.01, r=0.87),Kvert (p=0.03, r=0.70) but inversely with tc (p=0.02, r=-0.76), while MPJstand was correlated positively with the Kvert (p=0.02, r=0.77). These findings suggested that strength of toe plantar flexors provides stability and agility in the stance phase for more effective and faster forward movement.

Fatigue associated with prolonged graded running

Scientific experiments on running mainly consider level running. However, the magnitude and etiology of fatigue depend on the exercise under consideration, particularly the predominant type of contraction, which differs between level, uphill, and downhill running. The purpose of this review is to comprehensively summarize the neurophysiological and biomechanical changes due to fatigue in graded running. When comparing prolonged hilly running (i.e., a combination of uphill and downhill running) to level running, it is found that (1) the general shape of the neuromuscular fatigue-exercise duration curve as well as the etiology of fatigue in knee extensor and plantar flexor muscles are similar and (2) the biomechanical consequences are also relatively comparable, suggesting that duration rather than elevation changes affects neuromuscular function and running patterns. However, 'pure' uphill or downhill running has several fatigue-related intrinsic features compared with the level running. Downhill running induces severe lower limb tissue damage, indirectly evidenced by massive increases in plasma creatine kinase/myoglobin concentration or inflammatory markers. In addition, low-frequency fatigue (i.e., excitation-contraction coupling failure) is systematically observed after downhill running, although it has also been found in high-intensity uphill running for different reasons. Indeed, low-frequency fatigue in downhill running is attributed to mechanical stress at the interface sarcoplasmic reticulum/T-tubule, while the inorganic phosphate accumulation probably plays a central role in intense uphill running. Other fatigue-related specificities of graded running such as strategies to minimize the deleterious effects of downhill running on muscle function, the difference of energy cost versus heat storage or muscle activity changes in downhill, level, and uphill running are also discussed.

Validity and Reliability of a Commercial Fitness Watch for Measuring Running Dynamics

Study Design Validity and reliability study. Background Providing feedback on running mechanics is a common intervention to decrease the risk of injury or to restore running after an injury. Commercially available devices are able to measure running dynamics, such as cadence, ground contact time (GCT), and vertical oscillation (VO), but there is limited evidence on the validity and reliability of these measures. Objective To determine the validity and reliability of measures of cadence, GCT, and VO with a fitness watch compared to a motion-analysis system. Methods Twenty runners ran in 3 conditions: (1) baseline (self-selected speed and cadence), (2) higher cadence, and (3) decreased vertical motion (minimal oscillation). Ten runners also performed an additional baseline running session to measure intrasession reliability. For each condition, the average cadence, GCT, and VO were collected from a watch and from a motion-capture system. Intraclass correlation coefficients (ICCs) were used to assess validity between devices. An analysis of variance with 2 repeated measures was used to determine the ability of the watch and motion analysis to detect change in running dynamics. Results The ICCs between the 2 measuring systems were 0.931, 0.963, and 0.749 for cadence, VO, and GCT, respectively (P<.01). The minimal detectable changes at the 95% confidence interval for cadence, VO, and GCT were 2.53 steps per minute, 0.45 cm, and 0.01 seconds, respectively, for the watch. There were no interaction effects, but there was a main effect for condition; both devices detected changes in running dynamics. Conclusion The watch is a valid and reliable tool for detecting changes in cadence, VO, and GCT. J Orthop Sports Phys Ther 2016;46(6):471-476. Epub 26 Apr 2016. doi:10.2519/jospt.2016.6391.

Effect of the Fatigue Induced by a 110-km Ultramarathon on Tibial Impact Acceleration and Lower Leg Kinematics

