Muscle coordination of maximum-speed pedaling

A dynamic foot model for predictive simulations of human gait reveals causal relations between foot structure and whole-body mechanics

The unique structure of the human foot is seen as a crucial adaptation for bipedalism. The foot's arched shape enables stiffening the foot to withstand high loads when pushing off, without compromising foot flexibility. Experimental studies demonstrated that manipulating foot stiffness has considerable effects on gait. In clinical practice, altered foot structure is associated with pathological gait. Yet, experimentally manipulating individual foot properties (e.g. arch height or tendon and ligament stiffness) is hard and therefore our understanding of how foot structure influences gait mechanics is still limited. Predictive simulations are a powerful tool to explore causal relationships between musculoskeletal properties and whole-body gait. However, musculoskeletal models used in three-dimensional predictive simulations assume a rigid foot arch, limiting their use for studying how foot structure influences three-dimensional gait mechanics. Here, we developed a four-segment foot model with a longitudinal arch for use in predictive simulations. We identified three properties of the ankle-foot complex that are important to capture ankle and knee kinematics, soleus activation, and ankle power of healthy adults: (1) compliant Achilles tendon, (2) stiff heel pad, (3) the ability to stiffen the foot. The latter requires sufficient arch height and contributions of plantar fascia, and intrinsic and extrinsic foot muscles. A reduced ability to stiffen the foot results in walking patterns with reduced push-off power. Simulations based on our model also captured the effects of walking with anaesthetised intrinsic foot muscles or an insole limiting arch compression. The ability to reproduce these different experiments indicates that our foot model captures the main mechanical properties of the foot. The presented four-segment foot model is a potentially powerful tool to study the relationship between foot properties and gait mechanics and energetics in health and disease.

Intensive Aerobic Cycling Is Feasible and Elicits Improvements in Gait Velocity in Individuals With Multiple Sclerosis: A Preliminary Study

BACKGROUND

Aerobic exercise (AEx) has many potential benefits; however, it is unknown whether individuals with multiple sclerosis (MS) can attain the optimal intensity and duration to harness its effects. Forced-rate exercise (FE) is a novel paradigm in which the voluntary pedaling rate during cycling is supplemented to achieve a higher exercise intensity. The aim of this pilot trial was to investigate the feasibility and initial efficacy of a 12-week FE or voluntary exercise (VE) cycling intervention for individuals with MS.

METHODS

Twenty-two participants with MS (Expanded Disability Severity Scale [EDSS] 2.0-6.5) were randomly assigned to FE (n = 12) or VE (n = 10), each with twice weekly 45-minute sessions at a prescribed intensity of 60% to 80% of maximum heart rate (HR).

RESULTS

Eighteen individuals (FE = 11; VE = 7) completed the intervention, however, adaptations were required in both groups to overcome barriers to cycling. Overall, participants exercised for an average of 42.2 ± 2.3 minutes at an aerobic intensity of 65% ± 7% of maximum HR and a pedaling cadence of 67.3 ± 13.3 RPM. Cycling led to improved treadmill walking speed (0.61 to 0.68 m/sec, P = .010), with somewhat greater improvement with FE compared to VE (increase of 0.09 vs 0.03 m/s, respectively, P = .17) post intervention. Notably, the participant with the highest disability level (EDSS 6.5) tolerated FE but not VE.

CONCLUSIONS

Aerobic exercise is feasible for individuals with MS, although those with increased disability may require novel paradigms such as FE to achieve targeted intensity. Further trials are warranted to investigate the effects of FE across the MS disability spectrum.

Optimal fibre length and maximum isometric force are the most influential parameters when modelling muscular adaptations to unloading using Hill-type muscle models

Introduction: Spaceflight is associated with severe muscular adaptations with substantial inter-individual variability. A Hill-type muscle model is a common method to replicate muscle physiology in musculoskeletal simulations, but little is known about how the underlying parameters should be adjusted to model adaptations to unloading. The aim of this study was to determine how Hill-type muscle model parameters should be adjusted to model disuse muscular adaptations. Methods: Isokinetic dynamometer data were taken from a bed rest campaign and used to perform tracking simulations at two knee extension angular velocities (30°·s-1 and 180°·s-1). The activation and contraction dynamics were solved using an optimal control approach and direct collocation method. A Monte Carlo sampling technique was used to perturb muscle model parameters within physiological boundaries to create a range of theoretical and feasible parameters to model muscle adaptations. Results: Optimal fibre length could not be shortened by more than 67% and 61% for the knee flexors and non-knee muscles, respectively. Discussion: The Hill-type muscle model successfully replicated muscular adaptations due to unloading, and recreated salient features of muscle behaviour associated with spaceflight, such as altered force-length behaviour. Future researchers should carefully adjust the optimal fibre lengths of their muscle-models when trying to model adaptations to unloading, particularly muscles that primarily operate on the ascending and descending limbs of the force-length relationship.

The effects of cycling on walking outcomes in adults with stroke: a systematic review

BACKGROUND

Stationary cycling is often prescribed for survivors of stroke as a safe means of aerobic exercise to improve cardiovascular health. While cycling is typically not prescribed to restore ambulatory function, improvements in measures of walking after cycling interventions have been reported in the literature.

OBJECTIVE

To investigate the effects of cycling on walking outcomes in adults with stroke.

METHODS

Relevant databases were searched through 15 August. Walking-related outcomes were extracted. Correlation coefficients were computed to measure the relationship between exercise protocol parameters and change in walking outcomes.

RESULTS

Eleven articles were included in the review. Eight studies representing nine cycling intervention groups reported change in walking capacity measured by the six-minute walk test with improvements ranging from 6.1 to 63.0 m. Seven studies measured gait velocity, reporting improvements ranging from 0.01 to 0.21 m/sec. Protocols that yielded the greatest improvement in walking capacity prescribed moderate- to high-intensity aerobic training. Significant positive correlations were measured between change in gait velocity and number of exercise sessions and total minutes of exercise prescribed.

CONCLUSION

Considerable heterogeneity was observed across cycling protocols with respect to intensity, frequency, exercise duration and protocol duration. However, none of the studies reported declines in walking outcomes and improvements were measured in the absence of task-specific gait training. Cycling interventions employing moderate- to high-intensity aerobic training and 24 sessions or more may be optimal in improving gait velocity and walking capacity.

The Utilization of Forced-Rate Cycling to Facilitate Motor Recovery Following Stroke: A Randomized Clinical Trial

BACKGROUND

The potential for aerobic exercise (AE) to enhance neuroplasticity post-stroke has been theorized but not systematically investigated. Our aim was to determine the effects of forced-rate AE (FE) paired with upper extremity (UE) repetitive task practice (FE + RTP) compared to time-matched UE RTP (RTP only) on motor recovery.

METHODS

A single center randomized clinical trial was conducted from April 2019 to December 2022. Sixty individuals ≥6 months post-stroke with UE hemiparesis were randomized to FE + RTP (N = 30) or RTP only (N = 30), completing 90-minute sessions, 3×/week for 8 weeks. The FE + RTP group underwent 45-minute of FE (5-minute warm-up, 35-minute main set, and 5-minute cool down) followed by 45-minute of UE RTP. The RTP only group completed 90-minute of RTP. Primary outcomes were the Fugl-Meyer Assessment (FMA) and Action Research Arm Test (ARAT). The 6-minute Walk Test (6MWT, secondary outcome) assessed walking capacity.

RESULTS

Sixty individuals enrolled and 56 completed the study. The RTP only group completed more RTP in terms of repetitions (411.8 ± 44.4 vs 222.8 ± 28.4, P < .001) and time (72.7 ± 6.7 vs 37.8 ± 2.4 minutes, P < .001) versus FE + RTP. There was no significant difference between groups on the FMA (FE + RTP, 36.2 ± 10.1-44.0 ± 11.8 and RTP only, 34.4 ± 11.0-41.2 ± 13.4, P = .43) or ARAT (FE + RTP, 32.5 ± 16.6-37.7 ± 17.9 and RTP only, 32.8 ± 18.6-36.4 ± 18.5, P = .88). The FE + RTP group demonstrated greater improvements on the 6MWT (274.9 ± 122.0-327.1 ± 141.2 m) versus RTP only (285.5 ± 160.3-316.9 ± 170.0, P = .003).

CONCLUSIONS

There was no significant difference between groups in the primary outcomes. The FE + RTP improved more on the 6MWT, a secondary outcome.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT03819764.

