Contractile and elastic ankle joint muscular properties in young and older adults

Continuous neural control of a bionic limb restores biomimetic gait after amputation

For centuries scientists and technologists have sought artificial leg replacements that fully capture the versatility of their intact biological counterparts. However, biological gait requires coordinated volitional and reflexive motor control by complex afferent and efferent neural interplay, making its neuroprosthetic emulation challenging after limb amputation. Here we hypothesize that continuous neural control of a bionic limb can restore biomimetic gait after below-knee amputation when residual muscle afferents are augmented. To test this hypothesis, we present a neuroprosthetic interface consisting of surgically connected, agonist-antagonist muscles including muscle-sensing electrodes. In a cohort of seven leg amputees, the interface is shown to augment residual muscle afferents by 18% of biologically intact values. Compared with a matched amputee cohort without the afferent augmentation, the maximum neuroprosthetic walking speed is increased by 41%, enabling equivalent peak speeds to persons without leg amputation. Further, this level of afferent augmentation enables biomimetic adaptation to various walking speeds and real-world environments, including slopes, stairs and obstructed pathways. Our results suggest that even a small augmentation of residual muscle afferents restores biomimetic gait under continuous neuromodulation in individuals with leg amputation.

Scalable musculoskeletal model for dynamic simulations of lower body movement

A musculoskeletal (MSK) model is an important tool for analysing human motions, calculating joint torques during movement, enhancing sports activity, and developing exoskeletons and prostheses. To enable biomechanical investigation of human motion, this work presents an open-source lower body MSK model. The MSK model of the lower body consists of 7 body segments (pelvis, left/right thigh, left/right leg, and left/right foot). The model has 20 degrees of freedom (DoFs) and 28 muscle torque generators (MTGs), which are developed from experimental data. The model can be modified for different anthropometric measurements and subject body characteristics, including sex, age, body mass, height, physical activity, and skin temperature. The model is validated by simulating the torque within the range of motion (ROM) of isolated movements; all simulation findings exhibit a good level of agreement with the literature.

Shared and distinct mechanisms of skeletal muscle atrophy: A narrative review

Maintenance of skeletal muscle mass and function is an incredibly nuanced balance of anabolism and catabolism that can become distorted within different pathological conditions. In this paper we intend to discuss the distinct intracellular signaling events that regulate muscle protein atrophy for a given clinical occurrence. Aside from the common outcome of muscle deterioration, several conditions have at least one or more distinct mechanisms that creates unique intracellular environments that facilitate muscle loss. The subtle individuality to each of these given pathologies can provide both researchers and clinicians with specific targets of interest to further identify and increase the efficacy of medical treatments and interventions.

Effects of old age and contraction mode on knee extensor muscle ATP flux and metabolic economy in vivo

KEY POINTS

We used 31-phosphorus magnetic resonance spectroscopy to quantify in vivo skeletal muscle metabolic economy (ME; mass-normalized torque or power produced per ATP consumed) during three 24 s maximal-effort contraction protocols: (1) sustained isometric (MVIC), (2) intermittent isokinetic (MVDC_IsoK ), and (3) intermittent isotonic (MVDC_IsoT ) in the knee extensor muscles of young and older adults. ME was not different between groups during the MVIC but was lower in older than young adults during both dynamic contraction protocols. These results are consistent with an increased energy cost of locomotion, but not postural support, with age. The effects of old age on ME were not due to age-related changes in muscle oxidative capacity or ATP flux. Specific power was lower in older than young adults, despite similar total ATP synthesis between groups. Together, this suggests a dissociation between cross-bridge activity and ATP utilization with age.

