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

Relationship Between Supporting Leg Stiffness and Trunk Kinematics of the Kicking Leg During Soccer Kicking

The stiffness of the supporting leg may alter the energy transfer to the trunk and lower extremities of the kicking leg, which may affect kick performance. This study aimed to clarify whether the stiffness of the supporting leg affects the trunk kinematics during kicking and kicking performance in soccer players. Twenty-two male collegiate soccer players participated in the study. The data for the stiffness properties of the supporting leg and trunk kinematics were obtained and calculated using a 3-dimensional motion analysis system. The results showed that a greater leg stiffness of the supporting leg was associated with a lower trunk rotation angle during kicking. There were no significant correlations between the maximum swing speed and the stiffness of the supporting leg (P < .05). These results suggest that stiffness of the supporting leg may restrain trunk rotation during the kicking motion. However, the lack of a relationship with swing speed indicates the need for further investigation into its effects on kicking performance.

Effect of moxibustion on knee joint stiffness characteristics in recreational athletes pre- and post-fatigue

Objective

Joint stiffness results from the coupling of the nervous system and joint mechanics, and thus stiffness is a comprehensive representation of joint stability. It has been reported that moxibustion can alleviate general weakness and fatigue symptoms and subsequently may influence joint stiffness. This study investigated whether moxibustion could enhance knee joint stiffness in recreational athletes pre- and post-fatigue.

Methods

Eighteen participants were randomized into intervention (5 males: 20.6 ± 1.5 yr; 4 females: 20.8 ± 1.5 yr) and control groups (5 males: 19.4 ± 0.9 yr; 4 females: 20.5 ± 0.6 yr). The intervention group received indirect moxibustion applied to acupoints ST36 (bilateral) and CV4 for 30 min every other day for 4 consecutive weeks. The control group maintained regular exercise without moxibustion. Peak torque (PT) of right knee extensor, relaxed and contracted muscle stiffness (MS) of vastus lateralis, and knee extensor musculoarticular stiffness (MAS) was assessed with an isokinetic dynamometer (IsoMed 2000), myometer, and free oscillation technique, respectively. Measurements were taken at three time points: pre-intervention, post-intervention/pre-fatigue, and post-fatigue.

Results

MAS (P = 0.006) and PT (P = 0.007) in the intervention group increased more from pre-to post-intervention compared with the control group. Post-fatigue MAS (P = 0.016) and PT (P = 0.031) increased more in the intervention group than in the control group.

Conclusion

Moxibustion enhanced PT and knee MAS, suggesting that this intervention could be used in injury prevention and benefit fatigue resistance in young recreational athletes.

Jump Height and Hip Power Decrease During Cognitive Loading Regardless of Sex: Implications for Sport Performance Metrics

ABSTRACT

Shumski, EJ, Lempke, LB, Johnson, RS, Oh, J, Schmidt, JD, and Lynall, RC. Jump height and hip power decrease during cognitive loading regardless of sex: implications for sport performance metrics. J Strength Cond Res 37(4): 793-798, 2023-Sex and cognitive loading separately influence jumping performance. However, it is unknown how cognitive loading influences jump performance and how sex and cognitive loading interact. The purpose of our study was to determine if an interaction existed between sex and cognitive loading for the dependent variables jump height, ground contact time, reactive strength index, vertical stiffness, impulse, and lower extremity joint power during a double limb drop vertical jump. Twenty-one male (23.2 ± 2.5 years, 180.8 ± 8.4 cm, 80.4 ± 10.2 kg) and 20 female (21.7 ± 1.0 years, 163.7 ± 8.2 cm, 61.2 ± 9.4 kg) physically active individuals participated. Subjects jumped from a 30 cm box placed 50% of their height away from 2 force plates under single-task and dual-task (serial 6s or 7s) conditions. Separate 2 × 2 analyses of variance were used for all dependent variables (α = 0.05) with Bonferroni post hoc mean differences and 95% confidence intervals (CIs). There were no significant interactions for any outcomes ( p ≥ 0.190). Condition main effects demonstrated subjects jumped significantly higher (1.84 cm, 95% CI = 0.68-3.01, d = 0.26, p = 0.003) and with greater hip power (0.29 Watts·BW -1 ·HT -1 , 95% CI = 0.04-0.54, d = 0.21, p = 0.025) during single task compared with dual task. Sex main effects revealed males jumped higher (9.88 cm, 95% CI = 7.00-12.77, d = 2.17, p < 0.001), with greater reactive strength index (0.29, 95% CI = 0.17-0.41, d = 1.52, p < 0.001), greater ankle power (3.70 Watts·BW -1 ·HT -1 , 95% CI = 2.26-5.13, d = 1.64, p < 0.001), and greater knee power (5.00 Watts·BW -1 ·HT -1 , 95% CI = 3.25-6.75, d = 1.82, p < 0.001) compared with females. Jump performance is influenced by sex and dual-task conditions but not their interaction. To optimize jumping performance, testing should be completed without distractions (single task) to decrease cognitive loading.