A novel computational framework for the estimation of internal musculoskeletal loading and muscle adaptation in hypogravity

Introduction: Spaceflight is associated with substantial and variable musculoskeletal (MSK) adaptations. Characterisation of muscle and joint loading profiles can provide key information to better align exercise prescription to astronaut MSK adaptations upon return-to-Earth. A case-study is presented of single-leg hopping in hypogravity to demonstrate the additional benefit computational MSK modelling has when estimating lower-limb MSK loading. Methods: A single male participant performed single-leg vertical hopping whilst attached to a body weight support system to replicate five gravity conditions (0.17, 0.25, 0.37, 0.50, 1 g). Experimental joint kinematics, joint kinetics and ground reaction forces were tracked in a data-tracking direct collocation simulation framework. Ground reaction forces, sagittal plane hip, knee and ankle net joint moments, quadriceps muscle forces (Rectus Femoris and three Vasti muscles), and hip, knee and ankle joint reaction forces were extracted for analysis. Estimated quadriceps muscle forces were input into a muscle adaptation model to predict a meaningful increase in muscle cross-sectional area, defined in (DeFreitas et al., 2011). Results: Two distinct strategies were observed to cope with the increase in ground reaction forces as gravity increased. Hypogravity was associated with an ankle dominant strategy with increased range of motion and net plantarflexor moment that was not seen at the hip or knee, and the Rectus Femoris being the primary contributor to quadriceps muscle force. At 1 g, all three joints had increased range of motion and net extensor moments relative to 0.50 g, with the Vasti muscles becoming the main muscles contributing to quadriceps muscle force. Additionally, hip joint reaction force did not increase substantially as gravity increased, whereas the other two joints increased monotonically with gravity. The predicted volume of exercise needed to counteract muscle adaptations decreased substantially with gravity. Despite the ankle dominant strategy in hypogravity, the loading on the knee muscles and joint also increased, demonstrating this provided more information about MSK loading. Discussion: This approach, supplemented with muscle-adaptation models, can be used to compare MSK loading between exercises to enhance astronaut exercise prescription.

Intra- and interindividual reliability of muscle pain induced by an intramuscular injection of hypertonic saline injection into the quadriceps

BACKGROUND

Intramuscular injections of hypertonic saline are commonly used to induce experimental muscle pain, but reliability data on this technique are lacking. This study investigated the intra- and interindividual reliability of pain measures from a hypertonic saline injection into the vastus lateralis.

METHODS

Fourteen healthy participants (6 female) attended three laboratory visits where they received an intramuscular injection of 1 mL hypertonic saline into the vastus lateralis. Changes in pain intensity were recorded on an electronic visual analogue scale, and pain quality was assessed after pain had resolved. Reliability was assessed with the coefficient of variation (CV), minimum detectable change (MDC) and intraclass correlation coefficient (ICC) with 95% CIs.

RESULTS

Mean pain intensity displayed high levels of intraindividual variability (CV = 16.3 [10.5-22.0]%) and 'poor' to 'very good' relative reliability (ICC = 0.71 [0.45-0.88]) but had a MDC of 11 [8-16] au (out of 100). Peak pain intensity exhibited high levels of intraindividual variability (CV = 14.8 [8.8-20.8]%) with 'moderate' to 'excellent' levels of relative reliability (ICC = 0.81 [0.62-0.92]), whereas the MDC was 18 [14-26] au. Measures of pain quality exhibited good reliability. Interindividual variability in pain measures was high (CV > 37%).

CONCLUSIONS

Intramuscular injections of 1 mL of hypertonic saline into the vastus lateralis display substantial levels of interindividual variability, but MDC is below the clinically important changes in pain. This model of experimental pain is suitable for studies involving repeated exposures.

SIGNIFICANCE

Many pain research studies have performed intramuscular injections of hypertonic saline to investigate responses to muscle pain. However, the reliability of this technique is not well established. We examined the pain response over three repeated sessions of a hypertonic saline injection. The pain induced by hypertonic saline has considerable interindividual variability but has largely acceptable intraindividual reliability. Therefore, the injections of hypertonic saline to induce muscle pain are a reliable model of experimental muscle pain.

OpenCap: Human movement dynamics from smartphone videos

Measures of human movement dynamics can predict outcomes like injury risk or musculoskeletal disease progression. However, these measures are rarely quantified in large-scale research studies or clinical practice due to the prohibitive cost, time, and expertise required. Here we present and validate OpenCap, an open-source platform for computing both the kinematics (i.e., motion) and dynamics (i.e., forces) of human movement using videos captured from two or more smartphones. OpenCap leverages pose estimation algorithms to identify body landmarks from videos; deep learning and biomechanical models to estimate three-dimensional kinematics; and physics-based simulations to estimate muscle activations and musculoskeletal dynamics. OpenCap's web application enables users to collect synchronous videos and visualize movement data that is automatically processed in the cloud, thereby eliminating the need for specialized hardware, software, and expertise. We show that OpenCap accurately predicts dynamic measures, like muscle activations, joint loads, and joint moments, which can be used to screen for disease risk, evaluate intervention efficacy, assess between-group movement differences, and inform rehabilitation decisions. Additionally, we demonstrate OpenCap's practical utility through a 100-subject field study, where a clinician using OpenCap estimated musculoskeletal dynamics 25 times faster than a laboratory-based approach at less than 1% of the cost. By democratizing access to human movement analysis, OpenCap can accelerate the incorporation of biomechanical metrics into large-scale research studies, clinical trials, and clinical practice.

Increased Comfortable Gait Speed Is Associated With Improved Gait Biomechanics in Persons With Chronic Stroke Completing an 8-Week Forced-Rate Aerobic Cycling Intervention: A Preliminary Study

ABSTRACT

Task-specific gait training is recommended to improve locomotor function after stroke. Our objective was to determine the effects of a forced-rate aerobic exercise intervention on gait velocity and biomechanics in the absence of task-specific gait training. Individuals with chronic stroke ( N = 14) underwent 24 sessions of forced-rate aerobic exercise, at a targeted aerobic intensity of 60%-80% of their heart rate reserve. Change in comfortable walking speed in addition to spatiotemporal, kinematic, and kinetic variables were measured using three-dimensional motion capture. Overground walking capacity was measured by the 6-min walk test. To determine gait biomechanics associated with increased walking speed, spatiotemporal, kinematic, and kinetic variables were analyzed separately for those who met the minimal clinically important difference for change in gait velocity compared with those who did not. Participants demonstrated a significant increase in gait velocity from 0.61 to 0.70 m/sec ( P = 0.004) and 6-min walk test distance from 272.1 to 325.1 meters ( P < 0.001). Those who met the minimal clinically important difference for change in gait velocity demonstrated significantly greater improvements in spatiotemporal parameters ( P = 0.041), ground reaction forces ( P = 0.047), and power generation ( P = 0.007) compared with those who did not. Improvements in gait velocity were accompanied by normalization of gait biomechanics.

Strength capacity of lower-limb muscles in world-class cyclists: new insights into the limits of sprint cycling performance

This study aimed to determine the relationship between the torque-generating capacity in sprint cycling and the strength capacity of the six lower-limb muscle groups in male and female world-class sprint cyclists. Eleven female and fifteen male top-elite cyclists performed 5-s sprints at maximal power in seated and standing positions. They also performed a set of maximal voluntary ankle, knee and hip flexions and extensions to assess single-joint isometric and isokinetic torques. Isokinetic torques presented stronger correlations with cycling torque than isometric torques for both body positions, regardless of the group. In the female group, knee extension and hip flexion torques accounted for 81.2% of the variance in cycling torque, while the ability to predict cycling torque was less evident in males (i.e., 59% of variance explained by the plantarflexion torque only). The standing condition showed higher correlations than seated and a better predictive model in males (R² = 0.88). In addition to the knee extensors and flexors and hip extensors, main power producers, the strength capacity of lower-limb distal plantarflexor (and to a lesser extent dorsiflexor) muscles, as well as other non-measured qualities (e.g., the upper body), might be determinants to produce such extremely high cycling torque in males.

Sprint cycling rate of torque development associates with strength measurement in trained cyclists

PURPOSE

A cyclist's rate of force/torque development (RFD/RTD) and peak force/torque can be measured during single-joint or whole-body isometric tests, or during cycling. However, there is limited understanding of the relationship between these measures, and of the mechanisms that contribute to each measure. Therefore, we examined the: (i) relationship between quadriceps central and peripheral neuromuscular function with RFD/RTD in isometric knee extension, isometric mid-thigh pull (IMTP), and sprint cycling; and (ii) relationship among RFD/RTD and peak force/torque between protocols.

METHODS

Eighteen trained cyclists completed two familiarisation and two experimental sessions. Each session involved an isometric knee extension, IMTP, and sprint cycling protocol, where peak force/torque, average and peak RFD/RTD, and early (0-100 ms) and late (0-200 ms) RFD/RTD were measured. Additionally, measures of quadriceps central and peripheral neuromuscular function were assessed during the knee extension.

RESULTS

Strong relationships were observed between quadriceps early EMG activity (EMG₅₀/M) and knee extension RTD (r or ρ = 0.51-0.65) and IMTP late RFD (r = 0.51), and between cycling early or late RTD and peak twitch torque (r or ρ = 0.70-0.75). Strong-to-very strong relationships were observed between knee extension, IMTP, and sprint cycling for peak force/torque, early and late RFD/RTD, and peak RFD/RTD (r or ρ = 0.59-0.80).

CONCLUSION

In trained cyclists, knee extension RTD or IMTP late RFD are related to measures of quadriceps central neuromuscular function, while cycling RTD is related to measures of quadriceps peripheral neuromuscular function. Further, the strong associations among force/torque measures between tasks indicate a level of transferability across tasks.

