Enhanced force production in old age is not a far stretch: an investigation of residual force enhancement and muscle architecture

Age-related differences in the loss and recovery of serial sarcomere number following disuse atrophy in rats

BACKGROUND

Older adults exhibit a slower recovery of muscle mass following disuse atrophy than young adults. At a smaller scale, muscle fibre cross-sectional area (i.e., sarcomeres in parallel) exhibits this same pattern. Less is known, however, about age-related differences in the recovery of muscle fibre length, driven by increases in serial sarcomere number (SSN), following disuse. The purpose of this study was to investigate age-related differences in SSN adaptations and muscle mechanical function during and following muscle immobilization. We hypothesized that older adult rats would experience a similar magnitude of SSN loss during immobilization, however, take longer to recover SSN than young following cast removal, which would limit the recovery of muscle mechanical function.

METHODS

We casted the plantar flexors of young (8 months) and old (32 months) male rats in a shortened position for 2 weeks, and assessed recovery during 4 weeks of voluntary ambulation. Following sacrifice, legs were fixed in formalin for measurement of soleus SSN and physiological cross-sectional area (PCSA) with the un-casted soleus acting as a control. Ultrasonographic measurements of pennation angle (PA) and muscle thickness (MT) were conducted weekly. In-vivo active and passive torque-angle relationships were constructed pre-cast, post-cast, and following 4 weeks of recovery.

RESULTS

From pre- to post-cast, young and older adult rats experienced similar decreases in SSN (-20%, P < 0.001), muscle wet weight (-25%, P < 0.001), MT (-30%), PA (-15%, P < 0.001), and maximum isometric torque (-40%, P < 0.001), but there was a greater increase in passive torque in older (+ 180%, P < 0.001) compared to young adult rats (+ 68%, P = 0.006). Following cast removal, young exhibited quicker recovery of SSN and MT than old, but SSN recovered sooner than PA and MT in both young and old. PCSA nearly recovered and active torque fully recovered in young adult rats, whereas in older adult rats these remained unrecovered at ∼ 75%.

CONCLUSIONS

This study showed that older adult rats retain a better ability to recover longitudinal compared to parallel muscle morphology following cast removal, making SSN a highly adaptable target for improving muscle function in elderly populations early on during rehabilitation.

Ultrasonographic measurements of fascicle length overestimate adaptations in serial sarcomere number

Ultrasound-derived measurements of muscle fascicle length (FL) are often used to infer increases (chronic stretch or training) or decreases (muscle disuse or aging) in serial sarcomere number (SSN). Whether FL adaptations measured via ultrasound can truly approximate SSN adaptations has not been investigated. We casted the right hindlimb of 15 male Sprague-Dawley rats in a dorsiflexed position (i.e., stretched the plantar flexors) for 2 weeks, with the left hindlimb serving as a control. Ultrasound images of the soleus, lateral gastrocnemius (LG), and medial gastrocnemius (MG) were obtained with the ankle at 90° and full dorsiflexion for both hindlimbs pre and post-cast. Following post-cast ultrasound measurements, legs were fixed in formalin with the ankle at 90°, then muscles were dissected and fascicles were teased out for measurement of sarcomere lengths via laser diffraction and calculation of SSN. Ultrasound detected an 11% increase in soleus FL, a 12% decrease in LG FL, and an 8-11% increase in MG FL for proximal fascicles and at full dorsiflexion. These adaptations were partly reflected by SSN adaptations, with a 6% greater soleus SSN in the casted leg than the un-casted leg, but no SSN differences for the gastrocnemii. Weak relationships were observed between ultrasonographic measurements of FL and measurements of FL and SSN from dissected fascicles. Our results showed that ultrasound-derived FL measurements can overestimate an increase in SSN by ∼5%. Future studies should be cautious when concluding a large magnitude of sarcomerogenesis from ultrasound-derived FL measurements, and may consider applying a correction factor. NEW FINDINGS: What is the central question of this study? Measurements of muscle fascicle length via ultrasound are often used to infer changes in serial sarcomere number, such as increases following chronic stretch or resistance training, and decreases with ageing: does ultrasound-derived fascicle length accurately depict adaptations in serial sarcomere number? What is the main finding and its importance? Ultrasound detected an ∼11% increase in soleus fascicle length, but measurements on dissected fascicles showed the actual serial sarcomere number increase was only ∼6%; therefore, measurements of ultrasound-derived fascicle length can overestimate serial sarcomere number adaptations by as much as 5%.