Lower extremity joint stiffness of autistic adolescents during running at dual speeds

Running is one of the most common forms of physical activity for autistic adolescents. However, research examining their lower extremity dynamics is sparse. In particular, no information exists regarding lower extremity joint stiffness in autistic adolescents. This study compared knee and ankle joint stiffness during the absorption phase of running between autistic adolescents and non-autistic controls. Motion capture and ground reaction forces were recorded for 22 autistic adolescents and 17 non-autistic age, sex, and BMI matched peers who ran at self-selected and standardized (3.0 m/s) speeds. Group × speed knee and ankle joint stiffness, change in moment, and range of motion were compared using mixed-model ANOVAs. There were no group × speed interactions for any variable. Autistic adolescents presented with significant (12 % and 19 %) reduced knee and ankle joint stiffness, respectively. In addition, autistic adolescents had significant reduced changes in knee and ankle joint moments by 11 % and 21 %, respectively, compared to their non-autistic peers. Only knee joint stiffness and knee joint moments were sensitive to running speed, each significantly increasing with speed by 6 %. Current literature suggests joint stiffness is an important mechanism for stability and usage of the stretch shortening cycle (or elastic recoil); as such, it is possible that the reduced ankle plantar flexor and knee extensor stiffness found in autistic adolescents in this study could be indicative of reduced efficiency during running. As group differences existed across both speeds, autistic adolescents may benefit from therapeutic and/or educational interventions targeting efficient running mechanics.

Anxiety state impact on recovery of runners with lower extremity injuries

This prospective cohort study examined the impact of high anxiety levels on psychological state and gait performance during recovery in runners with lower body injuries. Recreational runners diagnosed with lower body injuries who had reduced running volume (N = 41) were stratified into groups using State Trait Anxiety Inventory (STAI) scores: high anxiety (H-Anx; STAI ≥40 points) and low anxiety (L-Anx; STAI <40 points). Runners were followed through rehabilitation to return-to-run using monthly surveys. Main outcome measures included kinesiophobia (Tampa Scale of Kinesiophobia, TSK-11), Positive and Negative Affect Schedule (PANAS; Positive and negative scores), Lower Extremity Function Scale (LEFS), running recovery (University of Wisconsin Running Injury and Recovery Index [UWRI]) and CDC Healthy Days modules for general health, days of anxiety/tension, disrupted sleep and work/usual activities. Running biomechanics were assessed at baseline and the final visit using 3D motion capture and a force-plated treadmill. The time to return-to-running for was 5.0±3.1 and 7.9±4.1 months for L-Anx and H-Anx, respectively and participants who withdrew (n = 15) did so at 7.7±6.2 months. L-Anx maintained low anxiety and H-Anx reduced anxiety from baseline to final visit (STAI = 31.5 to 28.4 points, 50.4 to 37.8 points, respectively), whereas the withdrawn runners remained clinically anxious at their final survey (41.5 to 40.3 points; p < .05). Group by time interactions were found for PANAS positive, LEFS UWRI, general health scores, and days feeling worry, tension and anxiety (all p < .05). Final running performance in L-Anx compared to H-Anx was most improved with cadence (8.6% vs 3.5%; p = .044), impact loading rate [-1.9% vs +8.9%] and lower body stiffness [+14.1% vs +3.2%; all p < .05). High anxiety may identify runners who will experience a longer recovery process, health-related functional disruptions, and less optimization of gait biomechanics during rehabilitation after a lower extremity injury.