Movement in low gravity environments (MoLo) programme-The MoLo-L.O.O.P. study protocol

BACKGROUND

Exposure to prolonged periods in microgravity is associated with deconditioning of the musculoskeletal system due to chronic changes in mechanical stimulation. Given astronauts will operate on the Lunar surface for extended periods of time, it is critical to quantify both external (e.g., ground reaction forces) and internal (e.g., joint reaction forces) loads of relevant movements performed during Lunar missions. Such knowledge is key to predict musculoskeletal deconditioning and determine appropriate exercise countermeasures associated with extended exposure to hypogravity.

OBJECTIVES

The aim of this paper is to define an experimental protocol and methodology suitable to estimate in high-fidelity hypogravity conditions the lower limb internal joint reaction forces. State-of-the-art movement kinetics, kinematics, muscle activation and muscle-tendon unit behaviour during locomotor and plyometric movements will be collected and used as inputs (Objective 1), with musculoskeletal modelling and an optimisation framework used to estimate lower limb internal joint loading (Objective 2).

METHODS

Twenty-six healthy participants will be recruited for this cross-sectional study. Participants will walk, skip and run, at speeds ranging between 0.56-3.6 m/s, and perform plyometric movement trials at each gravity level (1, 0.7, 0.5, 0.38, 0.27 and 0.16g) in a randomized order. Through the collection of state-of-the-art kinetics, kinematics, muscle activation and muscle-tendon behaviour, a musculoskeletal modelling framework will be used to estimate lower limb joint reaction forces via tracking simulations.

CONCLUSION

The results of this study will provide first estimations of internal musculoskeletal loads associated with human movement performed in a range of hypogravity levels. Thus, our unique data will be a key step towards modelling the musculoskeletal deconditioning associated with long term habitation on the Lunar surface, and thereby aiding the design of Lunar exercise countermeasures and mitigation strategies.

Quantifying the hip-ankle synergy in short-term maximal cycling

Simulation studies have demonstrated that the hip and ankle joints form a task-specific synergy during the downstroke in maximal cycling to enable the power produced by the hip extensor muscles to be transferred to the crank. The existence of the hip-ankle synergy has not been investigated experimentally. Therefore, we sought to apply a modified vector coding technique to quantify the strength of the hip-ankle moment synergy in the downstroke during short-term maximal cycling at a pedalling rate of 135 rpm. Twelve track sprint cyclists performed 3 × 4 s seated sprints at 135 rpm, interspersed with 2 × 4 s seated sprints at 60 rpm on an isokinetic ergometer. Data from the 60 rpm sprints were not analysed in this study. Joint moments were calculated via inverse dynamics, using pedal forces and limb kinematics. The hip-ankle moment synergy was quantified using a modified vector coding method. Results showed, for 28.8% of the downstroke the hip and ankle moments were in-phase, demonstrating the hip and ankle joints tend to work in synergy in the downstroke, providing some support findings from simulation studies of cycling. At a pedalling rate of 135 rpm the hip-phase was most frequent (42.5%) significantly differing from the in- (P = 0.044), anti- (P < 0.001), and ankle-phases (P = 0.004), demonstrating hip-dominant action. We believe this method shows promise to answer research questions on the relative strength of the hip-ankle synergy between different cycling conditions (e.g., power output and pedalling rates).

Effects of strength training on the biomechanics and coordination of short-term maximal cycling

The aim was to investigate the effects of a gym-based strength training intervention on biomechanics and intermuscular coordination patterns during short-term maximal cycling. Twelve track sprint cyclists performed 3 × 4 s seated sprints at 135 rpm, interspersed with 2 × 4 s seated sprints at 60 rpm on an isokinetic ergometer, repeating the session 11.6 ± 1.4 weeks later following a training programme that included two gym-based strength training sessions per week. Joint moments were calculated via inverse dynamics, using pedal forces and limb kinematics. EMG activity was measured for 9 lower limb muscles. Track cyclists 'leg strength" increased (7.6 ± 11.9 kg, P = 0.050 and ES = 0.26) following the strength training intervention. This was accompanied by a significant increase in crank power over a complete revolution for sprints at 135 rpm (26.5 ± 36.2 W, P = 0.028 and ES = 0.29). The increase in leg strength and average crank power was associated with a change in biceps femoris muscle activity, indicating that the riders successfully adapted their intermuscular coordination patterns to accommodate the changes in personal constraints to increase crank power.

Muscle synergies are modified with improved task performance in skill learning

How do muscle synergies change as motor skills are learned? The purpose of this study was to investigate the relationship between synergy number and skill acquisition, and to examine learning-related changes in synergy structure and activation patterns. We performed muscle synergy analysis using non-negative matrix factorization to identify muscle synergies from activation patterns of ten major leg muscles before and after recreational cyclists learned a novel one-legged pedal force aiming task (Park, Van Emmerik, & Caldwell, 2021). Synergy number was defined as the smallest number of factors from the matrix factorization algorithm that could explain more than the predefined threshold values. Improvements in pedal force direction after practice occurred without a change in the number of muscle synergies (four), suggesting that task constraints (e.g. the need for smooth pedaling motion) in this novel targeting task may limit the CNS to the same number of muscle synergies before and after practice. Improved task performance while continuing to satisfy multiple biomechanical tasks was obtained with changes in structure (muscle weightings) for one synergy, and activation amplitudes without changes in timing or pattern for three synergies. In each crank cycle quadrant, multiple synergies were altered in either structure or activation amplitude, suggesting that the cooperative changes may be essential for improving task performance while producing a smooth pedaling motion. Changes in both synergy structure and activation levels could be muscle coordination strategies in motor skill learning.

A direct collocation framework for optimal control simulation of pedaling using OpenSim

The direct collocation (DC) method has shown low computational costs in solving optimization problems in human movements, but it has rarely been used for solving optimal control pedaling problems. Thus, the aim of this study was to develop a DC framework for optimal control simulation of human pedaling within the OpenSim modeling environment. A planar bicycle-rider model was developed in OpenSim. The DC method was formulated in MATLAB to solve an optimal control pedaling problem using a data tracking approach. Using the developed DC framework, the optimal control pedaling problem was successfully solved in 24 minutes to ten hours with different objective function weightings and number of nodes from two different initial conditions. The optimal solutions for equal objective function weightings were successful in terms of tracking, with the model simulated pedal angles and pedal forces within ±1 standard deviation of the experimental data. With these weightings, muscle tendon unit (MTU) excitation patterns generally matched with burst timings and shapes observed in the experimental EMG data. Tracking quality and MTU excitation patterns were changed little by selection of node density above 31, and the optimal solution quality was not affected by initial guess used. The proposed DC framework could easily be turned into a predictive simulation with other objective functions such as fastest pedaling rate. This flexible and computationally efficient framework should facilitate the use of optimal control methods to study the biomechanics, energetics, and control of human pedaling.

Modeling toes contributes to realistic stance knee mechanics in three-dimensional predictive simulations of walking

Physics-based predictive simulations have been shown to capture many salient features of human walking. Yet they often fail to produce realistic stance knee and ankle mechanics. While the influence of the performance criterion on the predicted walking pattern has been previously studied, the influence of musculoskeletal mechanics has been less explored. Here, we investigated the influence of two mechanical assumptions on the predicted walking pattern: the complexity of the foot model and the stiffness of the Achilles tendon. We found, through three-dimensional muscle-driven predictive simulations of walking, that modeling the toes, and thus using two-segment instead of single-segment foot models, contributed to robustly eliciting physiological stance knee flexion angles, knee extension torques, and knee extensor activity. Modeling toes also slightly decreased the first vertical ground reaction force peak, increasing its agreement with experimental data, and improved stance ankle kinetics. It nevertheless slightly worsened predictions of ankle kinematics. Decreasing Achilles tendon stiffness improved the realism of ankle kinematics, but there remain large discrepancies with experimental data. Overall, this simulation study shows that not only the performance criterion but also mechanical assumptions affect predictive simulations of walking. Improving the realism of predictive simulations is required for their application in clinical contexts. Here, we suggest that using more complex foot models might contribute to such realism.

Quadriceps Muscle Fatigue Reduces Extension and Flexion Power During Maximal Cycling

Purpose: To investigate how quadriceps muscle fatigue affects power production over the extension and flexion phases and muscle activation during maximal cycling.

Methods: Ten participants performed 10-s maximal cycling efforts without fatigue and after 120 bilateral maximal concentric contractions of the quadriceps muscles. Extension power, flexion power and electromyographic (EMG) activity were compared between maximal cycling trials. We also investigated the associations between changes in quadriceps force during isometric maximal voluntary contractions (IMVC) and power output (flexion and extension) during maximal cycling, in addition to inter-individual variability in muscle activation and pedal force profiles.