ABSTRACT

Muscle metabolic economy (ME; mass-normalized torque or power produced per ATP consumed) is similar in young and older adults during some isometric contractions, but less is known about potential age-related differences in ME during dynamic contractions. We hypothesized that age-related differences in ME would exist only during dynamic contractions, due to the increased energetic demand of dynamic versus isometric contractions. Ten young (Y; 27.5 ± 3.9 years, 6 men) and 10 older (O; 71 ± 5 years, 5 men) healthy adults performed three 24 s bouts of maximal contractions: (1) sustained isometric (MVIC), (2) isokinetic (120°·s^-1 , MVDC_IsoK ; 0.5 Hz), and (3) isotonic (load = 20% MVIC, MVDC_IsoT ; 0.5 Hz). Phosphorus magnetic resonance spectroscopy of the vastus lateralis muscle was used to calculate ATP flux (mM ATP·s^-1 ) through the creatine kinase reaction, glycolysis and oxidative phosphorylation. Quadriceps contractile volume (cm³ ) was measured by MRI. ME was calculated using the torque-time integral (MVIC) or power-time integral (MVDC_IsoK and MVDC_IsoT ), total ATP synthesis and contractile volume. As hypothesized, ME was not different between Y and O during the MVIC (0.12 ± 0.03 vs. 0.12 ± 0.02 Nm^. s^. cm^{-3 .} mM ATP^-1 , mean ± SD, respectively; P = 0.847). However, during both MVDC_IsoK and MVDC_IsoT , ME was lower in O than Y adults (MVDC_IsoK : 0.011 ± 0.003 vs. 0.007 ± 0.002 J^. cm^{-3 .} mM ATP^-1 ; P < 0.001; MVDC_IsoT : 0.011 ± 0.002 vs. 0.008 ± 0.002; P = 0.037, respectively), despite similar muscle oxidative capacity, oxidative and total ATP flux in both groups. The lower specific power in older than young adults, despite similar total ATP synthesis between groups, suggests there is a dissociation between cross-bridge activity and ATP utilization with age.

Development of a Telemedicine Neurological Examination for Spine Surgery: A Pilot Trial

STUDY DESIGN

This was a prospective cohort study.

OBJECTIVE

The objective of this study was to design and test a novel spine neurological examination adapted for telemedicine.

SUMMARY OF BACKGROUND DATA

Telemedicine is a rapidly evolving technology associated with numerous potential benefits for health care, especially in the modern era of value-based care. To date, no studies have assessed whether.

METHODS

Twenty-one healthy controls and 20 patients with cervical or lumbar spinal disease (D) were prospectively enrolled. Each patient underwent a telemedicine neurological examination as well as a traditional in-person neurological examination administered by a fellowship trained spine surgeon and a physiatrist. Both the telemedicine and in-person tests consisted of motor, sensory, and special test components. Scores were compared via univariate analysis and secondary qualitative outcomes, including responses from a satisfaction survey, were obtained upon completion of the trial.

RESULTS

Of the 20 patients in the D group, 9 patients had cervical disease and 11 patients had lumbar disease. Comparing healthy control with the D group, there were no significant differences with respect to all motor scores, most sensory scores, and all special tests. There was a high rate of satisfaction among the cohort with 92.7% of participants feeling "very satisfied" with the overall experience.

CONCLUSIONS

This study presents the development of a viable neurological spine examination adapted for telemedicine. The findings in this study suggest that patients have comparable motor, sensory, and special test scores with telemedicine as with a traditional in-person examination administered by an experienced clinician, as well as reporting a high rate of satisfaction among participants. To our knowledge, this is the first telemedicine neurological examination for spine surgery. Further studies are warranted to validate these findings.

Estimation of the force-velocity properties of individual muscles from measurement of the combined plantarflexor properties

The force-velocity (F-V) properties of isolated muscles or muscle fibers have been well studied in humans and other animals. However, determining properties of individual muscles in vivo remains a challenge because muscles usually function within a synergistic group. Modeling has been used to estimate the properties of an individual muscle from the experimental measurement of the muscle group properties. While this approach can be valuable, the models and the associated predictions are difficult to validate. In this study, we measured the in situ F-V properties of the maximally activated kangaroo rat plantarflexor group and used two different assumptions and associated models to estimate the properties of the individual plantarflexors. The first model (Mdl1) assumed that the percent contributions of individual muscles to group force and power were based upon the muscles' cross-sectional area and were constant across the different isotonic loads applied to the muscle group. The second model (Mdl2) assumed that the F-V properties of the fibers within each muscle were identical, but because of differences in muscle architecture, the muscles' contributions to the group properties changed with isotonic load. We compared the two model predictions with independent estimates of the muscles' contributions based upon sonomicrometry measurements of muscle length. We found that predictions from Mdl2 were not significantly different from sonomicrometry-based estimates while those from Mdl1 were significantly different. The results of this study show that incorporating appropriate fiber properties and muscle architecture is necessary to parse the individual muscles' contributions to the group F-V properties.