Ultramarathon runners are exposed to a high number of impact shocks and to severe neuromuscular fatigue. Runners may manage mechanical stress and muscle fatigue by changing their running kinematics. Our purposes were to study (i) the effects of a 110-km mountain ultramarathon (MUM) on tibial shock acceleration and lower limb kinematics, and (ii) whether kinematic changes are modulated according to the severity of neuromuscular fatigue. Twenty-three runners participated in the study. Pre- and post-MUM, neuromuscular tests were performed to assess knee extensor (KE) and plantar flexor (PF) central and peripheral fatigue, and a treadmill running bouts was completed during which step frequency, peak acceleration, median frequency and impact frequency content were measured from tibial acceleration, as well as foot-to-treadmill, tibia-to-treadmill, and ankle flexion angles at initial contact, and ankle range of motion using video analysis. Large neuromuscular fatigue, including peripheral changes and deficits in voluntary activation, was observed in KE and PF. MVC decrements of ~35% for KE and of ~28% for PF were noted. Among biomechanical variables, step frequency increased by ~2.7% and the ankle range of motion decreased by ~4.1% post-MUM. Runners adopting a non rearfoot strike pre-MUM adopted a less plantarflexed foot strike pattern post-MUM while those adopting a rearfoot strike pre-MUM tended to adopt a less dorsiflexed foot strike pattern post-MUM. Positive correlations were observed between percent changes in peripheral PF fatigue and the ankle range of motion. Peripheral PF fatigue was also significantly correlated to both percent changes in step frequency and the ankle angle at contact. This study suggests that in a fatigued state, ultratrail runners use compensatory/protective adjustments leading to a flatter foot landing and this is done in a fatigue dose-dependent manner. This strategy may aim at minimizing the overall load applied to the musculoskeletal system, including impact shock and muscle stretch.

Sprint running: how changes in step frequency affect running mechanics and leg spring behaviour at maximal speed

The purpose of this study was to investigate the changes in selected biomechanical variables in 80-m maximal sprint runs while imposing changes in step frequency (SF) and to investigate if these adaptations differ based on gender and training level. A total of 40 athletes (10 elite men and 10 women, 10 intermediate men and 10 women) participated in this study; they were requested to perform 5 trials at maximal running speed (RS): at the self-selected frequency (SF_s) and at SF ±15% and ±30%SF_s. Contact time (CT) and flight time (FT) as well as step length (SL) decreased with increasing SF, while k_vert increased with it. At SF_s, k_leg was the lowest (a 20% decrease at ±30%SF_s), while RS was the largest (a 12% decrease at ±30%SF_s). Only small changes (1.5%) in maximal vertical force (F_max) were observed as a function of SF, but maximum leg spring compression (ΔL) was largest at SF_s and decreased by about 25% at ±30%SF_s. Significant differences in F_max, Δy, k_leg and k_vert were observed as a function of skill and gender (P < 0.001). Our results indicate that RS is optimised at SF_s and that, while k_vert follows the changes in SF, k_leg is lowest at SF_s.

Do Running Kinematic Characteristics Change over a Typical HIIT for Endurance Runners?

García-Pinillos, F, Soto-Hermoso, VM, and Latorre-Román, PÁ. Do running kinematic characteristics change over a typical HIIT for endurance runners?. J Strength Cond Res 30(10): 2907-2917, 2016-The purpose of this study was to describe kinematic changes that occur during a common high-intensity intermittent training (HIIT) session for endurance runners. Twenty-eight male endurance runners participated in this study. A high-speed camera was used to measure sagittal-plane kinematics at the first and the last run during a HIIT (4 × 3 × 400 m). The dependent variables were spatial-temporal variables, joint angles during support and swing, and foot strike pattern. Physiological variables, rate of perceived exertion, and athletic performance were also recorded. No significant changes (p ≥ 0.05) in kinematic variables were found during the HIIT session. Two cluster analyses were performed, according to the average running pace-faster vs. slower, and according to exhaustion level reached-exhausted group vs. nonexhausted group (NEG). At first run, no significant differences were found between groups. As for the changes induced by the running protocol, significant differences (p ≤ 0.05) were found between faster and slower athletes at toe-off in θhip and θknee, whereas some changes were found in NEG in θhip during toe-off (+4.3°) and θknee at toe-off (-5.2°) during swing. The results show that a common HIIT session for endurance runners did not consistently or substantially perturb the running kinematics of trained male runners. Additionally, although some differences between groups have been found, neither athletic performance nor exhaustion level reached seems to be determinant in the kinematic response during a HIIT, at least for this group of moderately trained endurance runners.

The Effects of Barefoot and Shod Running on Limb and Joint Stiffness Characteristics in Recreational Runners

The authors aimed to determine the effects of barefoot (BF) and several commercially available barefoot-inspired (BFIS) footwear models on limb and joint stiffness characteristics compared with conventional footwear (CF). Fifteen male participants ran over a force platform at 4.0 m.s(-1), in BF, BFIS, and CF conditions. Measures of limb and joint stiffness were calculated for each footwear. The results indicate that limb and knee stiffness were greater in BF and minimalist BFIS than in CF. CF and more structured BFIS were associated with a greater ankle stiffness compared with BF and minimalist BFIS. These findings serve to provide further insight into the susceptibility of runners to different injury mechanisms as a function of footwear.