Results: Quadriceps IMVC (−52 ± 21%, P = 0.002), voluntary activation (−24 ± 14%, P &lt; 0.001) and resting twitch amplitude (−45 ± 19%, P = 0.002) were reduced following the fatiguing task, whereas vastus lateralis (P = 0.58) and vastus medialis (P = 0.15) M-wave amplitudes were unchanged. The reductions in extension power (−15 ± 8%, P &lt; 0.001) and flexion power (−24 ± 18%, P &lt; 0.001) recorded during maximal cycling with fatigue of the quadriceps were dissociated from the decreases in quadriceps IMVC. Peak EMG decreased across all muscles while inter-individual variability in pedal force and EMG profiles increased during maximal cycling with quadriceps fatigue.

Conclusion: Quadriceps fatigue induced by voluntary contractions led to reduced activation of all lower limb muscles, increased inter-individual variability and decreased power production during maximal cycling. Interestingly, power production was further reduced over the flexion phase (24%) than the extension phase (15%), likely due to larger levels of peripheral fatigue developed in RF muscle and/or a higher contribution of the quadriceps muscle to flexion power production compared to extension power during maximal cycling.

Rates of neuromuscular excitation during cycling in Parkinson's disease compared to healthy young and older adults

BACKGROUND

Bradykinesia affects mobility in some people with Parkinson's. Fall risk makes the neural control of maximal speed ambulatory movements difficult to study in Parkinson's. Stationary recumbent bicycling favors the use of electromyography at high movement speeds, and may better reveal neuromuscular rate limiters.

METHODS

Subjects were 18 adults with Parkinson's, 14 older adults and 14 young adults. Electromyograms were recorded from two muscles during stationary recumbent bicycling at 60, 80, 100, 120 RPM and peak voluntary cadence. Rate of electromyogram rise was calculated. Subjects performed the timed up and go and four square step test. Parkinson's sub-groups were formed based on whether they could pedal ≥120 RPM. Mixed models were used to compare groups and spearman's correlations quantified relationships.

FINDINGS

Eight people with Parkinson's and four older adults could not complete the 120 RPM condition. Faster people with Parkinson's (n = 10) had greater maximum cadence (F = 42.85, P < 0.05), higher rates of electromyogram rise in both muscles (F > 16.9, P < 0.05), and faster mobility test times (F > 6.5, P < 0.05) than slower people with Parkinson's (n = 8). In Parkinson's, correlations between vastus lateralis rate of electromyogram rise and four square step test (ρ = -0.62), timed up and go (ρ = -0.53), and peak cadence (ρ = 0.76) were significant (all P < 0.05).

INTERPRETATION

People with Parkinson's with slower peak pedaling cadence had slower mobility performance and lower vastus lateralis excitation rates at higher cadences. Vastus lateralis excitation rates had moderate to strong relationships with peak cadence and mobility. Exercise interventions may seek to improve peak cadence or excitation rates in people with Parkinson's.

Predictive simulation of post-stroke gait with functional electrical stimulation

Post-stroke patients present various gait abnormalities such as drop foot, stiff-knee gait (SKG), and knee hyperextension. Functional electrical stimulation (FES) improves drop foot gait although the mechanistic basis for this effect is not well understood. To answer this question, we evaluated the gait of a post-stroke patient walking with and without FES by inverse dynamics analysis and compared the results to an optimal control framework. The effect of FES and cause-effect relationship of changes in knee and ankle muscle strength were investigated; personalized muscle–tendon parameters allowed the prediction of pathologic gait. We also predicted healthy gait patterns at different speeds to simulate the subject walking without impairment. The passive moment of the knee played an important role in the estimation of muscle force with knee hyperextension, which was decreased during FES and knee extensor strengthening. Weakening the knee extensors and strengthening the flexors improved SKG. During FES, weak ankle plantarflexors and strong ankle dorsiflexors resulted in increased ankle dorsiflexion, which reduced drop foot. FES also improved gait speed and reduced circumduction. These findings provide insight into compensatory strategies adopted by post-stroke patients that can guide the design of individualized rehabilitation and treatment programs.

Changes in Muscle Control After Learning to Direct Pedal Forces in One-Legged Pedaling

The aim of this study was to describe how major leg muscle activities are altered after learning a novel one-legged pedaling task. Fifteen recreational cyclists practiced one-legged pedaling trials during which they were instructed to match their applied pedal force to a target direction perpendicular to the crank arm. Activity in 10 major leg muscles was measured with surface electromyography electrodes. Improved upstroke task performance was obtained by greater activity in the hip and ankle flexor muscles, counteracting the negative effects of gravity. Greater quadriceps activities explained improved targeting near top dead center. Reduced uniarticular knee and ankle extensor downstroke activities were necessary to prevent freewheeling. Greater hamstring and tibialis anterior activities improved targeting performance near the bottom of the pedal stroke. The activity patterns of the biarticular plantarflexors changed little, likely due to their contributions as knee flexors for smooth upstroke pedaling motion. These results add to our understanding of how the degrees of freedom at the muscle level are altered in a cooperative manner to overcome gravitational effects in order to achieve the learning goal of the motor task while satisfying multiple constraints—in this case, the production of smooth one-legged pedaling motion at the designated mechanical task demands.

Maximal muscular power: lessons from sprint cycling

Maximal muscular power production is of fundamental importance to human functional capacity and feats of performance. Here, we present a synthesis of literature pertaining to physiological systems that limit maximal muscular power during cyclic actions characteristic of locomotor behaviours, and how they adapt to training. Maximal, cyclic muscular power is known to be the main determinant of sprint cycling performance, and therefore we present this synthesis in the context of sprint cycling. Cyclical power is interactively constrained by force-velocity properties (i.e. maximum force and maximum shortening velocity), activation-relaxation kinetics and muscle coordination across the continuum of cycle frequencies, with the relative influence of each factor being frequency dependent. Muscle cross-sectional area and fibre composition appear to be the most prominent properties influencing maximal muscular power and the power-frequency relationship. Due to the role of muscle fibre composition in determining maximum shortening velocity and activation-relaxation kinetics, it remains unclear how improvable these properties are with training. Increases in maximal muscular power may therefore arise primarily from improvements in maximum force production and neuromuscular coordination via appropriate training. Because maximal efforts may need to be sustained for ~15-60 s within sprint cycling competition, the ability to attenuate fatigue-related power loss is also critical to performance. Within this context, the fatigued state is characterised by impairments in force-velocity properties and activation-relaxation kinetics. A suppression and leftward shift of the power-frequency relationship is subsequently observed. It is not clear if rates of power loss can be improved with training, even in the presence adaptations associated with fatigue-resistance. Increasing maximum power may be most efficacious for improving sustained power during brief maximal efforts, although the inclusion of sprint interval training likely remains beneficial. Therefore, evidence from sprint cycling indicates that brief maximal muscular power production under cyclical conditions can be readily improved via appropriate training, with direct implications for sprint cycling as well as other athletic and health-related pursuits.

Passive Knee Orthoses Assistance in Functional Electrical Stimulation Cycling in an Individual With Spinal Cord Injury

Functional Electrical Stimulation (FES) may be used in rehabilitation and assistance of people with Spinal Cord Injury (SCI). One significant application is facilitating physical exercise, mainly when combining FES with mechanical platforms, such as tricycles. However, there are still technical challenges in FES cycling protocols, such as improving control and cycling performance. Here we show how passive elements in knee orthoses during FES cycling could increase the average cadence, taking advantage of the cycling movement. Our approach is twofold. First, we simulated the forward dynamics of a detailed musculoskeletal model with passive elements over the knees. Simulations showed that specific spring stiffness ranges increased the crankset speed during cycling by more than 50%. Using parameters found in simulations, we built a pair of passive orthoses and performed experiments with one individual with SCI. During two days, the volunteer cycled with similar stimulation magnitude with and without the passive elements. We observed that the average crankset speed was higher by more than 10% when the springs were attached to the passive orthoses. These results show the potential of using passive elements to increase cycling speed for FES cycling with similar or even lower stimulation magnitude, leading to longer exercise duration.

A Conceptual Blueprint for Making Neuromusculoskeletal Models Clinically Useful

The ultimate goal of most neuromusculoskeletal modeling research is to improve the treatment of movement impairments. However, even though neuromusculoskeletal models have become more realistic anatomically, physiologically, and neurologically over the past 25 years, they have yet to make a positive impact on the design of clinical treatments for movement impairments. Such impairments are caused by common conditions such as stroke, osteoarthritis, Parkinson’s disease, spinal cord injury, cerebral palsy, limb amputation, and even cancer. The lack of clinical impact is somewhat surprising given that comparable computational technology has transformed the design of airplanes, automobiles, and other commercial products over the same time period. This paper provides the author’s personal perspective for how neuromusculoskeletal models can become clinically useful. First, the paper motivates the potential value of neuromusculoskeletal models for clinical treatment design. Next, it highlights five challenges to achieving clinical utility and provides suggestions for how to overcome them. After that, it describes clinical, technical, collaboration, and practical needs that must be addressed for neuromusculoskeletal models to fulfill their clinical potential, along with recommendations for meeting them. Finally, it discusses how more complex modeling and experimental methods could enhance neuromusculoskeletal model fidelity, personalization, and utilization. The author hopes that these ideas will provide a conceptual blueprint that will help the neuromusculoskeletal modeling research community work toward clinical utility.