Estimating Tibial Stress throughout the Duration of a Treadmill Run

INTRODUCTION

Stress fractures of the tibia are a problematic injury among runners of all levels. Quantifying tibial stress using a modeling approach provides an alternative to invasive assessments that may be used to detect changes in tibial stress during running. This study aimed to assess the repeatability of a tibial stress model and to use this model to quantify changes in tibial stress that occur throughout the course of a 40-min prolonged treadmill run.

METHODS

Synchronized force and kinematic data were collected during prolonged treadmill running from 14 recreational male rearfoot runners on two separate occasions. During each session, participants ran at their preferred speed for two consecutive 20-min runs, separated by a 2-min pause. The tibia was modeled as a hollow ellipse and bending moments and stresses at the distal third of the tibia were estimated using beam theory combined with inverse dynamics and musculoskeletal modeling.

RESULTS

Intraclass correlation coefficients indicated good-to-excellent repeatability for peak stress values between sessions. Peak anterior and posterior stresses increased after 20 min of prolonged treadmill running and were 15% and 12% greater, respectively, after 40 min of running compared with the start of the run.

CONCLUSION

The hollow elliptical tibial model presented is a repeatable tool that can be utilized to assess within-participant changes in peak tibial stress during running. The increased stresses observed during a prolonged treadmill run may have implications for the development of tibial stress fracture.

Sensitivity analysis of muscle properties and impact parameters on head injury risk in American football

Head injuries frequently occur in American football and other contact sports. Uncertainty on the effects of cervical muscle properties on head injury risk may be due to the limitations of previous observational studies. This simulation study employs a musculoskeletal model of the head and neck to investigate the effect of several factors related to head injury metrics in American Football. These factors include isometric muscle strength, the eccentric multiplier (which is related to the athlete's ability to apply greater muscle force during eccentric contractions), posture, muscle activation patterns, and impact properties. Impact properties were based on the literature and tuned to reproduce peak linear and rotational accelerations of the skull. We hypothesized that active neck muscles significantly reduce head injury metrics. We systematically altered each model parameter to test our hypothesis. We then determined which model parameters affect head injury metrics the most. The results of this study indicate that active neck muscles have a statistically significant effect on head injury metrics. Increasing muscle strength and eccentric multiplier also resulted in a statistically significant reduction of head injury metrics. However, posture prior to impact had a much stronger effect than any other factor on head injury metrics. A comprehensive approach to athlete training protocols is recommended, including exercises aimed at increasing eccentric muscle strength and preparation for impacts. Future studies should investigate how targeted muscle strengthening and impact training (i.e. activation patterns and posture) modifies risk.

Comparison of neural excitation measures from the surface electromyogram during rate-dependent muscle contractions

Peak power and peak rate of isometric force development (RFD) predict performance and functional mobility. Surface electromyography (EMG) is used to quantify the amplitude and rate of neuromuscular excitation. To inform the selection of EMG measures in research on rate-dependent muscle contractions, this methodological study compared amplitude-, area- and rate-based measures based on their correlations with RFD. Considering populations in whom a quiet EMG baseline is challenging, we included measures that do not require the determination of EMG onset. Twenty-one young adults performed isometric dorsiflexion contractions to 40% of their maximal force at increasing RFD. EMG was recorded from tibialis anterior. Relationships between EMG measures and RFD were quantified with Spearman's rho. RMS amplitude of the initial 75 ms of EMG had the strongest correlation with peak RFD (ρ = 0.80) among measures computed from EMG onset. Peak rate of EMG rise (RER) had the strongest relationship with peak RFD (ρ = 0.69) among measures that did not require determination of EMG onset. The strength of the relationship between RER and RFD and the strong correlation between RER and RMS75 during rapid contractions (ρ = 0.86) supports the use of RER in experiments where neural excitation might not be initiated from a quiet baseline.

