Sex differences in relationship between passive ankle stiffness and leg stiffness during hopping

Can the recent sex-specific evolutions in elite running performances be attributed to advanced footwear technology?

Recent improvements in elite running performances across all distances have been largely attributed to the introduction of advanced footwear technology (AFT), which features a curved and stiff plate working synergistically with a new generation of midsole foams demonstrating enhanced resilience and compliance. These recent improvements appear to be considerably more pronounced in women's events, highlighted by improvements in road racing world records by an average of 3.7% (range: 2.6%-5.2%) compared to mean progressions of 1.5% (range: 1.3%-1.9%) in the same men's events. Although there is a growing body of research investigating the mechanisms underpinning running performance enhancements derived from AFT, there remains no explanation for potential sex-based differences in their benefits. We overview the currently available evidence and highlight why the recent direction of AFT research provides a barrier to progress by focusing primarily on male athletes. We subsequently provide our perspective on why women may be benefiting from the new generation of shoes more than men, suggest potential mechanisms leading to hypotheses that need to be further investigated in upcoming studies, and finally propose that factors outside of footwear innovation may have concurrently driven the recently observed performance evolutions.

Sex differences in dynamic joint stiffness during walking in older adults

BACKGROUND

Knee osteoarthritis (OA) often occurs in older women. Walking assistance such as knee brace is used to reduce mechanical stress on the knee, preventing OA onset. Dynamic joint stiffness (DJS) quantifies the resistance of an assistive device, providing a foundation for an objective bending stiffness prescription model. DJS may show sex differences among older adults.

RESEARCH QUESTION

This study aimed to investigate sex differences in lower limb DJS in the sagittal plane during walking in older adults.

METHODS

A total of 132 healthy older adults, aged 65 years or older (71 men and 61 women), were extracted from a public dataset. DJS of the hip, knee, and ankle joints in the sagittal plane was determined during the power absorption phase of the stance. DJS, joint angular excursion, and Δ joint moment were compared between older men and women using the Mann-Whitney U test. In addition, the r-value was calculated to represent the effect size of the differences in amplitude.

RESULTS

Ankle DJS in older women was significantly lower with a reduced Δ ankle plantar flexion moment compared with that into men (p < 0.001 and p = 0.001; r = 0.35 and 0.42, respectively). Additionally, knee DJS was lower in older women (p = 0.007). However, since the joint angular excursion and ΔMoment showed no differences (p = 0.624 and 0.222, respectively), the effect size was small (r = 0.24). Hip DJS showed no significant sex differences (p = 0.703).

SIGNIFICANCE

These results suggest that the decrease in ankle DJS in older women was caused by the reduced ankle plantarflexion moment. Thus, support for ankle DJS is necessary for healthy older women. Nonetheless, knee DJS does not elucidate the cause of knee OA in the older women.

Long-term static stretching can decrease muscle stiffness: A systematic review and meta-analysis

Stretch training increases the range of motion of a joint. However, to date, the mechanisms behind such a stretching effect are not well understood. An earlier meta-analysis on several studies reported no changes in the passive properties of a muscle (i.e., muscle stiffness) following long-term stretch training with various types of stretching (static, dynamic, and proprioceptive neuromuscular stretching). However, in recent years, an increasing number of papers have reported the effects of long-term static stretching on muscle stiffness. The purpose of the present study was to examine the long-term (≥2 weeks) effect of static stretching training on muscle stiffness. PubMed, Web of Science, and EBSCO published before December 28, 2022, were searched and 10 papers met the inclusion criteria for meta-analysis. By applying a mixed-effect model, subgroup analyses, which included comparisons of sex (male vs. mixed sex) and type of muscle stiffness assessment (calculated from the muscle-tendon junction vs. shear modulus), were performed. Furthermore, a meta-regression was conducted to examine the effect of total stretching duration on muscle stiffness. The result of the meta-analysis showed a moderate decrease in muscle stiffness after 3-12 weeks of static stretch training compared to a control condition (effect size = -0.749, p < 0.001, I2  = 56.245). Subgroup analyses revealed no significant differences between sex (p = 0.131) and type of muscle stiffness assessment (p = 0.813). Moreover, there was no significant relationship between total stretching duration and muscle stiffness (p = 0.881).

A Comparison of Plyometric and Jump Rope Training Programs for Improving Punching Performance in Junior Amateur Boxers

Improving lower extremity sports performance may contribute to punching performance in boxers. We compared the effects of two typical boxing routines for developing lower extremity sports performance and subsequent punching performance. Twenty-four high school amateur boxers between the ages of 12 and 18 performed training at least 3 days per week. All Athletes had 3–5 years of experience in boxing training. The participants separated into two groups to receive an 8-week plyometric or jump rope training program. They performed each training program for 30 min on 3 days/week. Lower extremity sports performance in countermovement jump (leg stiffness, jump power, and rate of force development) and jab-cross punching performance (punch velocity, punch force, reaction time, movement time, and ground reaction force) were assessed at pre-and post-training. The data were analyzed using a two-way mixed-design analysis of variance (ANOVA) (group × time). Both training programs improved the rate of force development in countermovement jump, the reaction time of punch, the peak ground reaction force of the rear leg during the jab punch, and the velocity of the jab punch. There were no group differences and interaction effects in all variables analyzed. It is concluded that 8 weeks of plyometric and rope jumping programs had a similar impact on improving lower extremity strength and punching performance. Both training programs may improve muscle strength and power, rate of force development, and reaction time. These improvements may contribute to lower extremity strength for driving a punch at the target with excellent performance.