Muscular activity patterns in 1-legged vs. 2-legged pedaling

BACKGROUND

One-legged pedaling is of interest to elite cyclists and clinicians. However, muscular usage in 1-legged vs. 2-legged pedaling is not fully understood. Thus, the study was aimed to examine changes in leg muscle activation patterns between 2-legged and 1-legged pedaling.

METHODS

Fifteen healthy young recreational cyclists performed both 1-legged and 2-legged pedaling trials at about 30 Watt per leg. Surface electromyography electrodes were placed on 10 major muscles of the left leg. Linear envelope electromyography data were integrated to quantify muscle activities for each crank cycle quadrant to evaluate muscle activation changes.

RESULTS

Overall, the prescribed constant power requirements led to reduced downstroke crank torque and extension-related muscle activities (vastus lateralis, vastus medialis, and soleus) in 1-legged pedaling. Flexion-related muscle activities (biceps femoris long head, semitendinosus, lateral gastrocnemius, medial gastrocnemius, tensor fasciae latae, and tibialis anterior) in the upstroke phase increased to compensate for the absence of contralateral leg crank torque. During the upstroke, simultaneous increases were seen in the hamstrings and uni-articular knee extensors, and in the ankle plantarflexors and dorsiflexors. At the top of the crank cycle, greater hip flexor activity stabilized the pelvis.

CONCLUSION

The observed changes in muscle activities are due to a variety of changes in mechanical aspects of the pedaling motion when pedaling with only 1 leg, including altered crank torque patterns without the contralateral leg, reduced pelvis stability, and increased knee and ankle stiffness during the upstroke.

HEART RATE AND LOWER LIMB MUSCLE ACTIVITY ON CYCLE ERGOMETER

ABSTRACT Introduction: Muscle activity in the pedal stroke movement on a cycle ergometer can be measured by surface electromyography, as an effective and improved method for studying muscle action and objectively determining the different action potentials of the muscles involved in specific movements. Heart rate behavior is an important factor during exercise with load. Objective: To identify heart rate behavior and pattern of muscle activity of the rectus femoris and vastus medialis in healthy subjects in the pedaling dynamic at different loads, submaximal test, on an instrumented cycle ergometer. Methods: 20 healthy adults were evaluated. Heart rate measurement was performed, together with electromyographic analysis, in the time domain, of the rectus femoris and vastus medialis muscles during incremental exercise of the lower limbs on the cycle ergometer. Results: Heart rate behavior presented significant difference for p≥0.05 in relation to increased loads. The EMG signal intensity from the vastus medialis muscle (normalized RMS value) in each quadrant of the pedaling cycle showed significant difference for p≥0.05 in relation to quadrants I, II and IV and significant difference for p≥0.05 in relation to quadrants III and IV. In the rectus femoris (RF) muscle, there was significant difference for p≥0.05 in relation to quadrants I, II and IV and significant difference for p≥0.05 in relation to quadrants I, II and III. Conclusion: An increase in heart rate proportional to the increase in load was observed, as well as an increase in the amplitude of the electromyographic signal proportional to the increase in load. It was possible to identify the pattern of muscle activation in the studied quadrants during pedal stroke movements, independent of load. Level of evidence III; Study of non-consecutive patients; without uniform application of the “gold” standard reference.

NIRS-derived skeletal muscle oxidative capacity is correlated with aerobic fitness and independent of sex

Near-infrared spectroscopy (NIRS) provides a simple and reliable measure of skeletal muscle oxidative capacity; however, its relationship to aerobic fitness and sex are unclear. We hypothesized that NIRS-derived oxidative capacity in the vastus lateralis (VL) and medial gastrocnemius (MG) would be correlated with indices of aerobic fitness and independent of sex. Twenty-six participants (13 males, 13 females) performed ramp- and step-incremental tests to volitional exhaustion on separate days to establish maximal oxygen uptake (V̇o2max), peak power output (PPO), lactate threshold (LT), gas exchange threshold (GET), respiratory compensation point (RCP), and maximal fat oxidation (MFO). Data were normalized to lean body mass to account for sex-based differences in body composition. Exercise tests were preceded by duplicate measurements of NIRS-derived oxidative capacity on the VL and MG muscles (i.e., repeated arterial occlusions following a brief set of muscle contractions). Skeletal muscle oxidative capacity for the VL (means ± SD: 21.9 ± 4.6 s) and MG (22.5 ± 6.1 s) were similar but unrelated (r2 = 0.03, P = 0.39). Skeletal muscle oxidative capacity for the VL, but not the MG (P > 0.05 for all variables), was significantly correlated with V̇o2max (r2 = 0.24; P = 0.01), PPO (r2 = 0.23; P = 0.01), LT (r2 = 0.23; P = 0.01), GET (r2 = 0.23; P = 0.01), and RCP (r2 = 0.27; P = 0.006). MFO was not correlated with VL or MG skeletal muscle oxidative capacity (P > 0.05). Females (54.9 ± 4.5 mL·kg LBM-1·min-1) and males (56.0 ± 6.2 mL·kg LBM-1·min-1), matched for V̇o2max (P = 0.62), had similar NIRS-derived oxidative capacities for VL (20.7 ± 4.4 vs. 23.2 ± 4.6 s; P = 0.18) and MG (24.4 ± 6.8 vs. 20.5 ± 4.8 s; P = 0.10). Overall, NIRS-derived skeletal muscle oxidative capacity in VL is indicative of aerobic fitness and independent of sex in humans.NEW & NOTEWORTHY Near-infrared spectroscopy (NIRS) can be used to measure skeletal muscle oxidative capacity. Here, we demonstrated that NIRS-derived skeletal muscle oxidative capacity of the vastus lateralis was independent of sex, reliable across and within days, and correlated with maximal and submaximal indices of aerobic fitness, including maximal oxygen uptake, lactate threshold, and respiratory compensation point. These findings highlight the utility of NIRS for investigating skeletal muscle oxidative capacity in females and males.

Biomechanical measures of short-term maximal cycling on an ergometer: a test-retest study

An understanding of test-retest reliability is important for biomechanists, such as when assessing the longitudinal effect of training or equipment interventions. Our aim was to quantify the test-retest reliability of biomechanical variables measured during short-term maximal cycling. Fourteen track sprint cyclists performed 3 × 4 s seated sprints at 135 rpm on an isokinetic ergometer, repeating the session 7.6 ± 2.5 days later. Joint moments were calculated via inverse dynamics, using pedal forces and limb kinematics. EMG activity was measured for 9 lower limb muscles. Reliability was explored by quantifying systematic and random differences within- and between-session. Within-session reliability was better than between-sessions reliability. The test-retest reliability level was typically moderate to excellent for the biomechanical variables that describe maximal cycling. However, some variables, such as peak knee flexion moment and maximum hip joint power, demonstrated lower reliability, indicating that care needs to be taken when using these variables to evaluate biomechanical changes. Although measurement error (instrumentation error, anatomical marker misplacement, soft tissue artefacts) can explain some of our reliability observations, we speculate that biological variability may also be a contributor to the lower repeatability observed in several variables including ineffective crank force, ankle kinematics and hamstring muscles' activation patterns.

Muscle pain induced by hypertonic saline in the knee extensors decreases single-limb isometric time to task failure

Purpose

Increased nociceptive activity and the experience of exercise-induced pain (EIP) may contribute to fatigue during endurance exercise. To investigate this, a pain model that produces pain similar to EIP and decouples its relationship to exercise intensity is required. This study (1) compared the quality of pain caused by a hypertonic saline injection into the vastus lateralis in resting and exercise conditions, and (2) investigated whether this pain contributes to changes in time to task failure.

Methods

On separate days, 18 participants completed a time to task failure at 20% maximal voluntary torque (MVT), a resting hypertonic saline intramuscular injection, and in a further three visits a time to task failure at 10% MVT following injection of isotonic saline, hypertonic saline or a control (no injection).

Results

In a subset of eligible participants (n = 12), the hypertonic saline combined with 10% MVT produced a qualitative experience of pain (assessed by the McGill Pain Questionnaire) that felt similar to EIP. 10% MVT with hypertonic saline significantly elevated pain intensity in the first 20% of the time to task failure and caused a significantly (P < 0.05) shorter time to task failure (448 ± 240 s) compared with the isotonic saline (605 ± 285 s) and control (514 ± 197 s) conditions.

Conclusion

These findings demonstrate that hypertonic saline increases the intensity of pain during exercise, which results in a faster occurrence of exercise-induced fatigue. These results provide important evidence supporting pain as a limiting factor in endurance performance.