Computational model to investigate the relative contributions of different neuromuscular properties of tibialis anterior on force generated during ankle dorsiflexion

This study describes a new model of the force generated by tibialis anterior muscle with three new features: single-fiber action potential, twitch force, and pennation angle. This model was used to investigate the relative effects and interaction of ten age-associated neuromuscular parameters. Regression analysis (significance level of 0.05) between the neuromuscular properties and corresponding simulated force produced at the footplate was performed. Standardized slope coefficients were computed to rank the effect of the parameters. The results show that reduction in the average firing rate is the reason for the sharp decline in the force and other factors, such as number of muscle fibers, specific force, pennation angle, and innervation ratio. The fast fiber ratio affects the simulated force through two significant interactions. This study has ranked the individual contributions of the neuromuscular factors to muscle strength decline of the TA and identified firing rate decline as the biggest cause followed by decrease in muscle fiber number and specific force. The strategy for strength preservation for the elderly should focus on improving firing rate. Graphical abstract Neuromuscular properties of Tibialis Anterior on force generated during ankle dorsiflexion.

A multi-joint model of quiet, upright stance accounts for the "uncontrolled manifold" structure of joint variance

The upright body in quiet stance is usually modeled as a single-link inverted pendulum. This agrees with most of the relevant sensory organs being at the far end of the pendulum, i.e., the eyes and the vestibular system in the head. Movement of the body in quiet stance has often been explained in terms of the "ankle strategy," where most movement is generated by the ankle musculature, while more proximal muscle groups are only rarely activated for faster movements or in response to perturbations, for instance, by flexing at the hips in what has been called the "hip strategy." Recent empirical evidence, however, shows that instead of being negligible in quiet stance, the movement in the knee and hip joints is even larger on average than the movement in the ankle joints (J Neurophysiol 97:3024-3035, 2007). Moreover, there is a strong pattern of covariation between movements in the ankle, knee and hip joints in a way that most of the observed movements leave the anterior-posterior position of the whole-body center of mass (CoM) invariant, i.e., only change the configuration of the different body parts around the CoM, instead of moving the body as a whole. It is unknown, however, where this covariation between joint angles during quiet stance originates from. In this paper, we aim to answer this question using a comprehensive model of the biomechanical, muscular and neural dynamics of a quietly standing human. We explore four different possible feedback laws for the control of this multi-link pendulum in upright stance that map sensory data to motor commands. We perform simulation studies to compare the generated inter-joint covariance patterns with experimental data. We find that control laws that actively coordinate muscle activation between the different joints generate correct variance patterns, while control laws that control each joint separately do not. Different specific forms of this coordination are compatible with the data.

Elucidating Sensorimotor Control Principles with Myoelectric Musculoskeletal Models

There is an old saying that you must walk a mile in someone's shoes to truly understand them. This mini-review will synthesize and discuss recent research that attempts to make humans "walk a mile" in an artificial musculoskeletal system to gain insight into the principles governing human movement control. In this approach, electromyography (EMG) is used to sample human motor commands; these commands serve as inputs to mathematical models of muscular dynamics, which in turn act on a model of skeletal dynamics to produce a simulated motor action in real-time (i.e., the model's state is updated fast enough produce smooth motion without noticeable transitions; Manal et al., 2002). In this mini-review, these are termed myoelectric musculoskeletal models (MMMs). After a brief overview of typical MMM design and operation principles, the review will highlight how MMMs have been used for understanding human sensorimotor control and learning by evoking apparent alterations in a user's biomechanics, neural control, and sensory feedback experiences.

Mechanisms of in vivo muscle fatigue in humans: investigating age-related fatigue resistance with a computational model

KEY POINTS

Muscle fatigue can be defined as the transient decrease in maximal force that occurs in response to muscle use. Fatigue develops because of a complex set of changes within the neuromuscular system that are difficult to evaluate simultaneously in humans. The skeletal muscle of older adults fatigues less than that of young adults during static contractions. The potential sources of this difference are multiple and intertwined. To evaluate the individual mechanisms of fatigue, we developed an integrative computational model based on neural, biochemical, morphological and physiological properties of human skeletal muscle. Our results indicate first that the model provides accurate predictions of fatigue and second that the age-related resistance to fatigue is due largely to a lower reliance on glycolytic metabolism during contraction. This model should prove useful for generating hypotheses for future experimental studies into the mechanisms of muscle fatigue.

