Interdependence of torque, joint angle, angular velocity and muscle action during human multi-joint leg extension

Single Leg Lateral and Horizontal Loaded Jump Testing: Reliability and Correlation With Long Track Sprint Speed Skating Performance

ABSTRACT

Zukowski, MH, Jordan, MJ, and Herzog, W. Single leg lateral and horizontal loaded jump testing: reliability and correlation with long track sprint speed skating performance. J Strength Cond Res 37(11): 2251-2259, 2023-This study examined the intraday reliability of 2 novel unilateral loaded jump protocols designed for long track speed skaters. Highly trained ( n = 26), national level athletes performed single leg jumps with a horizontal robotic resistance across 3 external load conditions (10 N, 7.5% of body mass and 15% of body mass) using their dominant limb. Jumps were performed in both the horizontal (Jump Horz ) and lateral (Jump Lat ) direction to replicate the body position and line of force application observed during the running and gliding phases of on-ice acceleration. Subjects completed 2 consecutive trials of the same jump protocol to examine the intraday reliability of the peak velocity achieved for each loading condition. Peak velocity across each jump type and loading condition had good reliability (intraclass correlation coefficient >0.8, coefficient of variation <5%). Significant positive relationships ( r = 0.5-0.8, p < 0.05; n = 22) were observed between all jump conditions and on-ice sprint race split times obtained including 100, 400, and 500 m. Our results indicate that unilateral loaded jump tests are reliable in speed skating athletes and may help practitioners diagnose and monitor lower-limb maximal muscle power capacity in a sport-specific manner.

Exploring the Low Force-High Velocity Domain of the Force-Velocity Relationship in Acyclic Lower-Limb Extensions

PURPOSE

To compare linear and curvilinear models describing the force-velocity relationship obtained in lower-limb acyclic extensions, considering experimental data on an unprecedented range of velocity conditions.

METHODS

Nine athletes performed lower-limb extensions on a leg-press ergometer, designed to provide a very broad range of force and velocity conditions. Previously inaccessible low inertial and resistive conditions were achieved by performing extensions horizontally and with assistance. Force and velocity were continuously measured over the push-off in six resistive conditions to assess individual force-velocity relationships. Goodness of fit of linear and curvilinear models (second-order polynomial function, Fenn and Marsh's, and Hill's equations) on force and velocity data were compared via the Akaike Information Criterion.

RESULTS

Expressed relative to the theoretical maximal force and velocity obtained from the linear model, force and velocity data ranged from 26.6 ± 6.6 to 96.0 ± 3.6% (16-99%) and from 8.3 ± 1.9 to 76.6 ± 7.0% (5-86%), respectively. Curvilinear and linear models showed very high fit (adjusted r2 = 0.951-0.999; SEE = 17-159N). Despite curvilinear models better fitting the data, there was a ~ 99-100% chance the linear model best described the data.

CONCLUSION

A combination between goodness of fit, degrees of freedom and common sense (e.g., rational physiologically values) indicated linear modelling is preferable for describing the force-velocity relationship during acyclic lower-limb extensions, compared to curvilinear models. Notably, linearity appears maintained in conditions approaching theoretical maximal velocity. Using horizontal and assisted lower-limb extension to more broadly explore resistive/assistive conditions could improve reliability and accuracy of the force-velocity relationship and associated parameters.

Tendon compliance and preload must be considered when determining the in vivo force-velocity relationship from the torque-angular velocity relation

In vivo, the force-velocity relation (F-v-r) is typically derived from the torque-angular velocity relation (T-ω-r), which is subject to two factors that may influence resulting measurements: tendon compliance and preload prior to contraction. The in vivo plantar flexors' T-ω-r was determined during preloaded maximum voluntary shortening contractions at 0-200°/s. Additionally, we used a two factor block simulation study design to independently analyze the effects of preload and tendon compliance on the resulting T-ω-r. Therefore, we replicated the in vivo experiment using a Hill-type muscle model of the gastrocnemius medialis. The simulation results matched a key pattern observed in our recorded in vivo experimental data: during preloaded contractions, torque output of the muscle was increased when compared with non-preloaded contractions from literature. This effect increased with increasing contraction velocity and can be explained by a rapidly recoiling tendon, allowing the contractile element to contract more slowly, thus developing higher forces compared with non-preloaded contractions. Our simulation results also indicate that a more compliant tendon results in increased ankle joint torques. The simulation and the experimental data clearly show that the deduction of the in vivo F-v-r from the T-ω-r is compromised due to the two factors preloading and tendon compliance.