Tensiomyographical responsiveness to peripheral fatigue in quadriceps femoris

Background

Fatigue influences athletic performance and can also increase the risk of injury in sports, and most of the methods to evaluate it require an additional voluntary effort. Tensiomyography (TMG), which uses electrical stimulation and a displacement sensor to evaluate muscle contraction properties of one or more muscle bellies, has emerged as a technique that can assess the presence of peripheral and central fatigue without requiring additional voluntary efforts. However, the evaluation of the TMG’s ability to detect fatigue is limited, both at the level of muscle bellies and statistical methods. Thus, the aim of the present study was twofold: (i) to examine and compare the tensiomyographical responsiveness to quadriceps femoris (QF) fatigue by multiple statistical methods and (ii) to analyze sex differences in the variation produced by fatigue in TMG parameters.

Methods

Thirty-nine recreational athletes participated (19 males/20 females; aged 22 ± 2 years). TMG parameters of QF bellies and maximal voluntary isometric contraction (MVIC) were measured before and after a fatigue protocol. TMG parameters used were maximum radial deformation (Dm), contraction time between 10–90% of the Dm (Tc), contraction velocity between 10–90% (Vc) and of the first 10% (V10) of the Dm. Internal responsiveness of TMG to fatigue was analyzed by paired t-test and standardized response mean (SRM). External responsiveness was examined by correlations, regression models, and receiver operating characteristic (ROC) curves.

Results

All TMG parameters, except for Tc of rectus femoris and vastus medialis, showed large internal responsiveness. In adjusted regression models by sex, only Dm and V10 of rectus femoris were statistically associated (p < 0.05) with b coefficients of 0.40 and 0.43, respectively. r2 explained the 22% of the total variance. In addition, these parameters could discriminate between QF with and without fatigue.

Conclusion

Since the QF is the main strength contributor during multiple physical activities, clinicians and trainers will be able to discriminate the presence of fatigue and the magnitude of changes in the QF strength by TMG evaluation.

Physics-Based Simulations to Predict the Differential Effects of Motor Control and Musculoskeletal Deficits on Gait Dysfunction in Cerebral Palsy: A Retrospective Case Study

Physics-based simulations of walking have the theoretical potential to support clinical decision-making by predicting the functional outcome of treatments in terms of walking performance. Yet before using such simulations in clinical practice, their ability to identify the main treatment targets in specific patients needs to be demonstrated. In this study, we generated predictive simulations of walking with a medical imaging based neuro-musculoskeletal model of a child with cerebral palsy presenting crouch gait. We explored the influence of altered muscle-tendon properties, reduced neuromuscular control complexity, and spasticity on gait dysfunction in terms of joint kinematics, kinetics, muscle activity, and metabolic cost of transport. We modeled altered muscle-tendon properties by personalizing Hill-type muscle-tendon parameters based on data collected during functional movements, simpler neuromuscular control by reducing the number of independent muscle synergies, and spasticity through delayed muscle activity feedback from muscle force and force rate. Our simulations revealed that, in the presence of aberrant musculoskeletal geometries, altered muscle-tendon properties rather than reduced neuromuscular control complexity and spasticity were the primary cause of the crouch gait pattern observed for this child, which is in agreement with the clinical examination. These results suggest that muscle-tendon properties should be the primary target of interventions aiming to restore an upright gait pattern for this child. This suggestion is in line with the gait analysis following muscle-tendon property and bone deformity corrections. Future work should extend this single case analysis to more patients in order to validate the ability of our physics-based simulations to capture the gait patterns of individual patients pre- and post-treatment. Such validation would open the door for identifying targeted treatment strategies with the aim of designing optimized interventions for neuro-musculoskeletal disorders.

Acute caffeine intake increases muscle oxygen saturation during a maximal incremental exercise test

AIMS

The main mechanism behind caffeine's ergogenicity lies in its tendency to bind to adenosine receptors, although other mechanisms might be involved. The aim of this investigation was to analyse the effects of caffeine on muscle oxygen saturation during exercise of increasing intensity.

METHODS

Thirteen healthy and active individuals volunteered to participate in a randomized, double blind, placebo-controlled crossover trial. During 2 different trials, participants either ingested a placebo (cellulose) or 3 mg/kg of caffeine. After waiting for 60 min to absorb the substances, participants underwent a maximal ramp cycle ergometer test (25 W/min). Near infrared spectrometers were positioned on each leg's vastus lateralis to monitor tissue O₂ saturation. Blood lactate concentration was measured 1 min after the end of the exercise test.

RESULTS

In comparison to the placebo, the ingestion of caffeine improved the maximal wattage (258 ± 50 vs 271 ± 54 W, respectively, P < .001, effect size [ES] = 0.27; 95% confidence interval [CI] 0.14-0.35) and blood lactate concentration (11.9 ± 3.8 vs 13.7 ± 3.5 mmol/L, P = .029, ES = 0.38; 95% CI 0.14-0.75) at the end of the test. Caffeine increased muscle oxygen saturation at several exercise workloads with a main effect found in respect to the placebo (F = 6.28, P = .029; ES = 0.30 to 0.54; 95% CI 0.01-0.78). Peak pulmonary ventilation (124 ± 29 vs 129 ± 23 L/min, P = 0.035, ES = 0.25; 95% CI 0.07-0.40) and peak oxygen uptake (3.18 ± 0.70 vs 3.33 ± 0.88 L/min, P = 0.032, ES = 0.26; 95% CI 0.08-0.51) were also increased with caffeine.

CONCLUSION

Acute ingestion of 3 mg/kg of caffeine improved peak aerobic performance and increased peak pulmonary ventilation. In addition, caffeine induced changes in muscle oxygen saturation during submaximal workloads, suggesting that this mechanism might also contribute to caffeine's ergogenic effect.

Comparison of Forward and Reverse Wingate Anaerobic Tests: A Brief Technical Note

Background

The Wingate anaerobic test (WAT) is traditionally performed in the forward pedaling direction on a cycle ergometer. However, reverse (backward) pedaling during a WAT test may be a novel way to convey meaningful information related to performance and rehabilitation. This study compared peak power measurements between 30-second forward pedaling WAT (FWAT) with a 30-second reverse pedaling WAT (RWAT).

Methods

10 male and 10 female participants (age 27.6 ± 7.31 yrs, mass 74.9 ± 21.3 kg and height 172.6 ± 10.9 cm) volunteered to participate. Participants performed one FWAT and one RWAT at 7.5% of body mass on a specially modified Monark cycle ergometer. Tests were separated 2 days of rest. Peak power output (PPO), mean power output (MPO), relative PPO (RPPO), relative MPO (RMPO), fatigue index (%FI), and rating of perceived exertion (RPE) were measured.

Results

The FWAT power measurements were all significantly greater (p < 0.05) than RWAT power measurements except MPO (p > 0.05); and that RPE was significantly greater (p < 0.05) in FWAT than RWAT. Specifically, FWAT vs. RWAT (M ± SD) are as follows: PPO watts (w) = 731.7 ± 237.1 vs. 529.6 ± 192.2; RPPO w/kg = 10.2 ± 2.3 vs. 7.2 ± 1.6; MPO w = 510.2 ± 162.1 vs. 415.1 ± 146.2; RMPO w/kg = 7.3 ± 1.5 vs. 5.8 ± 1.3; %FI = 49.2 ± 8.7 vs. 37.4 ± 13.7; and RPE = 19.4 ± 1.1 vs. 15.8 ± 1.5. Gender did not impact the relative differences in these relationships.

Conclusion

Practitioners and clinicians may use this information to begin to understand the power and perceived exertion relationships of forward versus reverse pedaling during a WAT; exercise prescription for rehabilitation and performance may benefit.

Algorithmic differentiation improves the computational efficiency of OpenSim-based trajectory optimization of human movement

Algorithmic differentiation (AD) is an alternative to finite differences (FD) for evaluating function derivatives. The primary aim of this study was to demonstrate the computational benefits of using AD instead of FD in OpenSim-based trajectory optimization of human movement. The secondary aim was to evaluate computational choices including different AD tools, different linear solvers, and the use of first- or second-order derivatives. First, we enabled the use of AD in OpenSim through a custom source code transformation tool and through the operator overloading tool ADOL-C. Second, we developed an interface between OpenSim and CasADi to solve trajectory optimization problems. Third, we evaluated computational choices through simulations of perturbed balance, two-dimensional predictive simulations of walking, and three-dimensional tracking simulations of walking. We performed all simulations using direct collocation and implicit differential equations. Using AD through our custom tool was between 1.8 ± 0.1 and 17.8 ± 4.9 times faster than using FD, and between 3.6 ± 0.3 and 12.3 ± 1.3 times faster than using AD through ADOL-C. The linear solver efficiency was problem-dependent and no solver was consistently more efficient. Using second-order derivatives was more efficient for balance simulations but less efficient for walking simulations. The walking simulations were physiologically realistic. These results highlight how the use of AD drastically decreases computational time of trajectory optimization problems as compared to more common FD. Overall, combining AD with direct collocation and implicit differential equations decreases the computational burden of trajectory optimization of human movement, which will facilitate their use for biomechanical applications requiring the use of detailed models of the musculoskeletal system.