The history-dependent features of muscle force production: A challenge to the cross-bridge theory and their functional implications

The cross-bridge theory predicts that muscle force is determined by muscle length and the velocity of active muscle length changes. However, before the formulation of the cross-bridge theory, it had been observed that the isometric force at a given muscle length is enhanced or depressed depending on active muscle length changes before that given length is reached. These enhanced and depressed force states are termed residual force enhancement (rFE) and residual force depression (rFD), respectively, and together they are known as the history-dependent features of muscle force production. In this review, we introduce early attempts in explaining rFE and rFD before we discuss more recent research from the past 25 years which has contributed to a better understanding of the mechanisms underpinning rFE and rFD. Specifically, we discuss the increasing number of findings on rFE and rFD which challenge the cross-bridge theory and propose that the elastic element titin plays a role in explaining muscle history-dependence. Accordingly, new three-filament models of force production including titin seem to provide better insight into the mechanism of muscle contraction. Complementary to the mechanisms behind muscle history-dependence, we also show various implications for muscle history-dependence on in-vivo human muscle function such as during stretch-shortening cycles. We conclude that titin function needs to be better understood if a new three-filament muscle model which includes titin, is to be established. From an applied perspective, it remains to be elucidated how muscle history-dependence affects locomotion and motor control, and whether history-dependent features can be changed by training.

Eccentric exercise-induced muscle weakness amplifies the history dependence of force

INTRODUCTION

Following active lengthening or shortening contractions, isometric steady-state torque is increased (residual force enhancement; rFE) or decreased (residual force depression; rFD), respectively, compared to fixed-end isometric contractions at the same muscle length and level of activation. Though the mechanisms underlying this history dependence of force have been investigated extensively, little is known about the influence of exercise-induced muscle weakness on rFE and rFD.

PURPOSE

Assess rFE and rFD in the dorsiflexors at 20%, 60%, and 100% maximal voluntary torque (MVC) and activation matching, and electrically stimulated at 20% MVC, prior to, 1 h following, and 24 h following 150 maximal eccentric dorsiflexion contractions.

METHODS

Twenty-six participants (13 male, 24.7 ± 2.0y; 13 female, 22.5 ± 3.6y) were seated in a dynamometer with their right hip and knee angle set to 110° and 140°, respectively, with an ankle excursion set between 0° and 40° plantar flexion (PF). MVC torque, peak twitch torque, and prolonged low frequency force depression were used to assess eccentric exercise-induced neuromuscular impairments. History-dependent contractions consisted of a 1 s isometric (40°PF or 0°PF) phase, a 1 s shortening or lengthening phase (40°/s), and an 8 s isometric (0°PF or 40°PF) phase.

RESULTS

Following eccentric exercise; MVC torque was decreased, prolonged low frequency force depression was present, and both rFE and rFD increased for all maximal and submaximal conditions.

CONCLUSIONS

The history dependence of force during voluntary torque and activation matching, and electrically stimulated contractions is amplified following eccentric exercise. It appears that a weakened neuromuscular system amplifies the magnitude of the history-dependence of force.