ABSTRACT

During repeated or sustained muscle activation, force-generating capacity becomes limited in a process referred to as fatigue. Multiple factors, including motor unit activation patterns, muscle fibre contractile properties and bioenergetic function, can impact force-generating capacity and thus the potential to resist fatigue. Given that neuromuscular fatigue depends on interrelated factors, quantifying their independent effects on force-generating capacity is not possible in vivo. Computational models can provide insight into complex systems in which multiple inputs determine discrete outputs. However, few computational models to date have investigated neuromuscular fatigue by incorporating the multiple levels of neuromuscular function known to impact human in vivo function. To address this limitation, we present a computational model that predicts neural activation, biomechanical forces, intracellular metabolic perturbations and, ultimately, fatigue during repeated isometric contractions. This model was compared with metabolic and contractile responses to repeated activation using values reported in the literature. Once validated in this way, the model was modified to reflect age-related changes in neuromuscular function. Comparisons between initial and age-modified simulations indicated that the age-modified model predicted less fatigue during repeated isometric contractions, consistent with reports in the literature. Together, our simulations suggest that reduced glycolytic flux is the greatest contributor to the phenomenon of age-related fatigue resistance. In contrast, oxidative resynthesis of phosphocreatine between intermittent contractions and inherent buffering capacity had minimal impact on predicted fatigue during isometric contractions. The insights gained from these simulations cannot be achieved through traditional in vivo or in vitro experimentation alone.

Improvement of isometric dorsiflexion protocol for assessment of tibialis anterior muscle strength

It is important to accurately estimate the electromyogram (EMG)/force relationship of triceps surae (TS) muscle for detecting strength deficit of tibalis anterior (TA) muscle. In literature, the protocol for recording EMG and force of dorsiflexion have been described, and the necessity for immobilizing the ankle has been explained. However, there is a significant variability of the results among researchers even though they report the fixation of the ankle.

We have determined that toe extension can cause significant variation in the dorsiflexion force and EMG of TS and this can occur despite following the current guidelines which require immobilizing the ankle. The results also show that there was a large increase in the variability of the force and the RMS of EMG of TS when the toes were not strapped compared with when they were strapped. Thus, with the current guidelines, where there are no instructions regarding the necessity of strapping the toes, the EMG/force relationship of TS could be incorrect and give an inaccurate assessment of the dorsiflexor TA strength.

In summary,

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Current methodology to estimate the dorsiflexor TA strength with respect to the TS activity, emphasizing on ankle immobilization is insufficient to prevent large variability in the measurements.

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Toe extension during dorsiflexion was found to be one source of variability in estimating the TA strength.

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It is recommended that guidelines for recording force and EMG from TA and TS muscles should require the strapping of the toes along with the need for immobilizing the ankle.

Velocity dependence of eccentric strength in young and old men: the need for speed!

Older adults better maintain eccentric strength relative to isometric strength, as indicated by a higher ratio of eccentric:isometric torque as compared with younger adults. The effect of increasing angular velocities (>200°/s) on the age-related maintenance of eccentric strength has not been tested and thus it is unknown whether the eccentric:isometric ratio is velocity dependent in old age. The purpose of this study was to investigate eccentric strength of the ankle dorsiflexors over a large range of lengthening angular velocities in young and older men. Isometric neuromuscular properties were assessed on a HUMAC NORM dynamometer. Nine young (∼24 years) and 9 older (∼76 years) healthy men performed maximal voluntary eccentric contractions at angular velocities of 15-360°/s. Despite near full voluntary activation (>95%), the older men were ∼30% weaker than the young men for isometric strength (P < 0.05). Across all lengthening velocities, older men had a greater eccentric:isometric ratio than young men (P < 0.05). Additionally, there was a velocity dependence of strength in both young and older men: eccentric strength increased as velocity increased up to 120°/s (P < 0.05) and plateaued thereafter. In young and older men, eccentric strength at 15°/s was ∼20% and ∼40% greater than isometric strength (P < 0.05), while at 360°/s eccentric strength was ∼50% and ∼90% greater, respectively (P < 0.05). These findings indicate that with increasing angular velocity, both young and older men have considerable increases in the eccentric:isometric ratio of torque production.