Inverse dynamic estimates of muscle recruitment and joint contact forces are more realistic when minimizing muscle activity rather than metabolic energy or contact forces

BACKGROUND

Assessment of contact forces is essential for a better understanding of mechanical factors affecting progression of osteoarthritis. Since contact forces cannot be measured non-invasively, computer simulations are often used to assess joint loading. Contact forces are to a large extent determined by muscle forces. These muscle forces are computed using optimization techniques that solve the muscle redundancy problem by assuming that muscles are coordinated in a way that optimizes performance (e.g., minimizes muscle activity or metabolic energy). However, it is unclear which of the many proposed performance criteria best describes muscle coordination.

RESEARCH QUESTION

Which performance criterion best describes muscle recruitment patterns and knee contact forces recorded using electromyography (EMG) and load cell instrumented prostheses?.

METHODS

We solved the muscle redundancy problem based on six different groups of performance criteria: muscle activations, volume-scaled activations, forces, stresses, metabolic energy, and joint contact forces. Computed muscle excitations and knee contact forces during over-ground walking were validated against recorded EMG signals and measured contact forces for four subjects with instrumented knee prostheses in the "Grand Challenge Competition to Predict in Vivo Knee Loads" dataset.

RESULTS

Performance criteria based on either stress or muscle activation (either unscaled or scaled by muscle volume), both to a power of 3 or 4, resulted in the best agreement between measured and simulated values. These performance criteria outperformed all other criteria in terms of agreement between simulated muscle excitations and EMG, whereas good agreement between measured and predicted contact forces was also observed for minimization of contact forces and metabolic energy.

SIGNIFICANCE

Given the large differences in accuracy obtained with different performance criteria (e.g., root mean square errors of contact forces differed up to 0.45 body weight), the results of our study are important to improve the validity of in silico assessment of joint loading.

Rapid predictive simulations with complex musculoskeletal models suggest that diverse healthy and pathological human gaits can emerge from similar control strategies

Physics-based predictive simulations of human movement have the potential to support personalized medicine, but large computational costs and difficulties to model control strategies have limited their use. We have developed a computationally efficient optimal control framework to predict human gaits based on optimization of a performance criterion without relying on experimental data. The framework generates three-dimensional muscle-driven simulations in 36 min on average—more than 20 times faster than existing simulations—by using direct collocation, implicit differential equations and algorithmic differentiation. Using this framework, we identified a multi-objective performance criterion combining energy and effort considerations that produces physiologically realistic walking gaits. The same criterion also predicted the walk-to-run transition and clinical gait deficiencies caused by muscle weakness and prosthesis use, suggesting that diverse healthy and pathological gaits can emerge from the same control strategy. The ability to predict the mechanics and energetics of a broad range of gaits with complex three-dimensional musculoskeletal models will allow testing novel hypotheses about gait control and hasten the development of optimal treatments for neuro-musculoskeletal disorders.

Muscle force adaptation to changes in upper body position during seated sprint cycling

Sprint cycling performance depends upon the balance between muscle and drag forces. This study assessed the influence of upper body position on muscle forces and aerodynamics during seated sprint cycling. Thirteen competitive cyclists attended two sessions. The first session was used to determine handlebar positions to achieve pre-determined hip flexion angles (70-110° in 10° increments) using dynamic bicycle fitting. In the second session, full body kinematics and pedal forces were recorded throughout 2x6-s seated sprints at the pre-determined handlebar positions, and frontal plane images were used to determine the projected frontal area. Leg work, joint work, muscle forces and frontal area were compared at three upper body positions, being optimum (maximum leg work), optimal+10° and optimal-10° of hip flexion. Larger hip (p = 0.01-0.02) and reduced knee (p = 0.02-0.03) contribution to leg work were observed at the optimal+10° position without changes at the ankle joint (p = 0.39). No differences were observed in peak muscle forces across the three body positions (p = 0.06-0.48). Frontal area was reduced at optimum+10° of hip flexion when compared to optimum (p = 0.02) and optimum-10° (p < 0.01). These findings suggest that large changes in upper body position can influence aerodynamics and alter contributions from the knee and hip joints, without influencing peak muscle forces.

Caffeine and Sprint Cycling Performance: Effects of Torque Factor and Sprint Duration

Purpose: To investigate the influence of torque factor and sprint duration on the effects of caffeine on sprint cycling performance. Methods: Using a counterbalanced, randomized, double-blind, placebo-controlled design, 13 men completed 9 trials. In trial 1, participants completed a series of 6-s sprints at increasing torque factors to determine the torque factor, for each individual, that elicited the highest (Toptimal) peak power output (PPO). The remaining trials involved all combinations of torque factor (0.8 N·m−1·kg−1 vs Toptimal), sprint duration (10 s vs 30 s), and supplementation (caffeine [5 mg·kg−1] vs placebo). Results: There was a significant effect of torque factor on PPO, with higher values at Toptimal (mean difference 168 W; 95% likely range 142–195 W). There was also a significant effect of sprint duration on PPO, with higher values in 10-s sprints (mean difference 52 W; 95% likely range 18–86 W). However, there was no effect of supplementation on PPO (P = .056). Nevertheless, there was a significant torque factor × sprint duration × supplement interaction (P = .036), with post hoc tests revealing that caffeine produced a higher PPO (mean difference 76 W; 95% likely range 19–133 W) when the sprint duration was 10 s and the torque factor was Toptimal. Conclusion: The results of this study show that when torque factor and sprint duration are optimized, to allow participants to express their highest PPO, there is a clear effect of caffeine on sprinting performance.

Performance criteria for generating predictive optimal control simulations of bicycle pedaling

The purpose of this study was to identify one or more performance-based criteria that may be used to generate predictive optimal control simulations of submaximal pedaling. Two-legged pedaling simulations were generated based on minimizing muscle activation, muscle stress, metabolic energy, time derivative of muscle force, and minimizing metabolic energy while pedaling smoothly. The simulations based on minimizing muscle activation and muscle stress most closely matched experimental pedaling data, with the activation criterion better matching experimental muscle activation timing. We conclude that predictive simulations of submaximal pedaling may be generated using a cost function based on minimizing muscle activation.

Effects of Exercise-Induced Muscle Damage in Well-Trained Cyclists' Aerobic and Anaerobic Performances

Karasiak, FC and Guglielmo, LGA. Effects of exercise-induced muscle damage in well-trained cyclists' aerobic and anaerobic performances. J Strength Cond Res 32(9): 2632-2640, 2018-The purpose of this study was to analyze the effect of exercise-induced muscle damage (EIMD) in gross efficiency and in aerobic and anaerobic cycling performances. Nine well-trained cyclists (30.8 ± 6.4 years, cycling experience 8.4 ± 5.6 years) visited the laboratory 5 times. During the first visit, they performed a maximal incremental test on a cycle ergometer, to identify V[Combining Dot Above]O2max (55.2 ± 4.9 ml·kg·min) and maximum aerobic power (Pmax; 327.0 ± 28.5 W). During the second visit (control), they cycled 5 minutes at 60% of Pmax, 5 minutes at 70% of Pmax, 5-minute time trial, and Wingate test. During the third visit, the athletes performed 10 sets of 10 countermovement jumps, to generate EIMD. The athletes repeated the second visit tests (control) 30 minutes, 48 hours (fourth visit), and 96 hours (fifth visit) after the jumps. The rated perceived exertion values increased 48 hours after EIMD (3.8 vs. 3.1) at 60% of Pmax. The ventilation and respiratory exchange ratio increased at 60% of Pmax (up to 4.3 L·min and 0.04, respectively) and at 70% of Pmax (up to 5.4 L·min and 0.05, respectively), mainly after 96 hours. There was no significant difference in V[Combining Dot Above]O2, V[Combining Dot Above]CO2, and heart rate in submaximal exercises, neither in time trial. No differences were observed in the Wingate tests. In conclusion, the EIMD did not impair gross efficiency, nor aerobic and anaerobic performances in trained cyclists. However, despite the benefits of strength training to improve cyclists' performance, coaches must be cautious to the days after the strength training sessions because EIMD may change the perception of maintaining a given submaximal intensity during training or competition.

Muscle Function and Coordination of Amputee Stair Ascent

Ascending stairs is challenging following transtibial amputation due to the loss of the ankle muscles, which are critical to human movement. Efforts to improve stair ascent following amputation are hindered by limited understanding of how prostheses and remaining muscles contribute to stair ascent. This study developed a three-dimensional muscle-actuated forward dynamics simulation of amputee stair ascent to identify contributions of individual muscles and passive prosthesis to the biomechanical subtasks of stair ascent. The prosthesis was found to provide vertical propulsion throughout stair ascent, similar to non-amputee plantarflexors. However, the timing differed considerably. The prosthesis also contributed to braking, similar to non-amputee soleus, but to a greater extent. In contrast, the prosthesis was unable to replicate the functions of non-amputee gastrocnemius which contributes to forward propulsion during the second half of stance and leg swing initiation. To compensate, hamstrings and vasti of the residual leg increased their contributions to forward propulsion during the first and second halves of stance, respectively. The prosthesis also contributed to medial control, consistent with the non-amputee soleus but not gastrocnemius. Therefore, prosthesis designs that provide additional vertical propulsion as well as forward propulsion, lateral control and leg swing initiation at appropriate points in the gait cycle could improve amputee stair ascent. However, because non-amputee soleus and gastrocnemius contribute oppositely to many subtasks, it may be necessary to couple the prosthesis, which functions most similarly to soleus, with targeted rehabilitation programs focused on muscle groups that can compensate for gastrocnemius.