Running economy and lower extremity stiffness in endurance runners: A systematic review and meta-analysis

Background: Lower extremity stiffness simulates the response of the lower extremity to landing in running. However, its relationship with running economy (RE) remains unclear. This study aims to explore the relationship between lower extremity stiffness and RE. Methods: This study utilized articles from the Web of Science, PubMed, and Scopus discussing the relationships between RE and indicators of lower extremity stiffness, namely vertical stiffness, leg stiffness, and joint stiffness. Methodological quality was assessed using the Joanna Australian Centre for Evidence-Based Care (JBI). Pearson correlation coefficients were utilized to summarize effect sizes, and meta-regression analysis was used to assess the extent of this association between speed and participant level. Result: In total, thirteen studies involving 272 runners met the inclusion criteria and were included in this review. The quality of the thirteen studies ranged from moderate to high. The meta-analysis results showed a negative correlation between vertical stiffness (r = -0.520, 95% CI, -0.635 to -0.384, p < 0.001) and leg stiffness (r = -0.568, 95% CI, -0.723 to -0.357, p < 0.001) and RE. Additional, there was a small negative correlation between knee stiffness and RE (r = -0.290, 95% CI, -0.508 to -0.037, p = 0.025). Meta-regression results showed that the extent to which leg stiffness was negatively correlated with RE was influenced by speed (coefficient = -0.409, p = 0.020, r 2 = 0.79) and participant maximal oxygen uptake (coefficient = -0.068, p = 0.010, r 2 = 0.92). Conclusion: The results of this study suggest that vertical, leg and knee stiffness were negatively correlated with RE. In addition, maximum oxygen uptake and speed will determine whether the runner can take full advantage of leg stiffness to minimize energy expenditure.

Test–Retest Reliability and Sensitivity of Common Strength and Power Tests over a Period of 9 Weeks

This study evaluated the reliability and sensitivity of a set of different common strength and power tests in a healthy adult population in a span of 9 weeks. Seventeen subjects (24.2 ± 2.2 years, 1.75 ± 0.10 m, 68.6 ± 14.2 kg, seven women) participated in the study. We tested countermovement jumps, reactive hops, and the maximal voluntary contraction (MVC) of handgrip and isometric knee extension. The tests were conducted in three separate sessions across a nine-week period, with one week between the first two sessions and eight weeks between the second and the third. Reliability and sensitivity statistics for each test were calculated for both the average of three trials and the best result during each session. The MVC of isometric knee extension and handgrip, as well as the countermovement jump test, demonstrated very high reliability and sensitivity over the nine-week period. The peak force of the reactive hops demonstrated high reliability but high sensitivity only for the average but not for the best result. The average contact time of reactive hops was neither a sensitive nor reliable measurement. In conclusion, isometric maximal knee extension and handgrip tests, as well as countermovement jumps and peak force of reactive hops, can be used as reliable and sensitive measurements of isometric and reactive strength and power over time periods of up to eight weeks. We recommend the use of the average results of three trials instead of the best performance value for longitudinal studies, as this procedure produces more consistent results and a lower measurement error.

Not Lower-Limb Joint Strength and Stiffness but Vertical Stiffness and Isometric Force-Time Characteristics Correlate With Running Economy in Recreational Male Runners

Neuromuscular characteristics, such as lower-limb joint strength, the ability to reuse elastic energy, and to generate force are essential factors influencing running performance. However, their relationship with running economy (RE) remains unclear. The aim of this study was to evaluate the correlations between isokinetic lower-limb joint peak torque (PT), lower-limb stiffness, isometric force-time characteristics and RE among recreational-trained male runners. Thirty male collegiate runners (aged 20–22 years, VO2max: 54.02 ± 4.67 ml·kg−1·min−1) participated in test sessions on four separate days. In the first session, the body composition and RE at 10 km·h−1 were determined. In the second session, leg and vertical stiffness (Kleg and Kvert), knee and ankle stiffness (Kknee and Kankle) were evaluated. In the third session, isokinetic knee and ankle joint PT at velocity of 60°s−1 were tested. The force-time characteristics of isometric mid-thigh pull (IMTP) were evaluated in the final session. The Pearson’s product-moment correlations analysis shows that there were no significant relationships between knee and ankle joint concentric and eccentric PT, Kknee and Kankle, Kleg, and RE at 10 km·h−1. However, Kvert (r = −0.449, p &lt; 0.05) and time-specific rate of force development (RFD) for IMTP from 0 to 50 to 0–300 ms (r = −0.434 to −0.534, p &lt; 0.05) were significantly associated with RE. Therefore, superior RE in recreational runners may not be related to knee and ankle joint strength and stiffness. It seems to be associated with vertical stiffness and the capacity to rapidly produce force within 50–300 ms throughout the lower limb.