The day-to-day reliability of residual force enhancement during voluntary and electrically stimulated contractions

Residual force enhancement (rFE) is characterized by increased steady-state isometric force following active muscle lengthening compared with a fixed-end isometric contraction at the same muscle length and level of neuromuscular activation. Many studies have characterized rFE in humans; however, the day-to-day reliability of rFE is unclear. We aimed to examine day-to-day reliability of rFE across various contraction types in the dorsiflexors in males and females. Twenty-five recreationally active young adults completed two visits, 1 week apart. Following determination of maximum voluntary contraction (MVC) strength, rFE was assessed during maximal voluntary effort, 20% MVC electrically stimulated, and 20% MVC torque-matching conditions. Each rFE condition was completed at two joint excursions: 0°-20° plantar flexion (PF) and 0°-40° PF. Intraclass correlation coefficients (ICC) assessed relative reliability and typical error of measurement (TEM), and the correlation variability of TEM (CV_TEM) assessed absolute reliability. Electrically stimulated contractions demonstrated the highest reliability at 40° PF (ICC: 0.9; CV_TEM: 22.8%) and 20° PF (ICC: 0.8; CV_TEM: 34.3%), followed by maximal voluntary contractions at 40° PF (ICC: 0.7; CV_TEM: 55.1%) and 20° PF (ICC: 0.1; CV_TEM: 81.1%). The torque-matching trials showed poor reliability for 20° and 40° PF (ICC: -0.1 to 0.3; CV_TEM: 118.1%-155.2%). Our results demonstrate higher reliability of rFE when stretching to the descending limb of the torque-angle relationship compared with the plateau region, and in electrically stimulated compared with voluntary contractions in the dorsiflexors for both males and females.

Residual force enhancement is affected more by quadriceps muscle length than stretch amplitude

Little is known about how muscle length affects residual force enhancement (rFE) in humans. We therefore investigated rFE at short, long, and very long muscle lengths within the human quadriceps and patellar tendon (PT) using conventional dynamometry with motion capture (rFE_TQ) and a new, non-invasive shear-wave tensiometry technique (rFE_WS). Eleven healthy male participants performed submaximal (50% max.) EMG-matched fixed-end reference and stretch-hold contractions across these muscle lengths while muscle fascicle length changes of the vastus lateralis (VL) were captured using B-mode ultrasound. We found significant rFE_TQ at long (7±5%) and very long (12±8%), but not short (2±5%) muscle lengths, whereas rFE_WS was only significant at the very long (38±27%), but not short (8±12%) or long (6±10%) muscle lengths. We also found significant relationships between VL fascicle length and rFE_TQ (r=0.63, p=0.001) and rFE_WS (r=0.52, p=0.017), but relationships were not significant between VL fascicle stretch amplitude and rFE_TQ (r=0.33, p=0.126) or rFE_WS (r=0.29, p=0.201). Squared PT shear-wave-speed-angle relationships did not agree with estimated PT force-angle relationships, which indicates that estimating PT loads from shear-wave tensiometry might be inaccurate. We conclude that increasing muscle length rather than stretch amplitude contributes more to rFE during submaximal voluntary contractions of the human quadriceps.

Residual force enhancement in human skeletal muscles: A systematic review and meta-analysis

OBJECTIVE

We reviewed and appraised the existing evidence of in vivo manifestations of residual force enhancement in human skeletal muscles and assessed, through a meta-analysis, the effect of an immediate history of eccentric contraction on the subsequent torque capacity of voluntary and electrically evoked muscle contractions.

METHODS

Our search was conducted from database inception to May 2020. Descriptive information was extracted from, and quality was assessed for, 45 studies. Meta-analyses and metaregressions were used to analyze residual torque enhancement and its dependence on the angular amplitude of the preceding eccentric contraction.

RESULTS

Procedures varied across studies with regards to muscle group tested, angular stretch amplitude, randomization of contractions, time window analyzed, and verbal command. Torque capacity in isometric (constant muscle tendon unit length and joint angle) contractions preceded by an eccentric contraction was typically greater compared to purely isometric contractions, and this effect was greater for electrically evoked muscle contractions than voluntary contractions. Residual torque enhancement differed across muscle groups for the voluntary contractions, with a significant enhancement in torque observed for the adductor pollicis, ankle dorsiflexors, ankle plantar flexors, and knee extensors, but not for the elbow and knee flexors. Meta-regressions revealed that the angular amplitude of the eccentric contraction (normalized to the respective joint's full range of motion) was not associated with the residual torque enhancement observed.