Improved lower extremity pedaling mechanics in individuals with stroke under maximal workloads

Background Individuals with stroke present with motor control deficits resulting in the abnormal activation and timing of agonist and antagonist muscles and inefficient movement patterns. The analysis of pedaling biomechanics provides a window into understanding motor control deficits, which vary as a function of workload. Understanding the relationship between workload and motor control is critical when considering exercise prescription during stroke rehabilitation. Objectives To characterize pedaling kinematics and motor control processes under conditions in which workload was systematically increased to an eventual patient-specific maximum. Methods A cohort study was conducted in which 18 individuals with chronic stroke underwent a maximal exertion cardiopulmonary exercise test on a stationary cycle ergometer, during which pedaling torque was continuously recorded. Measures of force production, pedaling symmetry, and pedaling smoothness were obtained. Results Mean Torque increased significantly (p < 0.05) for both legs from initial to terminal workloads. Mean torque Symmetry Index, calculated for down and upstroke portions of the pedaling action, improved from 0.37(0.29) to 0.29(0.35) during downstroke (p = 0.007), and worsened during the upstroke: -0.37(0.38) to -0.62(0.46) (p < 0.001) from initial to terminal workloads. Low Torque Duration improved from initial to terminal workloads, decreasing from 121.1(52.9) to 58.1(39.6) degrees (p < 0.001), respectively. Smoothness of pedaling improved significantly from initial to terminal workloads (p < 0.001). Conclusions Improved pedaling kinematics at terminal workloads indicate that individuals with stroke demonstrate improved motor control with respect to the timing, sequencing, and activation of hemiparetic lower extremity musculature compared to lower workloads. Therapeutic prescription involving higher resistance may be necessary to sufficiently engage and activate the paretic lower extremity.

Predictive Simulations of Neuromuscular Coordination and Joint-Contact Loading in Human Gait

We implemented direct collocation on a full-body neuromusculoskeletal model to calculate muscle forces, ground reaction forces and knee contact loading simultaneously for one cycle of human gait. A data-tracking collocation problem was solved for walking at the normal speed to establish the practicality of incorporating a 3D model of articular contact and a model of foot-ground interaction explicitly in a dynamic optimization simulation. The data-tracking solution then was used as an initial guess to solve predictive collocation problems, where novel patterns of movement were generated for walking at slow and fast speeds, independent of experimental data. The data-tracking solutions accurately reproduced joint motion, ground forces and knee contact loads measured for two total knee arthroplasty patients walking at their preferred speeds. RMS errors in joint kinematics were < 2.0° for rotations and < 0.3 cm for translations while errors in the model-computed ground-reaction and knee-contact forces were < 0.07 BW and < 0.4 BW, respectively. The predictive solutions were also consistent with joint kinematics, ground forces, knee contact loads and muscle activation patterns measured for slow and fast walking. The results demonstrate the feasibility of performing computationally-efficient, predictive, dynamic optimization simulations of movement using full-body, muscle-actuated models with realistic representations of joint function.

An Optimal Method of Training the Specific Lower Limb Muscle Group Using an Exoskeletal Robot

This paper suggests a novel method of strengthening specific muscle groups in the lower limb during a functional movement. When the foot of an user wearing an exoskeletal robot follows a given path, the contribution of each muscle group to generate the motion changes along the path of the trajectory. The efficiency of muscle training, which is defined as the ratio of the work of a specific muscle group to that of all groups, can be maximized by changing the training load along the path. Based on a musculoskeletal model, the contribution of each muscle group along the path can be calculated as a function of its position. When a specific muscle group is chosen for exercise or rehabilitation, the efficiency of training can be maximized by setting high load where its contribution is high and low load where its contribution is low. By doing so, the user can exercise longer with the same amount of energy consumption. The EXOWheel that features a lower limb exoskeleton is employed to verify the method and the hamstring muscle group is selected as the specific muscle group. Three healthy subjects participate in the experiment, and electromyogram sensors are employed to monitor the muscle power. The results indicate that the efficiency of the hamstring muscle group for a given circular foot path under the optimal training load is 46.2% compared with 32.5% with a constant load and it means the 42% higher efficiency of the specific muscle group training.

Force-velocity test on a stationary cycle ergometer: methodological recommendations

Force-velocity tests performed on stationary cycle ergometers are widely used to assess the torque- and power-generating capacities of the lower limbs. The aim of this study was to identify how testing and modeling procedures influence the assessment of individual torque-cadence and power-cadence relationships. Seventeen males completed 62 ± 16 pedal cycles from six 6-s all-out efforts interspersed with 5 min of rest. True measures of maximal power for a particular cadence were obtained for 24 ± 3 pedal cycles, while power was only 94 ± 3% of the true maximum in 19 ± 5 pedal cycles. Pedal cycles showing maximal levels of power also displayed higher levels of electromyography (EMG: 89 ± 7 vs . 87 ± 7%) and coactivation (34 ± 11 vs . 31 ± 10 arbitrary units), as well as lower variability in crank torque and EMG profiles. Compared with the linear and second-order polynomial models that are traditionally used, a better goodness of fit was obtained when the torque-cadence and power-cadence relationships were predicted using second- and third-order polynomials, respectively. The later modeling procedures also revealed an asymmetry in the power-cadence relationship in most participants (i.e., 15 out of 17) and provided a better estimation of maximal cadence [Cmax: 214 ± 20 revolutions/min (rpm)] from the x-intercept of power-cadence relationships (C0: 214 ± 14 rpm). Therefore, we recommend predicting the individual shapes of torque- and power-cadence relationships using second- and third-order polynomial regressions after having selected pedal cycles during which true measures of cadence-specific maximal power were recorded.

NEW & NOTEWORTHY This study is the first to demonstrate that suboptimal activation of the lower limb muscles accompanied reductions in cadence-specific levels of torque and power produced during a force-velocity test performed on a stationary cycle ergometer. This research is also the first to show that, in most noncyclist participants, torque-cadence relationships are not linear, whereas power-cadence relationships display asymmetric shapes, with power production decreasing rapidly when cadence increases beyond 180 revolutions/min.

Development and Evaluation of a Powered Artificial Gastrocnemius for Transtibial Amputee Gait

Existing robotic transtibial prostheses provide only ankle joint actuation and do not restore biarticular function of the gastrocnemius muscle. This paper presents the first powered biarticular transtibial prosthesis, which is a combination of a commercial powered ankle-foot prosthesis and a motorized robotic knee orthosis. The orthosis is controlled to emulate the human gastrocnemius based on neuromuscular models of matched nonamputees. Together with the ankle-foot prosthesis, the devices provide biarticular actuation. We evaluate differences between this biarticular condition and a monoarticular condition with the orthosis behaving as a free-joint. Six participants with transtibial amputation walk with the prosthesis on a treadmill while motion, force, and metabolic data are collected and analyzed for differences between conditions. The biarticular prosthesis reduces affected-side biological knee flexion moment impulse and hip positive work during late-stance knee flexion, compared to the monoarticular condition. The data do not support our hypothesis that metabolism decreases for all participants, but some participants demonstrate large metabolic reductions with the biarticular condition. These preliminary results suggest that a powered artificial gastrocnemius may be capable of providing large metabolic reductions compared to a monoarticular prosthesis, but further study is warranted to determine an appropriate controller for achieving more consistent metabolic benefits.

Muscle Function and Coordination of Stair Ascent

Stair ascent is an activity of daily living and necessary for maintaining independence in community environments. One challenge to improving an individual's ability to ascend stairs is a limited understanding of how lower-limb muscles work in synergy to perform stair ascent. Through dynamic coupling, muscles can perform multiple functions and require contributions from other muscles to perform a task successfully. The purpose of this study was to identify the functional roles of individual muscles during stair ascent and the mechanisms by which muscles work together to perform specific subtasks. A three-dimensional (3D) muscle-actuated simulation of stair ascent was generated to identify individual muscle contributions to the biomechanical subtasks of vertical propulsion, anteroposterior (AP) braking and propulsion, mediolateral control and leg swing. The vasti and plantarflexors were the primary contributors to vertical propulsion during the first and second halves of stance, respectively, while gluteus maximus and hamstrings were the primary contributors to forward propulsion during the first and second halves of stance, respectively. The anterior and posterior components of gluteus medius were the primary contributors to medial control, while vasti and hamstrings were the primary contributors to lateral control during the first and second halves of stance, respectively. To control leg swing, antagonistic muscles spanning the hip, knee, and ankle joints distributed power from the leg to the remaining body segments. These results compliment previous studies analyzing stair ascent and provide further rationale for developing targeted rehabilitation strategies to address patient-specific deficits in stair ascent.