Isokinetic strengthening and neuromuscular electrical stimulation protocol impact on physical performances, functional status and quality of life in knee osteoarthritis overweight/obese women

BACKGROUND

Knee muscle weakness associated with overweight/obesity can lead to impairment of vital daily function in knee osteoarthritis patients. This study investigated the effect of a knee eccentric isokinetic muscle strength (IMS) training program combined with neuromuscular electrical stimulation (NMES) on muscle strength and flexibility, joint ROM, functional status, physical performance, and quality of life in knee osteoarthritis overweight/obese women.

METHODS

Thirty-six women were randomized into three groups, two experimental groups (EG) and a control group following a classic rehabilitation program. During 6 weeks of two sessions/week, one of the two EGs performed an IMS program (ISO.G); the other underwent combined IMS and NMES training (ISO + NMES.G). All patients were evaluated with clinical examination, isokinetic test at 60°/s and 240°/s speeds, physical performance tests related to activities of daily living, and Knee injury and Osteoarthritis Outcome Score (KOOS) quality of life questionnaire, before and after the intervention.

RESULTS

In the 10-m walk, chair stand, stair climb and monopodal stance tests, muscle flexibility and quality of life scores showed significant improvement for ISO.G (P = 0.000) and ISO + NMES.G (P = 0.000). Concentric strength at 240°/s was improved in ISO + NMES.G (P = 0.000) unlike the muscle strength at 60°/s (quadriceps, P = 0.104; hamstrings, P = 0.171), force asymmetry (P = 0.481) and post-intervention joint ROM (P = 0.309).

CONCLUSIONS

The combination of IMS and NMES shows significant superiority over the usual rehabilitation program for the majority of the parameters measured for optimal management of knee osteoarthritis.

A novel equation that incorporates the linear and hyperbolic nature of the force-velocity relationship in lower and upper limb exercises

The purpose of this study is to provide a force-velocity (F-V) equation that combines a linear and a hyperbolic region, and to compare its derived results to those obtained from linear equations. A total of 10 cross-training athletes and 14 recreationally resistance-trained young men were assessed in the unilateral leg press (LP) and bilateral bench press (BP) exercises, respectively. F-V data were recorded using a force plate and a linear encoder. Estimated maximum isometric force (F₀), maximum muscle power (P_max), and maximum unloaded velocity (V₀) were calculated using a hybrid (linear and hyperbolic) equation and three different linear equations: one derived from the hybrid equation (linear_hyb), one applied to data from 0 to 100% of F₀ (linear_0-100), and one applied to data from 45 to 100% of F₀ (linear_45-100). The hybrid equation presented the best fit to the recorded data (R² = 0.996 and 0.998). Compared to the results derived from the hybrid equation in the LP, significant differences were observed in F₀ derived from linear_0-100; V₀ derived from linear_hyb, linear_0-100 and linear_45-100; and P_max derived from linear_hyb and linear_45-100 (all p < 0.05). For the BP, compared to the hybrid equation, significant differences were found in F₀ derived from linear_0-100; and V₀ and P_max derived from linear_hyb, linear_0-100 and linear_45-100 (all p < 0.05). An F-V equation combining a linear and a hyperbolic region showed to fit adequately recorded F-V data from ~ 20 to 100% of F₀, and overcame the limitations shown by linear equations while providing relevant results.

The Validity of the 2-Point Method for Assessing the Force-Velocity Relationship of the Knee Flexors and Knee Extensors: The Relevance of Distant Force-Velocity Testing

Over the past decade, force-velocity (F-v) profiling has emerged as a promising tool for assessing neuromuscular capacity to design individually tailored interventions in diverse populations. To date, a limited number of studies have addressed the optimization of the linear method for measuring F-v profiles of single-joint isokinetic movements. We aimed to simplify the measurement of knee extension (KE) and knee flexion (KF) isokinetic tasks by evaluating the most appropriate combination of two velocities (i.e., the 2-point method). Twenty-two healthy participants (11 males and 11 females) were included in the study. Isokinetic peak torque was measured at nine angular velocities (30-60-90-120-150-180-210-240-300°/s) and under isometric conditions (at 150° and 120° of KF for KE, and KF, respectively). Maximal theoretical force (F₀), maximal theoretical velocity (v₀), slope of the relationship (S_fv) and maximal theoretical power (P_max) were derived from the linear F-v profiles of KE and KF and compared between the 9-point method and all possible combinations (36 in total) of the 2-point methods. The F-v profiles obtained from nine points were linear for KE (R2 = 0.95; 95% CI = 0.94–0.96) and KF (R2 = 0.93; 95% CI = 0.90–0.95), with F₀ underestimating isometric force. Further analyses revealed great to excellent validity (range: ICCs = 0.89–0.99; CV = 2.54%–4.34%) and trivial systematic error (range: ES = −0.11–0.24) of the KE 2-point method when force from distant velocities (30°/s, 60°/s or 90°/s combined with 210°/s, 240°/s or 300°/s) was used. Similarly, great to excellent validity and trivial systematic error of the KF 2-point method for F0 and Pmax (range: ICC = 0.90–0.96; CV = 2.94%–6.38%; ES = −0.07–0.14) were observed when using the previously described combinations of velocities. These results suggest that practitioners should consider using more distant velocities when performing simplified isokinetic 2-point single-joint F-v profiling. Furthermore, the F-v profile has the potential to differentiate between the mechanical properties of knee extensors and flexors and could therefore serve as a potential descriptor of performance.