Balance decrements are associated with age-related muscle property changes

In this study, a comprehensive evaluation of static and dynamic balance abilities was performed in young and older adults and regression analysis was used to test whether age-related variations in individual ankle muscle mechanical properties could explain differences in balance performance. The mechanical properties included estimates of the maximal isometric force capability, force-length, force-velocity, and series elastic properties of the dorsiflexors and individual plantarflexor muscles (gastrocnemius and soleus). As expected, the older adults performed more poorly on most balance tasks. Muscular maximal isometric force, optimal fiber length, tendon slack length, and velocity-dependent force capabilities accounted for up to 60% of the age-related variation in performance on the static and dynamic balance tests. In general, the plantarflexors had a stronger predictive role than the dorsiflexors. Plantarflexor stiffness was strongly related to general balance performance, particularly in quiet stance; but this effect did not depend on age. Together, these results suggest that age-related differences in balance performance are explained in part by alterations in muscular mechanical properties.

In vivo evidence of an age-related increase in ATP cost of contraction in the plantar flexor muscles

Impaired skeletal muscle efficiency potentially contributes to the age-related decline in exercise capacity and may explain the altered haemodynamic response to exercise in the elderly. Thus we examined whether (i) the ATP cost of contraction increases with age, and (ii) this results in altered convective O(2) delivery to maintain microvascular oxygenation in the calf muscle. To this aim, we used an integrative experimental approach combining (31)P-MRS (magnetic resonance spectroscopy), Doppler ultrasound imaging and NIRS (near-IR spectroscopy) during dynamic plantar flexion exercise at 40% of WR(max) (maximal power output) in 20 healthy young and 20 older subjects matched for physical activity. The ATP cost of contraction was significantly higher in the old (7.2±4.1 mM/min per W) compared with the young (2.4±1.9 mM/min per W; P<0.05) and this was only significantly correlated with the plantar flexion WR(max) value in the old subjects (r=-0.52; P<0.05). Even when differences in power output were taken into account, end-exercise blood flow (old, 259±168 ml/min per W and young, 134±40 ml/min per W; P<0.05) and convective O(2) delivery (old, 0.048±0.031 l/min per W and young, 0.026±0.008 l/min per W; P<0.05) were greater in the old in comparison with the young subjects. In contrast, the NIRS oxyhaemoglobin, deoxyhaemoglobin and microvascular oxygenation indices were not significantly different between the groups (P>0.05). Therefore the present study reveals that, although the peripheral haemodynamic responses to plantar flexion exercise appear to be appropriate, the elevated energy cost of contraction and associated reduction in the WR(max) value in this muscle group may play a role in limiting exercise capacity with age.

Advanced age and the mechanics of uphill walking: a joint-level, inverse dynamic analysis

We sought to gain insight into age-related muscular limitations that may restrict the uphill walking ability of old adults. We hypothesized that: (1) old adults would exhibit smaller peak ankle joint kinetics and larger peak hip joint kinetics than young adults during both level and uphill walking and (2) these age-related differences in ankle and hip joint kinetics would be greatest during uphill vs. level walking. We quantified the sagittal plane ankle, knee, and hip joint kinetics of 10 old adults (mean ± SD, age: 72 ± 5 yrs) and 8 young adults (age: 27 ± 5 yrs) walking at 1.25 m/s on a dual-belt, force-measuring treadmill at four grades (0°, +3°, +6°, +9°). As hypothesized, old adults walked with smaller peak ankle joint kinetics (e.g., power generation: -18% at +9°) and larger peak hip joint kinetics (e.g., power generation: +119% at +9°) than young adults, most evident during the late stance phase of both level and uphill conditions. Old adults performed two to three times more single support positive work than young adults via muscles crossing the knee. In partial support of our second hypothesis, the age-related reduction in peak ankle joint moments was greater during uphill (-0.41 Nm/kg) vs. level (-0.30 Nm/kg) walking. However, old adults that exhibited reduced propulsive ankle function during level walking could perform 44% more trailing leg positive ankle joint work to walk uphill. Our findings indicate that maintaining ankle power generation and trailing leg propulsive function should be the primary focus of "prehabilitation" strategies for old adults to preserve their uphill walking ability.