CONCLUSION

There is evidence of residual torque enhancement for most, but not all, muscle groups, and residual torque enhancement is greater for electrically evoked than for voluntary contractions. Contrary to our hypothesis, and contrary to generally accepted findings on isolated muscle preparations, residual torque enhancement in voluntary and electrically evoked contractions does not seem to depend on the angular amplitude of the preceding eccentric contraction.

The effect of stretch-shortening magnitude and muscle-tendon unit length on performance enhancement in a stretch-shortening cycle

Stretch-induced residual force enhancement (rFE) is associated with increased performance in a stretch–shortening cycle (SSC). Although the influence of different range of motions and muscle–tendon unit lengths has been investigated in pure stretch-hold experiments in vivo, the contribution to a SSC movement in human muscles remains unclear. In two sessions, 25 healthy participants performed isometric reference (ISO), shortening hold (SHO) and SSC contractions on an isokinetic dynamometer. We measured the net knee-joint torque, rotational mechanical work, knee kinematics and fascicle behavior (m. vastus lateralis) of the upper right leg. In session 1 the SHO- and SSC-magnitude was changed respectively (SHO: 50°–20°, 80°–20° and 110°–20°; SSC: 20°–50°–20°, 20°–80°–20° and 20°–110°–20°) and in session 2 the muscle–tendon unit length (SHO: 50°–20°, 80°–50° and 110°–80°; SSC: 20°–50°–20°, 50°–80°–50° and 80°–110°–80°; straight leg = 0°). In both sessions, rotational work was significantly (p < 0.05) increased in the SSC compared to the SHO contractions (in the range of 8.1–17.9%). No significant difference of joint torque was found in the steady-state for all SSC-magnitudes compared to the corresponding SHO contractions in session 1. In session 2, we found only significantly (p < 0.05) less depressed joint torque in the SSC at the longest muscle–tendon unit length compared to the corresponding SHO condition, without any differences in knee kinematics and fascicle behavior. Therefore, the physiological relevance of rFE might be particularly important for movements at greater muscle–tendon unit lengths.

Energy Cost of Force Production After a Stretch-Shortening Cycle in Skinned Muscle Fibers: Does Muscle Efficiency Increase?

Muscle force is enhanced during shortening when shortening is preceded by an active stretch. This phenomenon is known as the stretch-shortening cycle (SSC) effect. For some stretch-shortening conditions this increase in force during shortening is maintained following SSCs when compared to the force following a pure shortening contraction. It has been suggested that the residual force enhancement property of muscles, which comes into play during the stretch phase of SSCs may contribute to the force increase after SSCs. Knowing that residual force enhancement is associated with a substantial reduction in metabolic energy per unit of force, it seems reasonable to assume that the metabolic energy cost per unit of force is also reduced following a SSC. The purpose of this study was to determine the energy cost per unit of force at steady-state following SSCs and compare it to the corresponding energy cost following pure shortening contractions of identical speed and magnitude. We hypothesized that the energy cost per unit of muscle force is reduced following SSCs compared to the pure shortening contractions. For the SSC tests, rabbit psoas fibers (n = 12) were set at an average sarcomere length (SL) of 2.4 μm, activated, actively stretched to a SL of 3.2 μm, and shortened to a SL of 2.6 or 3.0 μm. For the pure shortening contractions, the same fibers were activated at a SL of 3.2 μm and actively shortened to a SL of 2.6 or 3.0 μm. The amount of ATP consumed was measured over a 40 s steady-state total isometric force following either the SSCs or the pure active shortening contractions. Fiber stiffness was determined in an additional set of 12 fibers, at steady-state for both experimental conditions. Total force, ATP consumption, and stiffness were greater following SSCs compared to the pure shortening contractions, but ATP consumption per unit of force was the same between conditions. These results suggest that the increase in total force observed following SSCs was achieved with an increase in the proportion of attached cross-bridges and titin stiffness. We conclude that muscle efficiency is not enhanced at steady-state following SSCs.