T2 mapping for knee cartilage degeneration in young patients with mild symptoms

BACKGROUND

We aimed to analyze the distribution of knee cartilage degeneration in young patients with mild symptoms using quantitative magnetic resonance imaging (MRI) T2 mapping.

MATERIALS AND METHODS

This study included sixty six patients (case group) and twenty eight healthy volunteers (control group). The participants underwent 3.0 T conventional MRI plus a multi-echo sequence. The cartilage of each participant was divided into twenty eight subregions. We then calculated the T2 mean values and standard deviation or median and quartile range for each subregion according to whether the normal distribution was satisfied. Besides, we employed Kruskal-Wallis test to determine the statistical differences of each subregion in the control group while the Mann-Whitney U test was used to define the statistical difference between the case group and the control group and between the control group and subjects aged less than or equal to 35 years in the case group.

RESULTS

In the case group, age of 30 male patients was 31.5 ± 9.3 and age of 36 female patients was 35.7 ± 8.3. In the two groups, the superficial central lateral femoral region exhibited relatively high T2 values (control/case group: 49.6 ± 2.7/55.9 ± 8.8), and the deep medial patellar region exhibited relatively low T2 values (control/case group: 34.2 ± 1.3/33.5(32.2, 35.5)). Comparison of the T2 values between the case and the control group demonstrated a statistically significant increase in nine subregions (P₁ < 0.05) and there were five subregions in the case group with age ≤ 35 years (P₂ < 0.05). In particular, the p-values for four subregions of the patellofemoral joint were all less than 0.05 (P₁ = 0.002, 0.015, 0.036, 0.005).

CONCLUSION

T2 values of patients were significantly different with values of healthy groups, especially in the superficial cartilage of the patellofemoral joint. It made T2 mapping helpful to early identify patients with knee cartilage degeneration.

Maximum isometric torque at individually-adjusted joint angles exceeds eccentric and concentric torque in lower extremity joint actions

Background

Previous research indicates the high relevance of optimal joint angles for individual isometric strength assessment. The objective was to compare lower limb peak isometric muscle strength abilities at the strongest joint angles with those of dynamic contractions in healthy young adults.

Methods

Eighteen young male adults performed maximum concentric, isometric, and eccentric contractions of the ankle, knee, and hip flexors and extensors, and hip adductors and abductors in a randomized sequence on an isokinetic dynamometer (ISOMED 2000). Angular velocity was set at 60°/s. The peak of concentric contraction torque curves was used to define optimal joint angles best suited to generate maximum torque during isometric contractions. Maximum voluntary contraction torque of all contraction conditions was adjusted for limb weight and analyzed via a generalized linear mixed gamma regression model (GLMM).

Results

The gamma GLMM revealed strongly significant effects for all three categorical covariates (contraction types, muscle group, and test order) (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$p < 2 \times 10^{ - 16}$$\end{document}p<2×10-16). Eccentric contraction increases the muscle torque (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\beta }_{k} = 0.147$$\end{document}β^k=0.147) compared to concentric contraction, and isometric contraction even more (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\beta }_{k} = 0.258$$\end{document}β^k=0.258). A moderate individual-specific variation was found (random effects standard deviation \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sigma_{b} = 0.093$$\end{document}σb=0.093).

Conclusion

The results support the importance of optimal joint angles for isometric maximum strength assessment. When such conditions are given, isometric contractions can produce higher muscle torques than eccentric contractions in the lower body.

Determining concentric and eccentric force-velocity profiles during squatting

Purpose

The force–velocity relationship of muscular contraction has been extensively studied. However, previous research has focussed either on isolated muscle or single-joint movements, whereas human movement consists of multi-joint movements (e.g. squatting). Therefore, the purpose of this study was to investigate the force–velocity relationship of isovelocity squatting.

