Biology subjects

Lichtwark, G. A.

Publications and source records attributed to Lichtwark, G. A..

12 recordsLinked to original sources

bioRxiv2026

A time-dependent mechano-bioenergetics model of muscle contraction

Predictions of skeletal muscle energy consumption under a diverse range of muscle contractile conditions are critical for improving our understanding of locomotion. Existing mathematical models, while capturing the mechanical dependence of energy consuming processes, neglect the time-dependent behaviour and recovery costs associated with regenerating ATP. This time-dependence is important for predicting the energetic response of muscles during repetitive or cyclical tasks like locomotion, where muscle undergoes many contraction cycles. This study presents a novel model to predict energetic rates based on physiological processes: Ca2+ transport costs, cross-bridge cycling costs, and ATP regeneration. Previous mathematical models include the dependence on Ca2+ transport and cross-bridge cycling, but neglect the time-dependent response and the subsequent recovery of ATP following the contraction. Model parameters were obtained from existing data on isolated muscle preparations, and predicted energetic rates were validated on separate datasets across a range of contractile conditions including dynamic, sub-maximal, and twitch contractions. The time-dependent model was able to capture the influence of contraction frequency on peak energetic rates and the time-course of energetic recovery observed experimentally. The model captures key physiological processes while maintaining a minimal number of free parameters and low computational cost. This enables generalis-ability across muscles and species, and implementation into larger scale musculoskeletal models.

physiology↗

bioRxiv2025

The impact of mechanical requirements on the neural control of skeletal muscle and subsequent energetic rates

1The energetic cost of skeletal muscle contraction is a fundamental driver in the selection of locomotor strategies. Muscle energy consumption depends on muscle-fibre typology, neural drive, and mechanical state. While the influence of fibre-type and mechanical state on energy use has been extensively characterised in isolated muscle preparations, these experiments fail to capture the influence of realistic in vivo motor unit recruitment strategies. Hence, this study aims to capture neural drive in the tibialis anterior, in particular motor unit recruitment thresholds and rate coding, and the corresponding changes in energy use across a range of mechanical demands. Fixed ankle angle dorsiflexion contractions were performed at varying rates of torque development, while high density electromyography characterized motor unit spiking activity, B-mode ultrasound captured muscle fascicle dynamics, and indirect calorimetry measured energetic rates. Faster rates of torque development required earlier recruitment of motor units with increased motor unit firing rates which coincided with increased fascicle strain rates. At higher rates of torque development, these altered motor unit discharge patterns and muscle mechanics coincided with increased muscle energy use. Together, these findings highlight that in vivo muscle energetics emerge from the dynamic interplay between neural control and mechanical demands.

physiology↗

bioRxiv2024

Dissecting the metabolic costs of up- and down-hill walking

Work- and collision-based models of locomotion are often used to describe the relationship between the bodys mechanical work requirements and metabolic energy expenditure. While work- and collision-based models do a reasonable job of relating mechanical work to metabolic cost at a system level, these models may not map to the underlying force and work demands of muscle, which directly affects energy expenditure. We collected motion capture, force, electromyography and ultrasound data from the main power producing muscles during uphill and downhill walking between +/- 15% grade. These data were used to evaluate a musculoskeletal modelling approach to simulate muscle force- and work-related costs that could be compared to metabolic power that we measured using indirect calorimetry. Muscle force-related costs (activation heat rate + maintenance heat rate) increased at steeper up- and down-hill grades and were moderately correlated with mean joint moments. Muscle work-related costs (mechanical work rate + shortening / lengthening heat rate) increased as grade became more positive and were strongly correlated with net joint work. Compared to traditional models, the inclusion of a term to account for muscle force-related costs should lead to a more explanatory cost model that maps directly to the mechanical demands of muscle.

physiology↗

bioRxiv2024

Cycling Cadence Selections at Different Saddle Heights Minimize Muscle Activation Rather Than Energy Cost