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

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

Muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions

Although high-velocity contractions elicit greater muscle fatigue in older than young adults, the cause of this difference is unclear. We examined the potential roles of resting muscle architecture and baseline contractile properties, as well as changes in voluntary activation and low-frequency fatigue in response to high-velocity knee extensor work. Vastus lateralis muscle architecture was determined in quiescent muscle by ultrasonography in 8 young (23.4±1.8 yrs) and 8 older women (69.6±1.1). Maximal voluntary dynamic (MVDC) and isometric (MVIC), and stimulated (80Hz and 10Hz, 500ms) isometric contractions were performed before and immediately after 120 MVDCs (240°.s-1, one every 2s). Architecture variables did not differ between groups (p≥0.209), but the half-time of torque relaxation (T1/2) was longer in older than young women at baseline (151.9±6.0 vs. 118.8±4.4 ms, respectively, p = 0.001). Older women fatigued more than young (to 33.6±4.7% vs. 55.2±4.2% initial torque, respectively; p = 0.004), with no evidence of voluntary activation failure (ΔMVIC:80Hz torque) in either group (p≥0.317). Low-frequency fatigue (Δ10:80Hz torque) occurred in both groups (p<0.001), as did slowing of T1/2 (p = 0.001), with no differences between groups. Baseline T1/2 was inversely associated with fatigue in older (r2 = 0.584, p = 0.045), but not young women (r2 = 0.147, p = 0.348). These results indicate that differences in muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions. The inverse association between baseline T1/2 and fatigue in older women suggests that factors related to slower muscle contractile properties may be protective against fatigue during fast, repetitive contractions in aging.

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

Purpose

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

Methods

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

Results

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

Conclusion

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

Electronic supplementary material

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

The influence of training-induced sarcomerogenesis on the history dependence of force

The increase or decrease in isometric force following active muscle lengthening or shortening, relative to a reference isometric contraction at the same muscle length and level of activation, are referred to as residual force enhancement (rFE) and residual force depression (rFD), respectively. The purpose of these experiments was to investigate the trainability of rFE and rFD on the basis of serial sarcomere number (SSN) alterations to history-dependent force properties. Maximal rFE/rFD measures from the soleus and extensor digitorum longus (EDL) of rats were compared after 4 weeks of uphill or downhill running with a no-running control. SSN adapted to the training: soleus SSN was greater with downhill compared with uphill running, while EDL demonstrated a trend towards more SSN for downhill compared with no running. In contrast, rFE and rFD did not differ across training groups for either muscle. As such, it appears that training-induced SSN adaptations do not modify rFE or rFD at the whole-muscle level.

Age-related differences in vastus lateralis fascicle behavior during fast accelerative leg-extension movements

Leg-extensor rate of power development (RPD) decreases during aging. This study aimed to identify the underlying mechanism of the age-related decline in RPD during a fast acceleration in terms of in vivo vastus lateralis (VL) fascicle shortening behavior. Thirty-nine men aged between 25 and 69 years performed three maximal isokinetic leg-extensor tests with a fixed initial acceleration of 45° knee extension in 150 ms until 340°/s knee angular velocity. RPD, VL activity, and ultrasound images were recorded to assess (relative) fascicle shortening and mean shortening velocity for the phases of electromechanical delay, pretension, and acceleration. Our findings show that fascicle shortening and mean shortening velocity during a fast action increase with aging (0.002 per year, P = .035 and 0.005 s^-1 per year, P = .097, respectively), mainly due to a higher amount of shortening in the phase of electromechanical delay. The ratio of VL fascicle length over upper leg length at rest showed a negative correlation (r = -.46, P = .004) with RPD/body mass, while pennation angle at rest showed a trend toward a positive correlation (r = .28, P = .089). To conclude, our findings indicate that the ability to reach high VL fascicle shortening velocities in vivo is not reduced in older men while performing preprogrammed fast accelerations. The greater amount of fascicle shortening in old age is probably the result of age-related differences in the tendinous properties of the muscle-tendon complex, forcing the fascicles to shorten more in order to transmit the muscle force to the segment.