Methods

Fifteen male participants (24 ± 2 years, 79.8 ± 9.1 kg, 177.5 ± 6 cm) performed isovelocity squats on a novel motorised isovelocity device (Kineo Training System) at three concentric (0.25, 0.5, and 0.75 m s⁻¹) and three eccentric velocities (− 0.25, − 0.5, and − 0.75 m s⁻¹). Peak vertical ground reaction forces, that occurred during the isovelocity phase, were collected using dual force plates (2000 Hz) (Kistler, Switzerland).

Results

The group mean squat force–velocity profile conformed to the typical in vivo profile, with peak vertical ground reaction forces during eccentric squatting being 9.5 ± 19% greater than isometric (P = 0.037), and occurring between − 0.5 and − 0.75 m s⁻¹. However, large inter-participant variability was identified (0.84–1.62 × isometric force), with some participants being unable to produce eccentric forces greater than isometric. Sub-group analyses could not identify differences between individuals who could/could not produce eccentric forces above isometric, although those who could not tended to be taller.

Conclusions

These finding suggest that variability exists between participants in the ability to generate maximum eccentric forces during squatting, and the magnitude of eccentric increase above isometric cannot be predicted solely based on a concentric assessment. Therefore, an assessment of eccentric capabilities may be required prior to prescribing eccentric-specific resistance training.

Vision-based human joint angular velocity estimation during squat and walking on a treadmill actions

Elderly health monitoring, rehabilitation training, and sport supervision could benefit from continuous assessment of joint angle, and angular velocity to identify the joint movement patterns. However, most of the measurement systems are designed based on special kinematic sensors to estimate angular velocities. The study aims to measure the lower limb joint angular velocity based on a 2D vision camera system during squat and walking on treadmill action using deep convolution neural network (CNN) architecture. Experiments were conducted on 12 healthy adults, and six digital cameras were used to capture the videos of the participant actions in lateral and frontal view. The normalized cross-correlation (Cc_norm) analysis was performed to obtain a degree of symmetry of the ground truth and estimated angular velocity waveform patterns. Mean Cc_norm for angular velocity estimation by deep CNN model has higher than 0.90 in walking on the treadmill and 0.89 in squat action. Furthermore, joint-wise angular velocities at the hip, knee, and ankle joints were observed and compared. The proposed system gets higher estimation performance under the lateral view and the frontal view of the camera. This study potentially eliminates the requirement of wearable sensors and proves the applicability of using video-based system to measure joint angular velocities during squat and walking on a treadmill actions.

Contribution of Stretch-Induced Force Enhancement to Increased Performance in Maximal Voluntary and Submaximal Artificially Activated Stretch-Shortening Muscle Action

In everyday muscle action or exercises, a stretch-shortening cycle (SSC) is performed under different levels of intensity. Thereby, compared to a pure shortening contraction, the shortening phase in a SSC shows increased force, work, and power. One mechanism to explain this performance enhancement in the SSC shortening phase is, besides others, referred to the phenomenon of stretch-induced increase in muscle force (known as residual force enhancement; rFE). It is unclear to what extent the intensity of muscle action influences the contribution of rFE to the SSC performance enhancement. Therefore, we examined the knee torque, knee kinematics, m. vastus lateralis fascicle length, and pennation angle changes of 30 healthy adults during isometric, shortening (CON) and stretch-shortening (SSC) conditions of the quadriceps femoris. We conducted maximal voluntary contractions (MVC) and submaximal electrically stimulated contractions at 20%, 35%, and 50% of MVC. Isometric trials were performed at 20° knee flexion (straight leg: 0°), and dynamic trials followed dynamometer-driven ramp profiles of 80°–20° (CON) and 20°–80°–20° (SSC), at an angular velocity set to 60°/s. Joint mechanical work during shortening was significantly (p < 0.05) enhanced by up to 21% for all SSC conditions compared to pure CON contractions at the same intensity. Regarding the steady-state torque after the dynamic phase, we found significant torque depression for all submaximal SSCs compared to the isometric reference contractions. There was no difference in the steady-state torque after the shortening phases between CON and SSC conditions at all submaximal intensities, indicating no stretch-induced rFE that persisted throughout the shortening. In contrast, during MVC efforts, the steady-state torque after SSC was significantly less depressed compared to the steady-state torque after the CON condition (p = 0.034), without significant differences in the m. vastus lateralis fascicle length and pennation angle. From these results, we concluded that the contribution of the potential enhancing factors in SSCs of the m. quadriceps femoris is dependent on the contraction intensity and the type of activation.

Force enhancement in the human vastus lateralis is muscle-length-dependent following stretch but not during stretch

Purpose

Force enhancement is the phenomenon of increased forces during (transient force enhancement; tFE) and after (residual force enhancement; rFE) eccentric muscle actions compared with fixed-end contractions. Although tFE and rFE have been observed at short and long muscle lengths, whether both are length-dependent remains unclear in vivo.