Influence of stride frequency and length on running mechanics: a systematic review

Context:

A high number of recreational runners sustain a running-related injury each year. To reduce injury risk, alterations in running form have been suggested. One simple strategy for running stride frequency or length has been commonly advocated.

Objective:

To characterize how running mechanics change when stride frequency and length are manipulated.

Data Sources:

In January 2012, a comprehensive search of PubMed, CINAHL Plus, SPORTDiscus, PEDro, and Cochrane was performed independently by 2 reviewers. A second search of the databases was repeated in June 2012 to ensure that no additional studies met the criteria after the initial search.

Study Selection:

Inclusion criteria for studies were an independent variable including manipulation of stride frequency or length at a constant speed with outcome measures of running kinematics or kinetics.

Study Design:

Systematic review.

Level of Evidence:

Level 3.

Data Extraction:

Two reviewers independently appraised each article using a modified version of the Quality Index, designed for assessing bias of nonrandomized studies.

Results:

Ten studies met the criteria for this review. There was consistent evidence that increased stride rate resulted in decreased center of mass vertical excursion, ground reaction force, shock attenuation, and energy absorbed at the hip, knee, and ankle joints. All but 1 study had a limited number of participants, with several methodological differences existing among studies (eg, overground and treadmill running, duration of test conditions). Although speed was held constant during testing, it was individually self-selected or fixed. Most studies used only male participants.

Conclusion:

Despite procedural differences among studies, an increased stride rate (reduced stride length) appears to reduce the magnitude of several key biomechanical factors associated with running injuries.

Is midsole thickness a key parameter for the running pattern?

Many studies have highlighted differences in foot strike pattern comparing habitually shod runners who ran barefoot and with running shoes. Barefoot running results in a flatter foot landing and in a decreased vertical ground reaction force compared to shod running. The aim of this study was to investigate one possible parameter influencing running pattern: the midsole thickness. Fifteen participants ran overground at 3.3 ms(-1) barefoot and with five shoes of different midsole thickness (0 mm, 2 mm, 4 mm, 8 mm, 16 mm) with no difference of height between rearfoot and forefoot. Impact magnitude was evaluated using transient peak of vertical ground reaction force, loading rate, tibial acceleration peak and rate. Hip, knee and ankle flexion angles were computed at touch-down and during stance phase (range of motion and maximum values). External net joint moments and stiffness for hip, knee and ankle joints were also observed as well as global leg stiffness. No significant effect of midsole thickness was observed on ground reaction force and tibial acceleration. However, the contact time increased with midsole thickness. Barefoot running compared to shod running induced ankle in plantar flexion at touch-down, higher ankle dorsiflexion and lower knee flexion during stance phase. These adjustments are suspected to explain the absence of difference on ground reaction force and tibial acceleration. This study showed that the presence of very thin footwear upper and sole was sufficient to significantly influence the running pattern.

Changes in leg spring behaviour, plantar loading and foot mobility magnitude induced by an exhaustive treadmill run in adolescent middle-distance runners

OBJECTIVES

This study aimed to determine adjustments in spring-mass model characteristics, plantar loading and foot mobility induced by an exhaustive run.

DESIGN

Within-participants repeated measures.

METHODS

Eleven highly-trained adolescent middle-distance runners ran to exhaustion on a treadmill at a constant velocity corresponding to 95% of velocity associated with VO₂max (17.8 ± 1.4 kmh(-1), time to exhaustion=8.8 ± 3.4 min). Contact time obtained from plantar pressure sensors was used to estimate spring-mass model characteristics, which were recorded (during 30 s) 1 min after the start and prior to exhaustion using pressure insoles. Foot mobility magnitude (a composite measure of vertical and medial-lateral mobility of the midfoot) was measured before and after the run.

RESULTS

Mean contact area (foot to ground), contact time, peak vertical ground reaction force, centre of mass vertical displacement and leg compression increased significantly with fatigue, while flight time, leg stiffness and mean pressure decreased. Leg stiffness decreased because leg compression increased to a larger extent than peak vertical ground reaction forces. Step length, step frequency and foot mobility magnitude did not change at exhaustion.