Application of Leg, Vertical, and Joint Stiffness in Running Performance: A Literature Overview

Stiffness, the resistance to deformation due to force, has been used to model the way in which the lower body responds to landing during cyclic motions such as running and jumping. Vertical, leg, and joint stiffness provide a useful model for investigating the store and release of potential elastic energy via the musculotendinous unit in the stretch-shortening cycle and may provide insight into sport performance. This review is aimed at assessing the effect of vertical, leg, and joint stiffness on running performance as such an investigation may provide greater insight into performance during this common form of locomotion. PubMed and SPORTDiscus databases were searched resulting in 92 publications on vertical, leg, and joint stiffness and running performance. Vertical stiffness increases with running velocity and stride frequency. Higher vertical stiffness differentiated elite runners from lower-performing athletes and was also associated with a lower oxygen cost. In contrast, leg stiffness remains relatively constant with increasing velocity and is not strongly related to the aerobic demand and fatigue. Hip and knee joint stiffness are reported to increase with velocity, and a lower ankle and higher knee joint stiffness are linked to a lower oxygen cost of running; however, no relationship with performance has yet been investigated. Theoretically, there is a desired “leg-spring” stiffness value at which potential elastic energy return is maximised and this is specific to the individual. It appears that higher “leg-spring” stiffness is desirable for running performance; however, more research is needed to investigate the relationship of all three lower limb joint springs as the hip joint is often neglected. There is still no clear answer how training could affect mechanical stiffness during running. Studies including muscle activation and separate analyses of local tissues (tendons) are needed to investigate mechanical stiffness as a global variable associated with sports performance.

IMMEDIATE EFFECTS OF SEMI-CUSTOM INSOLES AND STRUCTURED KNEE SLEEVES ON LOWER EXTREMITY KINETICS AND KINEMATICS IN RECREATIONAL MALE ATHLETES WITH PATELLOFEMORAL PAIN

The aim of this experiment was to provide insight into the immediate influence of both semi-custom insoles and knee sleeves in recreational male runners/athletes suffering from patellofemoral pain and also to explore the association between the extent of patellofemoral pain and psychological wellbeing. Experiment 1 examined 17 male recreational runners with patellofemoral pain, in semi-custom insole and no-insole conditions. Experiment 2 examined 13 male recreational athletes with patellofemoral pain, undertaking run, [Formula: see text] cut and single-leg hop movements in knee sleeve and no-sleeve conditions. In both experiments, motion capture and ground reaction forces were collected, allowing kinetics and three-dimensional kinematics to be calculated alongside patellofemoral joint loading quantified using musculoskeletal modeling. In both experiments, patellofemoral pain symptoms were examined using the KOOS patellofemoral pain subscale and psychological wellbeing using the COOP-WONCA questionnaire. The findings from both experiments showed that pain symptoms significantly predicted psychological wellbeing ([Formula: see text] in experiment 1 and [Formula: see text] in experiment 2). Experiment 1 showed that orthoses significantly reduced tibial internal rotation range of motion (no-[Formula: see text] and [Formula: see text]) whilst also increasing the peak knee adduction moment (no-[Formula: see text][Formula: see text]N[Formula: see text]m/kg and [Formula: see text][Formula: see text]N[Formula: see text]m/kg). The findings from experiment 2 revealed that the knee sleeve reduced the peak patellofemoral force (no-[Formula: see text][Formula: see text]BW and [Formula: see text][Formula: see text]BW) in the run movement and the patellofemoral load rate in the cut movement (no-[Formula: see text][Formula: see text]BW/s and [Formula: see text][Formula: see text]BW/s). Overall, the findings confirm that pain symptoms are predictive of psychological wellbeing in recreational male athletes with patellofemoral pain. Furthermore, the findings suggest that both insoles and knee sleeves may provide immediate biomechanical benefits in recreationally active individuals with patellofemoral pain, although when wearing insoles this may be at the expense of an increased knee adduction moment during running.

Fit to Burst: Toward Noninvasive Estimation of Achilles Tendon Load Using Burst Vibrations

OBJECTIVE

Tendons are essential components of the musculoskeletal system and, as with any mechanical structure, can fail under load. Tendon injuries are common and can be debilitating, and research suggests that a better understanding of their loading conditions could help mitigate injury risk and improve rehabilitation. To that end, we present a novel method of noninvasively assessing parameters related to mechanical load in the Achilles tendon using burst vibrations.