Methods

We determined maximal-effort vastus lateralis (VL) force-angle relationships of eleven healthy males and selected one knee joint angle at a short and long muscle lengths where VL produced approximately the same force (85% of maximum). We then examined tFE and rFE at these two lengths during and following the same amount of knee joint rotation.

Results

We found tFE at both short (11.7%, P = 0.017) and long (15.2%, P = 0.001) muscle lengths. rFE was only observed at the long (10.6%, P < 0.001; short: 1.3%, P = 0.439) muscle length. Ultrasound imaging revealed that VL muscle fascicle stretch magnitude was greater at long compared with short muscle lengths (mean difference: (tFE) 1.7 mm, (rFE) 1.9 mm, P ≤ 0.046), despite similar isometric VL forces across lengths (P ≥ 0.923). Greater fascicle stretch magnitude was likely to be due to greater preload forces at the long compared with short muscle length (P ≤ 0.001).

Conclusion

At a similar isometric VL force capacity, tFE was not muscle-length-dependent at the lengths we tested, whereas rFE was greater at longer muscle length. We speculate that the in vivo mechanical factors affecting tFE and rFE are different and that greater stretch of a passive component is likely contributing more to rFE at longer muscle lengths.

Electronic supplementary material

The online version of this article (10.1007/s00421-020-04488-1) contains supplementary material, which is available to authorized users.

Comparison of linear, hyperbolic and double-hyperbolic models to assess the force-velocity relationship in multi-joint exercises

AbstractThis study assessed the validity of linear, hyperbolic and double-hyperbolic models to fit measured force-velocity (F-V) data in multi-joint exercises and the influence of muscle excitation on the F-V relationship. The force-joint angle and F-V relationships were assessed in 10 cross-training athletes and 14 recreationally resistance-trained subjects in the unilateral leg press (LP) and bilateral bench press (BP) exercises, respectively. A force plate and a linear encoder were installed to register external force and velocity, respectively. Muscle excitation was assessed by surface EMG recording of the quadriceps femoris, biceps femoris and gluteus maximus muscles during the unilateral LP. Linear, Hill's (hyperbolic) and Edman's (double-hyperbolic) equations were fitted to the measured F-V data and compared. Measured F-V data were best fitted by double-hyperbolic models in both exercises (p < 0.05). F-V data deviated from the rectangular hyperbola above a breakpoint located at 90% of measured isometric force (F₀) and from the linearity at ≤45% of F₀ (both p < 0.05). Hyperbolic equations overestimated F₀ values by 13 ± 11% and 6 ± 6% in the LP and BP, respectively (p < 0.05). No differences were found between muscle excitation levels below and above the breakpoint (p > 0.05). Large associations between variables obtained from linear and double-hyperbolic models were noted for F₀, maximum muscle power, and velocity between 25% and 100% of F₀ (r = 0.70-0.99; all p < 0.05). The F-V relationship in multi-joint exercises was double-hyperbolic, which was unrelated with lower muscle excitation levels. However, linear models may be valid to assess F₀, maximal muscle power and velocity between 25% and 100% of F₀.

Is Test Standardization Important when Arm and Leg Muscle Mechanical Properties are Assessed through the Force-Velocity Relationship?

The force‐velocity (F‐V) relationship observed in multi‐joint tasks proved to be strong and approximately linear. Recent studies showed that mechanical properties of muscles: force (F), velocity (V) and power (P) could be assessed through the F‐V relationship although the testing methods have not been standardized. The aim of the present study was to evaluate and compare F‐V relationships assessed from two tests performed on a modified Smith machine that standardizes kinematics of the movement pattern. Fifteen participants were tested on the maximum performance bench press throws and squat jumps performed against a variety of different loads. In addition, their strength properties were assessed through maximum isometric force (Fiso) and one repetition maximum (1 RM). The observed individual F‐V relationships were exceptionally strong and approximately linear (r = 0.98 for bench press throws; r = 0.99 for squat jumps). F‐V relationship parameter depicting maximum force (F0) revealed high correlations with both Fiso and 1 RM indicating high concurrent validity (p < 0.01). However, the generalizability of F‐V relationship parameters depicting maximum force (F0), velocity (V0) and power (P0) of the tested muscle groups was inconsistent and on average low (i.e. F0; r = ‐0.24) to moderate (i.e. V0 and P0; r = 0.54 and r = 0.64, respectively; both p < 0.05). We concluded that the F‐V relationship could be used for the assessment of arm and leg muscle mechanical properties when standard tests are applied, since the typical outcome is an exceptionally strong and linear F‐V relationship, as well as high concurrent validity of its parameters. However, muscle mechanical properties could be only partially generalized across different tests and muscles.