The Inhibitory Tendon-Evoked Reflex Is Increased in the Torque-Enhanced State Following Active Lengthening Compared to a Purely Isometric Contraction

Residual torque enhancement (rTE) is a history-dependent property of muscle, which results in an increase in steady-state isometric torque production following an active lengthening contraction as compared to a purely isometric (ISO) contraction at the same muscle length and level of activation. Once thought to be only an intrinsic property of muscle, recent evidence during voluntary contractions indicates a neuromechanical coupling between motor neuron excitability and the contractile state of the muscle. However, the mechanism by which this occurs has yet to be elucidated. The purpose of this study was to investigate inhibition arising from tendon-mediated feedback (e.g., Golgi tendon organ; GTO) through tendon electrical stimulation (TStim) in the ISO and rTE states during activation-matching and torque-matching tasks. Fourteen male participants (22 ± 2 years) performed 10 activation-matching contractions at 40% of their maximum tibialis anterior electromyography amplitude (5 ISO/5 rTE) and 10 torque-matching contractions at 40% of their maximum dorsiflexion torque (5 ISO/5 rTE). During both tasks, 10 TStim were delivered during the isometric steady state of all contractions, and the resulting tendon-evoked inhibitory reflexes were averaged and analyzed. Reflex amplitude increased by ~23% in the rTE state compared to the ISO state for the activation-matching task, and no differences were detected for the torque-matching task. The current data indicate an important relationship between afferent feedback in the torque-enhanced state and voluntary control of submaximal contractions. The history-dependent properties of muscle is likely to alter motor neuron excitability through modifications in tension- or torque-mediated afferent feedback arising from the tendon.

Effects of High-Intensity Interval Training and Isoinertial Training on Leg Extensors Muscle Function, Structure, and Intermuscular Adipose Tissue in Older Adults

We compared the effects of aerobic high-intensity training (HIT) and isoinertial resistance training (IRT) on the strength, mass, architecture, intermuscular adipose tissue (IMAT) quality, and neuromuscular activation of the quadriceps in elderly subjects. Twelve healthy men (69.3 ± 4.2 years; 77.8 ± 10.4 kg; 1.72 ± 0.05 m) were exposed to 8 weeks of HIT (7 × 2-min cycling repetitions at 90% of V.O_2peak, 3 times/week) and, after 4 months (detraining), to IRT (4 × 7 maximal concentric–eccentric knee extensions, 3 times/week). Before and after trainings, we measured knee extension isometric (T_MVC) and dynamic (T_C) maximal concentric torque, anatomical cross-sectional area (ACSA) at 25, 50, and 75% of femur length, quadriceps volume (Vol), IMAT, pennation angle (θ_p) of the fibers from the vastus lateralis, and voluntary activation (%Act). T_MVC and T_C were significantly larger only after IRT (P = 0.008); IRT was able to elicit a greater increase of ACSA than HIT; Vol increases similarly and significantly after HIT and IRT (P = 0.003–0.001); IMAT at 50% of femur length decreased after both HIT and IRT (P = 0.001–0.003); physiological cross-sectional area (PCSA) was larger after IRT than before (P = 0.025); specific torque did not change throughout the study (45.5 N cm^–2 ± 12.0); %Act of the quadriceps was significantly affected only by IRT (P = 0.011). Both HIT and IRT are able to elicit beneficial modifications of muscular mass, architecture, and quality (reducing IMAT) in elderly subjects in connection with an amelioration of strength. HIT and IRT caused a homogeneous increase of ACSA and of Vol of the quadriceps. PCSA increases, but specific strength per unit of PCSA did not change. The increases of functional torque seemed to be attributed to a parallel increase of %Act and muscle hypertrophy only after IRT. Data suggest that IMAT may be a prominent indicator to track metabolic-dependent activity and skeletal muscle quality.