Unlike walking and running, people do not consistently choose cadences that minimize energy consumption when cycling. Assuming a common objective function for all forms of locomotion, this suggests either that the neural control system relies on indirect sensorimotor cues to energetic cost that are approximately accurate during walking but not cycling, or that an alternative objective function applies that correlates with energy expenditure in walking but not cycling. This study compared how objective functions derived as proxies to 1) energy cost or 2) an avoidance of muscle fatigue predicted self-selected cycling cadences (SSC) at different saddle heights. Saddle height systematically affected SSC, with lower saddles increasing SSC and higher saddles decreasing SSC. Both fatigue-avoidance and energy-expenditure cost functions derived from muscle activation measurements showed minima that closely approximated the SSCs. By contrast, metabolic power derived from VO2 uptake was minimal at cadences well below the SSC across all saddle height variations. The mismatch between the cadence versus muscle activation and the cadence versus metabolic energy relations is likely due to additional energy costs associated with performing mechanical work at higher cadences. The results suggest that the nervous system places greater emphasis on muscle activation than on energy consumption for action selections in cycling.

evolutionary biology↗

bioRxiv2024

Using physiologically-based models to predict in vivo skeletal muscle energetics

Understanding how muscles use energy is essential for elucidating the role of skeletal muscle in animal locomotion. Yet, experimental measures of in vivo muscle energetics are challenging to obtain, so physiologically-based muscle models are often used to estimate energy use. These predictions of individual muscle energy expenditure are not often compared to indirect whole body measures of energetic cost. Here, we examined and illustrated the capability of physiologically-based muscle models to predict in vivo measures of energy use. To improve model predictions and ensure a physiological basis for model parameters, we refined our model to include data from isolated muscle experiments. Simulations were performed to capture three different experimental protocols, which involved varying contraction frequency, duty cycle, and muscle fascicle length. Our results demonstrated that these models are able capture the general features of whole body energetics across contractile conditions, but tended to under predict the magnitude of energetic cost. Our analysis revealed that when predicting in vivo energetic rates across contractile conditions, the model was most sensitive to the force-velocity parameters and the data informing the energetic rates when predicting in vivo energetic rates across contractile conditions. This work highlights it is the mechanics of skeletal muscle contraction that govern muscle energy use.

bioengineering↗

bioRxiv2024

Unlocking the secrets of kangaroo locomotor energetics: Postural adaptations underpin increased tendon stress in hopping kangaroos

Hopping kangaroos exhibit remarkably little change in their rate of metabolic energy expenditure with locomotor speed compared to other running animals. This phenomenon may be related to greater elastic energy savings due to increasing tendon stress; however, the mechanisms which enable the rise in stress without additional muscle work remain poorly understood. In this study, we created a three-dimensional (3D) kangaroo musculoskeletal model, integrating 3D motion capture and force plate data, to analyse the kinematics and kinetics of hopping red and grey kangaroos. Using our model, we evaluated how body mass and speed influence (i) hindlimb posture, (ii) effective mechanical advantage (EMA), (iii) the associated tendon stress in the ankle extensors, and (iv) ankle work during hopping. We found that increasing ankle dorsiflexion and metatarsophalangeal plantarflexion likely played an important role in decreasing ankle EMA by altering both the muscle and external moment arms, which subsequently increased energy absorption and peak tendon stress at the ankle. Surprisingly, kangaroo hindlimb posture changes appeared to contribute to increased tendon stress, allowing more elastic energy storage at faster speeds. These posture-mediated increases in elastic energy storage and return could be a key factor enabling kangaroos to achieve energetic benefits at faster hopping speeds, but may limit the performance of large kangaroos due to the risk of tendon rupture.

zoology↗

bioRxiv2023

Increased force and elastic energy storage are not the mechanisms that improve jump performance with accentuated eccentric loading during a constrained vertical jump

ObjectiveAccentuated eccentric loading (AEL) involves higher load applied during the eccentric phase of a stretch-shortening cycle movement, followed by a sudden removal of load before the concentric phase. Previous studies suggest that AEL enhances human countermovement jump performance, however the mechanism is not fully understood. Here we explore whether isolating additional load during the countermovement is sufficient to increase ground reaction force, and hence elastic energy stored, at the start of the upward movement and whether this leads to increased jump height or power generation. MethodsWe conducted a trunk-constrained vertical jump test on a custom-built device to isolate the effect of additional load while controlling for effects of squat depth, arm swing, and coordination. Twelve healthy, recreationally active adults (7 males, 5 females) performed maximal jumps without AEL, followed by randomised AEL conditions prescribed as a percentage of body mass (10%, 20%, and 30%), before repeating jumps without AEL. Results. No significant changes in vertical ground reaction force at the turning point were observed. High load AEL conditions (20% and 30% body weight) led to slight reductions in jump height, primarily due to decreased hip joint and centre of mass work. AEL conditions did not alter peak or integrated activation levels of the knee extensor muscles. ConclusionThese findings suggest that increased elastic energy return may not be the primary mechanism behind the potentiating effects of AEL on jump performance, and other factors such as rate of descent, squat depth, or body configuration may contribute to effective AEL.