On the Shape of the Force-Velocity Relationship in Skeletal Muscles: The Linear, the Hyperbolic, and the Double-Hyperbolic

The shape of the force-velocity (F-V) relationship has important implications for different aspects of muscle physiology, such as muscle efficiency and fatigue, the understanding of the pathophysiology of several myopathies or the mechanisms of muscle contraction per se, and may be of relevance for other fields, such as the development of robotics and prosthetic applications featuring natural muscle-like properties. However, different opinions regarding the shape of the F-V relationship and the underlying mechanisms exist in the literature. In this review, we summarize relevant evidence on the shape of the F-V relationship obtained over the last century. Studies performed at multiple scales ranging from the sarcomere to the organism level have described the concentric F-V relationship as linear, hyperbolic or double-hyperbolic. While the F-V relationship has most frequently been described as a rectangular hyperbola, a large number of studies have found deviations from the hyperbolic function at both ends of the F-V relation. Indeed, current evidence suggests that the F-V relation in skeletal muscles follows a double-hyperbolic pattern, with a breakpoint located at very high forces/low velocities, which may be a direct consequence of the kinetic properties of myofilament cross-bridge formation. Deviations at low forces/high velocities, by contrast, may be related to a recently discovered, calcium-independent regulatory mechanism of muscle contraction, which may also explain the low metabolic cost of very fast muscle shortening contractions. Controversial results have also been reported regarding the eccentric F-V relationship, with studies in prepared muscle specimens suggesting that maximum eccentric force is substantially greater than isometric force, whereas in vivo studies in humans show only a modest increase, no change, or even a decrease in force in lengthening contractions. This review discusses possible reasons reported in the literature for these discrepant findings, including the testing procedures (familiarization, pre-load condition, and temperature) and a potential neural inhibition at higher lengthening velocities. Finally, some unresolved questions and recommendations for F-V testing in humans are reported at the end of this document.

Shortening-induced residual force depression in humans

When an isometric muscle contraction is immediately preceded by an active shortening contraction, a reduction in steady-state isometric force is observed relative to an isometric reference contraction at the same muscle length and level of activation. This shortening-induced reduction in isometric force, termed “residual force depression” (rFD), has been under investigation for over a half century. Various experimental models have revealed the positive relationship between rFD and the force and displacement performed during shortening, with rFD values ranging from 5 to 39% across various muscle groups, which appears to be due to a stress-induced inhibition of cross-bridge attachments. The current review will discuss the findings of rFD in humans during maximal and submaximal contractions.

Force-velocity profiling of sprinting athletes: single-run vs. multiple-run methods

PURPOSE

This study explored the agreement between a single-run and a multiple-run method for force-velocity (Fv) profiling of sprinting athletes; we evaluated both absolute values and changes over time caused by sprint training.

METHODS

Seventeen female handball players (23 ± 3 years, 177 ± 7 cm, 73 ± 6 kg) performed 30 m un-resisted and resisted sprints (50, 80 and 110 N resistance) before and after an 8-week sprint training intervention. Two approaches were used to calculate theoretical maximal velocity (v₀), horizontal force (F₀), power (P_max), and the force-velocity slope (S_Fv): (1) the single-run method, based on inverse dynamics applied to the centre-of-mass movement, was calculated from anthropometric and sprint split time data; and (2) the multiple-run method, where peak velocity from un-resisted and resisted sprints were plotted against the horizontal resistances.

RESULTS

Trivial differences in v₀ (0.7%) were observed between the two calculation methods. Corresponding differences for F₀, P_max and S_Fv were 16.4, 15.6 and 17.6%, respectively (most likely; very large effect size). F₀ showed poor agreement between the methods (r = 0.26 and 0.16 before and after the intervention). No substantial correlation between the changes (from pre- to post-training tests) in SFV calculated with the single-run and the multiple-run methods were observed (r = 0.03) [corrected].

CONCLUSIONS

This study revealed poor agreement between the Fv relationships of the investigated calculation methods. In practice, both methods may have a purpose, but the single-run and the multiple-run methods appear to measure somewhat different sprint properties and cannot be used interchangeably.

A versatile approach to determine instantaneous co-activation: Development, implementation and comparison to existing measures

An adaptable method of computing muscle co-activity is presented. Instantaneous muscle activities from multiple muscles are reduced into commonality and activity level dimensions. Both dimensions are weighted independently and combined into a co-activation measure, Φ. Myoelectric data from relaxed and braced simulated rear vehicle impacts were used to compare Φ to two existing co-activity measures in muscle pair and multi-muscle cases, Φ showed greater sensitivity in the muscle pair case and could better discriminate the relaxed and braced conditions in the multi-muscle case than previous measures. The flexibility of Φ allowed tailoring to reflect the current purpose.