Human neuromuscular structure and function in old age: A brief review

Natural adult aging is associated with many functional impairments of the human neuromuscular system. One of the more observable alterations is the loss of contractile muscle mass, termed sarcopenia. The loss of muscle mass occurs primarily due to a progressive loss of viable motor units, and accompanying atrophy of remaining muscle fibers. Not only does the loss of muscle mass contribute to impaired function in old age, but alterations in fiber type and myosin heavy chain isoform expression also contribute to weaker, slower, and less powerful contracting muscles. This review will focus on motor unit loss associated with natural adult aging, age-related fatigability, and the age-related differences in strength across contractile muscle actions.

Neuromechanical properties of the triceps surae in young and older adults

The aim of this study was to compare voluntary and involuntary force generating capacity of the triceps surae muscles in healthy young and older adult participants during isometric and isokinetic contractions. Ultrasound was used to measure medial gastrocnemius (MG) fascicle length during maximal voluntary isometric contractions and supra-maximal isometric twitch contractions at five ankle angles throughout the available range of motion, as well as isokinetic concentric and eccentric contractions at four ankle velocities. Maximum voluntary activation of the plantar flexors was assessed using the twitch interpolation technique. Peak plantar flexor torque was significantly lower in older adults compared to young participants by 42%, 28% and 43% during maximal voluntary isometric contractions, supra-maximal isometric twitch and concentric contractions respectively. No age-related differences in eccentric torque production were detected. When age-related differences in triceps surae muscle volume determined from MRI were taken into account, the age-related peak plantar flexor torque deficits for maximum voluntary isometric, supra-maximal twitch, and concentric contractions were 24%, 19% and 24% respectively. These age-related differences in torque were not explained by torque-length-velocity behaviour of the MG muscle fascicles, passive plantar flexor torque-angle properties, decreased neural drive of the plantar flexor muscles or antagonistic co-activation of the tibialis anterior muscle. The residual deficit in isometric and concentric plantar flexor torques in healthy older adults may involve reduced muscle quality. A significant reduction in supra-maximal twitch torque at longer MG fascicle lengths as well as a lower MG fascicle velocity during eccentric contractions in older adults was detected, which could possibly be a function of the reported increased Achilles tendon compliance in older adults.

Increased residual force enhancement in older adults is associated with a maintenance of eccentric strength

Despite an age-related loss of voluntary isometric and concentric strength, muscle strength is well maintained during lengthening muscle actions (i.e., eccentric strength) in old age. Additionally, in younger adults during lengthening of an activated skeletal muscle, the force level observed following the stretch is greater than the isometric force at the same muscle length. This feature is termed residual force enhancement (RFE) and is believed to be a combination of active and passive components of the contractile apparatus. The purpose of this study was to provide an initial assessment of RFE in older adults and utilize aging as a muscle model to explore RFE in a system in which isometric force production is compromised, but structural mechanisms of eccentric strength are well-maintained. Therefore, we hypothesised that older adults will experience greater RFE compared with young adults. Following a reference maximal voluntary isometric contraction (MVC) of the dorsiflexors in 10 young (26.1±2.7y) and 10 old (76.0±6.5y) men, an active stretch was performed at 15°/s over a 30° ankle joint excursion ending at the same muscle length as the reference MVCs (40° of plantar flexion). Any additional torque compared with the reference MVC therefore represented RFE. In older men RFE was ∼2.5 times greater compared to young. The passive component of force enhancement contributed ∼37% and ∼20% to total force enhancement, in old and young respectively. The positive association (R² = 0.57) between maintained eccentric strength in old age and RFE indicates age-related mechanisms responsible for the maintenance of eccentric strength likely contributed to the observed elevated RFE. Additionally, as indicated by the greater passive force enhancement, these mechanisms may be related to increased muscle series elastic stiffness in old age.