CONCLUSIONS

The stride pattern of adolescents when running on a treadmill at high constant velocity deteriorates near exhaustion, as evidenced by impaired leg-spring behaviour (leg stiffness) and altered plantar loading.

Leg stiffness decreases during a run to exhaustion at the speed at VO2max

Vertical and leg stiffness are related to running speed. In endurance running, the ability to maintain stiffness might be more important than the absolute stiffness magnitude. The purpose of this study was to examine changes in vertical and leg stiffness during an exhaustive. Six sub-elite runners (24.2, s = 4.2 years; 1.81, s = 0.03 m; 73.4, s = 4.4 kg) participated in this study. They performed preliminary tests to determine lactate threshold, lactate turnpoint, [Formula: see text]O2max, s[Formula: see text]O2max and a series of isokinetic endurance tests. During the run to exhaustion runners were videoed (50 Hz) to determine contact and flight times, from which leg (Kleg) and vertical (Kvert) stiffness were calculated. During the run Kleg showed a significant decrease [P = 0.030, effect size statistics (ES) = 0.74], however, the decrease in Kvert was non-significant and of a small magnitude (P = 0.051, ES = 0.32). The distance covered during the run was correlated with ΔKleg (r = -0.868) but not ΔKvert (r = 0.684). ΔKleg was very strongly related to Δ ground contact time (r = -0.937) and Δ step length (r = -0.957). The Δ ground contact time had a near perfect relationship with Δ step length (r = 0.995). Isokinetic measures were not significantly correlated with either ΔKleg. The ability to maintain a short ground contact time appears to be a key determinant of maintaining performance during a run to exhaustion. Minimising this is important for maintaining Kleg. Kleg was not significantly related to isokinetic measures.

Leg stiffness: comparison between unilateral and bilateral hopping tasks

Leg stiffness is a predictor of athletic performance and injury and typically evaluated during bilateral hopping. The contribution of each limb to bilateral leg stiffness, however, is not well understood. This study investigated leg stiffness during unilateral and bilateral hopping to address the following research questions: (1) does the magnitude and variability of leg stiffness differ between dominant and non-dominant legs? (2) Does unilateral leg stiffness differ from bilateral leg stiffness? and (3) Is bilateral leg stiffness determined by unilateral leg stiffness? Thirty-two physically active males performed repeated hopping tests on a force platform for each of the three conditions: bilateral hopping, unilateral hopping on the dominant leg, and unilateral hopping on the non-dominant leg. Leg stiffness was estimated as the ratio of the peak vertical force and the maximum displacement using a simple 1-D mass-spring model. Neither the magnitude nor variability of leg stiffness differed between dominant and non-dominant limbs. Unilateral leg stiffness was 24% lower than bilateral stiffness and showed less variability between consecutive hops and subjects. Unilateral leg stiffness explained 76% of the variance in bilateral leg stiffness. We conclude that leg stiffness estimates during unilateral hopping are preferable for intervention studies because of their low variability.

Amputee locomotion: spring-like leg behavior and stiffness regulation using running-specific prostheses

Carbon fiber running-specific prostheses (RSPs) have allowed individuals with lower extremity amputation (ILEA) to participate in running. It has been established that as running speed increases, leg stiffness (Kleg) remains constant while vertical stiffness (Kvert) increases in able-bodied runners. The Kvert further depends on a combination of the torsional stiffnesses of the joints (joint stiffness; Kjoint) and the touchdown joint angles. Thus, an increased understanding of spring-like leg function and stiffness regulation in ILEA runners using RSPs is expected to aid in prosthetic design and rehabilitation strategies. The aim of this study was to investigate stiffness regulation to various overground running speeds in ILEA wearing RSPs. Eight ILEA performed overground running at a range of running speeds. Kleg, Kvert and Kjoint were calculated from kinetic and kinematic data in both the intact and prosthetic limbs. Kleg and Kvert in both the limbs remained constant when running speed increased, while intact limbs in ILEA running with RSPs have a higher Kleg and Kvert than residual limbs. There were no significant differences in Kankle, Kknee and touchdown knee angle between the legs at all running speeds. Hip joints in both the legs did not demonstrate spring-like function; however, distinct impact peaks were observed only in the intact leg hip extension moment at the early stance phase, indicating that differences in Kvert between limbs in ILEA are due to attenuating shock with the hip joint. Therefore, these results suggest that ILEA using RSPs has a different stiffness regulation between the intact and prosthetic limbs during running.