biophysics↗

bioRxiv2023

An in-silico investigation of the effect of changing cycling crank power and cadence on muscle energetics and active muscle volume

This study used musculoskeletal modelling to explore the relationship between cycling conditions (power output and cadence) and muscle activation and metabolic power. We hypothesized that the cadence that minimized the simulated average active muscle volume would be higher than that which minimized the simulated metabolic power. We validated the simulation by comparing predicted muscle activation and fascicle velocities from select muscles with experimental records of electromyography and ultrasound images. We found strong correlations for averaged muscle activations and moderate to good correlations for fascicle dynamics. These correlations tended to weaken when analyzed at the individual participant level. Our study revealed a curvilinear relationship between average active muscle volume and cadence, with the minimum active volume being aligned to the self-selected cadence. The simulated metabolic power was consistent with previous results and was minimized at lower cadences than that which minimized active muscle volume across power outputs. Whilst there are some limitations to the musculoskeletal modelling approach, the findings suggest that minimizing active muscle volume may be a more important factor than minimizing metabolic power for self-selected cycling cadence preferences. Further research is warranted to explore the potential of an active muscle volume based objective function for control schemes across a wider range of cycling conditions.

bioengineering↗

bioRxiv2023

Linking Muscle Mechanics to the Metabolic Cost of Human Hopping

Many models have been developed to predict metabolic energy expenditure based on biomechanical proxies of muscle function. However, current models may only perform well for select forms of locomotion, not only because the models are rarely rigorously tested across subtle and broad changes in locomotor task, but also because previous research has not adequately characterised different forms of locomotion to account for the potential variability in muscle function and thus metabolic energy expenditure. To help to address the latter point, the present study imposed frequency and height constraints to hopping and quantified gross metabolic power as well as the activation requirements of medial gastrocnemius, lateral gastrocnemius (GL), soleus (SOL), tibialis anterior, vastus lateralis (VL), rectus femoris (RF) and biceps femoris (BF), and the work requirements GL, SOL and VL. Gross metabolic power increased with a decrease in hop frequency and increase in hop height. There was no hop frequency or hop height effect on the mean electromyography (EMG) of ankle musculature, however, the mean EMG of VL and RF increased with a decrease in hop frequency and that of BF increased with an increase in hop height. With a reduction in hop frequency, GL, SOL and VL fascicle shortening, fascicle shortening velocity and fascicle to MTU shortening ratio increased, whereas with an increase in hop height, only SOL fascicle shortening velocity increased. Therefore, within the constraints that we imposed, decreases in hop frequency and increases in hop height resulted in increases in metabolic power that could be explained by increases in the activation requirements of knee musculature and/or increases in the work requirements of both knee and ankle musculature. Summary StatementThis study directly measures activation and work requirements of lower-limb musculature and whole-body metabolic energy requirements across a wide variety of human hopping conditions, helping to guide biomechanical models of energy expenditure.

physiology↗

bioRxiv2022

The effects of crank power and cadence on muscle fascicle shortening velocities, muscle activations and joint-powers during cycling.

Whilst people typically chose to locomote in most economical fashion, during cycling on a bicycle they will, unusually, chose cadences that are higher than metabolically optimal. Empirical measurements of the intrinsic contractile properties of the vastus lateralis (VL) muscle during submaximal cycling suggest that the cadences that people prefer (i.e., self-selected cadences: SSC) allow for optimal muscle fascicle shortening velocity for the production of knee extensor muscle power. It remains unclear, however, whether this is consistent across different power outputs where SSC is known to might be affected. We examined the effect of cadence and external power requirements on muscle neuromechanics and joint powers during cycling. VL fascicle shortening velocities, muscle activations and joint-specific powers were measured during cycling between 60 and 120rpm (and the SSC), while participants produced 10%, 30%, and 50% of peak maximal power. VL shortening velocity increased as cadence increased but was similar across the different power outputs. Although no differences were found in the distribution of joint powers across cadence conditions, the absolute knee joint power increased with increasing crank power output. Muscle fascicle shortening velocities increase in VL at the SSC as pedal power demands increase from submaximal to maximal cycling. It therefore seems highly unlikely that preferred cadence is primarily driven by the desire to maintain "optimal" muscle fascicle shortening velocities. A secondary analysis of muscle activation patterns revealed that minimizing muscle activation is likely more important when choosing a cadence for given pedal power demand.