Stretching the limits of maximal voluntary eccentric force production in vivo

During eccentric contractions, muscular force production capacity is enhanced compared to isometric contractions. Although this is well accepted in terms of muscle mechanics, maximal voluntary eccentric contractions are associated with neural inhibition that prevents increased force production of in vivo human muscles. However, because it was shown that maximal voluntary eccentric forces can exceed maximum isometric forces by a factor of 1.2-1.4, this review focuses on the question of whether the absent eccentric force enhancement, as observed in many studies, can unambiguously be attributed to an inherent neural inhibition. First, we demonstrate that participant familiarization, preload, and fascicle behavior are crucial factors influencing maximal voluntary eccentric force production. Second, we show that muscle mechanics such as muscle length, lengthening velocity, and stretch amplitude interact when it comes to maximal voluntary eccentric force production. Finally, we discuss the diverging findings on neural inhibition during maximal voluntary eccentric contractions. Because there was no inhibition of the major motor pathways in the presence of enhanced maximal voluntary eccentric forces, further research is needed to test the concept of neural inhibition and to understand why maximal voluntary force production is reduced compared to the force capacity of isolated muscle preparations.

Joint angle and movement velocity effects on muscle activity of elderly with knee osteoarthritis - Categorized and probabilistic analysis

The aim of the study was to determine the effects of joint angle position and angular velocity on concentric and eccentric knee muscles activity of elderly with osteoarthritis (OA) in a deterministic and probabilistic approach compared to matched controls. Concentric and eccentric muscle activation of vastus medialis (VM) and semitendinous (ST) muscles were recorded of eleven elderly women with knee OA (median (Md (25-75%)) age of 62 years (60-72) and Md of body mass index (BMI) of 26 kg/m² (24.5-27.2)) and ten controls (Md 65 years (62-69) and Md of BMI 24.5 kg/m² (23.6-28.9), during twenty-five knee extension-flexion movements. Activation type, angular velocities (90° s^-1 and 240° s^-1) and joint angle intervals were categorized into groups. The cumulative frequency distributions of the normalized sEMG envelope were computed and the probability to be out of specific norm-reference limits (controls) was calculated. No statistical differences between groups were found. Higher probabilities were found for VM and ST (concentric) and ST (eccentric) activation to be out of norm (55%, 53% and 84%, respectively) at 240 s^-1 in different joint angles. During dynamic contractions, concentric and eccentric activity of medial knee muscles of elderly with OA were affected in a different way by joint angles and angular velocity compared to matched controls. The probabilistic analysis provided an additional understanding of the muscle activation between elderly with knee OA and healthy older people.

Muscle Force-Velocity Relationships Observed in Four Different Functional Tests

The aims of the present study were to investigate the shape and strength of the force-velocity relationships observed in different functional movement tests and explore the parameters depicting force, velocity and power producing capacities of the tested muscles. Twelve subjects were tested on maximum performance in vertical jumps, cycling, bench press throws, and bench pulls performed against different loads. Thereafter, both the averaged and maximum force and velocity variables recorded from individual trials were used for force–velocity relationship modeling. The observed individual force-velocity relationships were exceptionally strong (median correlation coefficients ranged from r = 0.930 to r = 0.995) and approximately linear independently of the test and variable type. Most of the relationship parameters observed from the averaged and maximum force and velocity variable types were strongly related in all tests (r = 0.789-0.991), except for those in vertical jumps (r = 0.485-0.930). However, the generalizability of the force-velocity relationship parameters depicting maximum force, velocity and power of the tested muscles across different tests was inconsistent and on average moderate. We concluded that the linear force-velocity relationship model based on either maximum or averaged force-velocity data could provide the outcomes depicting force, velocity and power generating capacity of the tested muscles, although such outcomes can only be partially generalized across different muscles.

MUSCLE FORCE MAGNITUDES IN THE HUMAN LEG FOR ISOMETRIC EXERCISES WITH VARIOUS RESULTANT FORCE DIRECTIONS AND JOINT ANGLES

Using a gradient-based numerical optimization routine, the force magnitudes required of 10 major sagittal plane leg muscles to create a constant magnitude isometric resultant force against a fixed surface at the toe directed anteriorly, posteriorly, superiorly, and inferiorly were quantitatively predicted for three sets of joint angles: a straight leg configuration, with the knee flexed, and with both the hip and knee flexed. Comparisons over the conditions studied for each individual system muscle found that the maximum variation occurred in knee and hip extensor forces (up to two orders of magnitude). Comparisons within the set of active muscles for each studied condition identified dominant muscles and muscle functions. All anteriorly-directed and posteriorly-directed resultants required a small number of muscles with common functions (mainly knee or hip extensors) and large force magnitudes (O(1000[Formula: see text]N)). In contrast, a large number of muscles, with wide-ranging synergistic and antagonistic functions, acting across multiple joints with relatively small magnitudes (O(100[Formula: see text]N)) were needed to create the superiorly-directed resultant with flexed hip and knee. With good correlation to experimentally measured trends in the interrelationships between leg joint angles and isometric forces, the systematic muscle force prediction and analysis presented in this work can be used to guide the design of targeted muscle strengthening exercises and study of muscle-specific injury.