METHODS

These vibrations, produced by a small vibration motor on the skin superficial to the tendon, are sensed by a skin-mounted accelerometer, which measures the tendon's response to burst excitation under varying tensile load. In this study, twelve healthy subjects performed a variety of everyday tasks designed to expose the Achilles tendon to a range of loading conditions. To approximate the vibration motor-tendon system and provide an explanation for observed changes in tendon response, a 2-degree-of-freedom mechanical systems model was developed.

RESULTS

Reliable, characteristic changes in the burst response profile as a function of Achilles tendon tension were observed during all loading tasks. Using a machine learning-based approach, we developed a regression model capable of accurately estimating net ankle moment-which captures general trends in tendon tension-across a range of walking speeds and across subjects (R² = 0.85). Simulated results of the mechanical model accurately recreated behaviors observed in vivo. Finally, preliminary, proof-of-concept results from a fully wearable system demonstrated trends similar to those observed in experiments conducted using benchtop equipment.

CONCLUSION

These findings suggest that an untethered, unobtrusive system can effectively assess tendon loading during activities of daily life.

SIGNIFICANCE

Access to such a system would have broad implications for injury recovery and prevention, athletic training, and the study of human movement.

Leg Stiffness and Vertical Stiffness of Habitual Forefoot and Rearfoot Strikers during Running

Foot strike patterns influence the running efficiency and may be an injury risk. However, differences in the leg stiffness between runners with habitual forefoot (hFFS) and habitual rearfoot (hRFS) strike patterns remain unclear. This study aimed at determining the differences in the stiffness, associated loading rate, and kinematic performance between runners with hFFS and hRFS during running. Kinematic and kinetic data were collected amongst 39 runners with hFFS and 39 runners with hRFS running at speed of 3.3 m/s, leg stiffness (Kleg), and vertical stiffness (Kvert), and impact loads were calculated. Results found that runners with hFFS had greater Kleg (P = 0.010, Cohen′s d = 0.60), greater peak vertical ground reaction force (vGRF) (P = 0.040, Cohen′s d = 0.47), shorter contact time(t_c) (P < 0.001, Cohen′s d = 0.85), and smaller maximum leg compression (ΔL ) (P = 0.002, Cohen′s d = 0.72) compared with their hRFS counterparts. Runners with hFFS had lower impact peak (IP) (P < 0.001, Cohen′s d = 1.65), vertical average loading rate (VALR) (P < 0.001, Cohen′s d = 1.20), and vertical instantaneous loading rate (VILR) (P < 0.001, Cohen′s d = 1.14) compared with runners with hRFS. Runners with hFFS landed with a plantar flexed ankle, whereas runners with hRFS landed with a dorsiflexed ankle (P < 0.001, Cohen′s d = 3.35). Runners with hFFS also exhibited more flexed hip (P = 0.020, Cohen′s d = 0.61) and knee (P < 0.001, Cohen′s d = 1.15) than runners with hRFS at initial contact. These results might indicate that runners with hFFS were associated with better running economy through the transmission of elastic energy.

Validation of Spatiotemporal and Kinematic Measures in Functional Exercises Using a Minimal Modeling Inertial Sensor Methodology

This study proposes a minimal modeling magnetic, angular rate and gravity (MARG) methodology for assessing spatiotemporal and kinematic measures of functional fitness exercises. Thirteen healthy persons performed repetitions of the squat, box squat, sandbag pickup, shuffle-walk, and bear crawl. Sagittal plane hip, knee, and ankle range of motion (ROM) and stride length, stride time, and stance time measures were compared for the MARG method and an optical motion capture (OMC) system. The root mean square error (RMSE), mean absolute percentage error (MAPE), and Bland–Altman plots and limits of agreement were used to assess agreement between methods. Hip and knee ROM showed good to excellent agreement with the OMC system during the squat, box squat, and sandbag pickup (RMSE: 4.4–9.8°), while ankle ROM agreement ranged from good to unacceptable (RMSE: 2.7–7.2°). Unacceptable hip and knee ROM agreement was observed for the shuffle-walk and bear crawl (RMSE: 3.3–8.6°). The stride length, stride time, and stance time showed good to excellent agreement between methods (MAPE: (3.2 ± 2.8)%–(8.2 ± 7.9)%). Although the proposed MARG-based method is a valid means of assessing spatiotemporal and kinematic measures during various exercises, further development is required to assess the joint kinematics of small ROM, high velocity movements.