Application of an Accelerometric System for Determination of Stiffness during a Hopping Task

Currently, there are several computational methods for stiffness during a hopping task, but they do not necessarily yield the same values. Therefore, it is essential that the simplicity of the equipment used does not affect the measurement validity. The aim of this study is to compare the stiffness values during a hopping task recorded in a laboratory environment and those acquired using the Myotest accelerometer. The measurements were performed on a group of 30 untrained female students (age: 23.0 ± 1.7 years, body height: 1.72 ± 0.07 m, and body mass: 64.8 ± 10.0 kg). According to the manual for the Myotest accelerometric system, each study participant performed three sets of 5 hops. Vertical stiffness was determined based on two measurement methods, one using the Myotest accelerometer and the other using Kistler force plates. The mean value (±SD) of vertical stiffness was 19.0 ± 9.3 kN/m in the countermovement phase and 15.1 ± 5.9 kN/m in the take-off phase. Furthermore, the stiffness determined using the Myotest was 30.7 ± 13.3 kN/m. However, significant relationships between the vertical stiffness in the countermovement phase and the Myotest stiffness (r = 0.79) and between the vertical stiffness in the take-off phase and the Myotest stiffness (r = 0.89) were found. The relationships between the vertical stiffness (in the countermovement and take-off phases) and the stiffness estimated using the Myotest allow us to conclude that despite the significantly overestimated stiffness value, the Myotest accelerometer can still be used for determination of the stiffness trends, e.g., following training. The overestimated stiffness values can result both from inaccuracy in the determination of ground contact time and flight time by the Myotest accelerometer and from the use of an equation that assumes that the movement of the center of mass has a harmonic profile.

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.

Female Athletes With Varying Levels of Vertical Stiffness Display Kinematic and Kinetic Differences During Single-Leg Hopping

Vertical stiffness may contribute to lower-extremity injury risk; however, it is unknown whether athletes with different stiffness levels display differences in biomechanics. This study compared differences in biomechanics between female athletes (n = 99) with varying stiffness levels during a repetitive, single-leg, vertical hopping task. Vertical stiffness was calculated as the ratio of peak vertical ground-reaction force to maximum center-of-mass displacement. Tertiles were established using stiffness values, and separate 1-way ANOVAs were used to evaluate between-group differences. Stance times decreased, and flight times, ground-reaction force, and stiffness increased, from the low- to high-stiffness group (P < .050). The high-stiffness group displayed: (1) greater lateral trunk flexion (P = .009) and lesser hip adduction (P = .022) at initial ground contact compared to the low- and moderate-stiffness groups, respectively; (2) lesser peak hip adduction compared to the low-stiffness group (P = .040); (3) lesser lateral trunk-flexion (P = .046) and knee-flexion (P = .010) excursion compared to the moderate- and low-stiffness groups, respectively; and (4) greater peak hip-flexion (P = .001), ankle-dorsiflexion (P = .002), and ankle-eversion (P = .038) moments compared to the low-stiffness group. A wide range of variability in stiffness exists within a relatively homogenous population. Athletes with varying stiffness levels display biomechanical differences that may help identify the potential mechanism(s) by which stiffness contributes to injury risk.

Transitioning to Minimal Footwear: a Systematic Review of Methods and Future Clinical Recommendations

BACKGROUND

Recent interest in barefoot running has led to the development of minimalist running shoes that are popular in distance runners. A careful transition to these shoes has been suggested and examined in the literature. However, no guidelines based on systematic evidence have been presented. The purpose of this review is to systematically examine the methods employed in the literature to transition to minimal footwear (MFW), as well as the outcomes to these studies in distance runners. In addition, MFW transition guidelines for future clinical practice will be presented based on observations from this review.

METHODS

A systematic database search was employed using PubMed online as the primary database. Twenty papers were included in the final review.

RESULTS

All studies implemented a prospective transition design to MFW with a detail of this transition provided, which increased MFW exposure up to an average of 60% (30-100%) at completion. Only 8/20 studies included injury prevention exercises, and 9/20 included gait retraining. The main outcomes of this transition included limited positive evidence of transitioning into MFW for running economy (n = 4 studies) and muscle development (n = 5). The injury incidence comparing running during the MFW transition (17.9 injuries per 100 participants) to matched participants in conventional running shoes (13.4 injuries per 100) appears equivocal (p = 0.219; effect size phi (φ) = 0.06 [very small]). Finally, several important recommendations for clinical practice and future research have been presented.

CONCLUSIONS

It is hoped that this paper will present important first steps in unifying the process of transitioning to MFW, both for academic and clinical use.

A Comparison of Computation Methods for Leg Stiffness During Hopping

Despite the presence of several different calculations of leg stiffness during hopping, little is known about how the methodologies produce differences in the leg stiffness. The purpose of this study was to directly compareKlegduring hopping as calculated from three previously published computation methods. Ten male subjects hopped in place on two legs, at four frequencies (2.2, 2.6, 3.0, and 3.4 Hz). In this article, leg stiffness was calculated from the natural frequency of oscillation (method A), the ratio of maximal ground reaction force (GRF) to peak center of mass displacement at the middle of the stance phase (method B), and an approximation based on sine-wave GRF modeling (method C). We found that leg stiffness in all methods increased with an increase in hopping frequency, butKlegvalues using methods A and B were significantly higher than when using method C at all hopping frequencies. Therefore, care should be taken when comparing leg stiffness obtained by method C with those calculated by other methods.

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.