physiology↗

bioRxiv2021

Concurrent assessment of motor unit firing properties and fascicle length changes with high-density surface electromyography ultrasound-transparent electrodes

The integration of electromyography (EMG) and ultrasound imaging has provided important information about the mechanisms of muscle activation and contraction. Unfortunately, EMG does not allow an accurate assessment of the interplay between the neural drive received by muscles, changes in fascicle length (FL) and the force/torque produced. We aimed to assess the relationship between modulations in tibialis anterior (TA) motor unit (MU) firing rate, FL and dorsiflexion torque (DT) using ultrasound-transparent high-density EMG electrodes. EMG and ultrasound images were recorded simultaneously from TA, using a 32-electrode silicon matrix, while performing isometric dorsiflexion, at diverse ankle joint positions (0{degrees} and 30{degrees} plantar flexion) and torques (20% and 40% of maximum). EMG signals were decomposed into individual MUs and changes in FL were assessed with a fascicle-tracking algorithm. MU firings were converted into a cumulative spike train (CST) that was cross-correlated with DT (CST-DT) and FL (CST-FL). High cross-correlations were found for CST-FL, 0.60 (range: 0.31-0.85) and CST-DT 0.71 (range: 0.31-0.88). Cross-correlation lags revealed that the delay between CST-FL (~75ms) was significantly smaller than CST-DT (~150ms, p<0.001). These delays affected the interpretation of MU recruitment/de-recruitment thresholds, with FL showing similar lengths for both recruitment and de-recruitment. This study is the first to demonstrate that changes in TA FL are closely related to both modulations in MU firing frequency and DT. These relationships allow assessment of the interplay between neural drive, muscle contraction and resultant torque, thereby providing a better understanding of the mechanisms responsible for the generation of muscle force. NEW AND NOTEWORTHYBy employing ultrasound-transparent high-density surface EMG electrodes, we show that modulations in tibialis anterior motor unit discharge rate were closely related to both changes in its fascicle length and resultant torque. These relationships allowed quantifying delays between neural drive and muscle shortening as well as muscle shortening and torque during submaximal isometric contractions, providing an accurate estimation of the time required to generate muscle force and subsequent production of torque via the tendon.

physiology↗

bioRxiv2021

Biceps femoris long head sarcomere and fascicle length adaptations after three weeks of eccentric exercise training

PurposeEccentric exercise is widely used to increase muscle fascicle lengths and thus decrease the risk of muscle strain injuries. However, the mechanisms behind this protection are still unknown. The aim of this study was to determine whether Biceps femoris long head (BFlh) fascicle length increases in response to three weeks of eccentric exercise training are the result of addition of in-series sarcomeres within muscle fibres. MethodsTen recreationally active participants (age: 27 {+/-} 3 years, mass: 70 {+/-} 14 kg, height: 174 {+/-} 9 cm) completed three weeks of Nordic hamstring exercise (NHE) training. We collected in vivo sarcomere and muscle fascicle images of the BFlh in two regions (central and distal), utilising microendoscopy and 3D ultrasonography. These images allowed us to estimate sarcomere length, sarcomere number and fascicle lengths before and after the training intervention. ResultsEccentric knee flexion strength increased after the training (15%, P < 0.001, {eta}p2= 0.75). Further, we found a significant increase in fascicle (21%, P < 0.001, {eta}p2 = 0.81) and sarcomere (17%, P < 0.001, {eta}p2 = 0.9) lengths in the distal but not in the central portion of the muscle. The estimated number of in series sarcomeres did not change in either region. ConclusionFascicle length adaptations appear to be heterogeneous in the BFlf in response to three weeks of NHE training. An increase in sarcomere length, rather than the addition of sarcomeres in series, appears to be underlying this adaptation. The mechanism driving regional increases in fascicle and sarcomere length remain unknown, but we speculate it may be driven by regional changes in the passive tension of muscle or connective tissue adaptations.

physiology↗