The effect of proprioceptive knee bracing on knee stability during three different sport related movement tasks in healthy subjects and the implications to the management of Anterior Cruciate Ligament (ACL) injuries

INTRODUCTION

Proprioceptive knee braces have been shown to improve knee mechanics, however much of the work to date has focused on tasks such as slow step down tasks rather than more dynamic sporting tasks.

OBJECTIVE

This study aimed to explore if such improvements in stability may be seen during faster sports specific tasks as well as slower tasks.

METHOD

Twelve subjects performed a slow step down, single leg drop jump and pivot turn jump with and without a silicone web brace. 3D kinematics of the knee were collected using a ten camera Qualisys motion analysis system. Reflective markers were placed on the foot, shank, thigh and pelvis using the Calibrated Anatomical Systems Technique. A two way ANOVA with repeated measures was performed with post-hoc pairwise comparison to explore the differences between the two conditions and three tasks.

RESULTS

Significant differences were seen in the knee joint angles and angular velocities in the sagittal, coronal and transverse planes between the tasks. The brace showed a reduction in knee valgum and internal rotation across all tasks, with the most notable effect during the single leg drop jump and pivot turn jump. The transverse plane also showed a significant reduction in the external rotation knee angular velocity when wearing the brace.

DISCUSSION

The brace influenced the knee joint kinematics in coronal and transverse planes which confirms that such braces can have a significant effect on knee control during dynamic tasks. Further studies are required exploring the efficacy of proprioceptive braces in athletic patient cohort.

Examination of the torque required to passively palmar abduct the thumb CMC joint in a pediatric population with hemiplegia and stroke

Many activities of daily living involve precision grasping and bimanual manipulation, such as putting toothpaste on a toothbrush or feeding oneself. However, children afflicted by stroke, cerebral palsy, or traumatic brain injury may have lost or never had the ability to actively and accurately control the thumb. To translate insights from adult rehabilitation robotics to innovative therapies for hand rehabilitation in pediatric care, specifically for thumb deformities, an understanding of the torque needed to abduct the thumb to assist grasping tasks is required. Participants (n=16, 10 female, 13.2±3.1 years) had an upper extremity evaluation and measures were made of their passive range of motion, anthropometrics, and torques to abduct the thumb for both their affected and non-affected sides. Torque measures were made using a custom wrist orthosis that was adjusted for each participant. The torque to achieve maximum abduction was 1.47±0.61inlb for the non-affected side and 1.51±0.68inlb for the affected side, with a maximum recorded value of 4.87inlb. The overall maximum applied torque was observed during adduction and was 5.10inlb. We saw variation in the applied torque, which could have been due to the applied torques by the Occupational Therapist or the participant actively assisting or resisting the motion rather than remaining passive. We expect similar muscle and participant variation to exist with an assistive device. Thus, the data presented here can be used to inform the specifications for the development of an assistive thumb orthosis for children with "thumb-in-palm" deformity.

Deconstructing the power resistance relationship for squats: A joint-level analysis

Generating high leg power outputs is important for executing rapid movements. Squats are commonly used to increase leg strength and power. Therefore, it is useful to understand factors affecting power output in squatting. We aimed to deconstruct the mechanisms behind why power is maximized at certain resistances in squatting. Ten male rowers (age = 20 ± 2.2 years; height = 1.82 ± 0.03 m; mass = 86 ± 11 kg) performed maximal power squats with resistances ranging from body weight to 80% of their one repetition maximum (1RM). Three-dimensional kinematics was combined with ground reaction force (GRF) data in an inverse dynamics analysis to calculate leg joint moments and powers. System center of mass (COM) velocity and power were computed from GRF data. COM power was maximized across a range of resistances from 40% to 60% 1RM. This range was identified because a trade-off in hip and knee joint powers existed across this range, with maximal knee joint power occurring at 40% 1RM and maximal hip joint power at 60% 1RM. A non-linear system force-velocity relationship was observed that dictated large reductions in COM power below 20% 1RM and above 60% 1RM. These reductions were due to constraints on the control of the movement.