Shortening-induced residual force depression in humans

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

Modifiability of the history dependence of force through chronic eccentric and concentric biased resistance training

The increase and decrease in steady-state isometric force following active muscle lengthening and shortening are referred to as residual force enhancement (RFE) and force depression (FD), respectively. The RFE and FD states are associated with decreased (activation reduction; AR) and increased (activation increase; AI) neuromuscular activity, respectively. Although the mechanisms have been discussed over the last 60 years, no studies have systematically investigated the modifiability of RFE and FD with training. The purpose of the present study was to determine whether RFE and FD could be modulated through eccentric and concentric biased resistance training. Fifteen healthy young adult men (age: 24 ± 2 yr, weight: 77 ± 8 kg, height: 178 ± 5 cm) underwent 4 wk of isokinetic dorsiflexion training, in which one leg was trained eccentrically (-25°/s) and the other concentrically (+25°/s) over a 50° ankle excursion. Maximal and submaximal (40% maximum voluntary contraction) steady-state isometric torque and EMG values following active lengthening and shortening were compared to purely isometric values at the same joint angles and torque levels. Residual torque enhancement (rTE) decreased by ~36% after eccentric training ( P < 0.05) and increased by ~89% after concentric training ( P < 0.05), whereas residual torque depression (rTD), AR, AI, and optimal angles for torque production were not significantly altered by resistance training ( P ≥ 0.05). It appears that rTE, but not rTD, for the human ankle dorsiflexors is differentially modifiable through contraction type-dependent resistance training. NEW & NOTEWORTHY The history dependence of force production is a property of muscle unexplained by current cross bridge and sliding filament theories. Whether a muscle is actively lengthened (residual force enhancement; RFE) or shortened (force depression) to a given length, the isometric force should be equal to a purely isometric contraction-but it is not! In this study we show that eccentric training decreased RFE, whereas concentric training increased RFE and converted all nonresponders (i.e., not exhibiting RFE) into responders.

The influence of residual force enhancement on spinal and supraspinal excitability

Background

Following active muscle lengthening, there is an increase in steady-state isometric force as compared with a purely isometric contraction at the same muscle length and level of activation. This fundamental property of skeletal muscle is known as residual force enhancement (RFE). While the basic mechanisms contributing to this increase in steady-state isometric force have been well documented, changes in central nervous system (CNS) excitability for submaximal contractions during RFE are unclear. The purpose of this study was to investigate spinal and supraspinal excitability in the RFE isometric steady-state following active lengthening of the ankle dorsiflexor muscles.

Methods

A total of 11 male participants (20–28 years) performed dorsiflexions at a constant level of electromyographic activity (40% of maximum). Half of the contractions were purely isometric (8 s at an ankle angle of 130°), and the other half were during the RFE isometric steady-state following active lengthening (2 s isometric at 90°, a 1 s lengthening phase at 40°/s, and 5 s at 130°). Motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and compound muscle action potentials (M-waves) were recorded from the tibialis anterior during the purely isometric contraction and RFE isometric steady-state.

Results

Compared to the purely isometric condition, following active lengthening, there was 10% RFE (p < 0.05), with a 17% decrease in normalized CMEP amplitude (CMEP/M_max) (p < 0.05) and no change in normalized MEP amplitude (MEP/CMEP) (p > 0.05).

Discussion

These results indicate that spinal excitability is reduced during submaximal voluntary contractions in the RFE state with no change in supraspinal excitability. These findings may have further implications to everyday life offering insight into how the CNS optimizes control of skeletal muscle following submaximal active muscle lengthening.

Activation reduction following an eccentric contraction impairs torque steadiness in the isometric steady-state

BACKGROUND

The isometric steady-state following active lengthening is associated with greater torque production and lower activation, as measured by electromyographic activity (EMG), in comparison with a purely isometric contraction (ISO) at the same joint angle. This phenomenon is termed residual force enhancement (RFE). While there has been a great deal of research investigating the basic mechanisms of RFE, little work has been performed to understand the everyday relevance of RFE. The purpose of this study was to investigate whether neuromuscular control strategies differ between ISO and RFE by measuring torque steadiness of the human ankle plantar flexors.

METHODS

Following ISO maximal voluntary contractions in 12 males (25 ± 4 years), an active lengthening contraction was performed at 15°/s over a 30° ankle excursion, ending at the same joint angle as ISO (5° dorsiflexion; RFE). Surface EMG of the tibialis anterior and soleus muscles was recorded during all tasks. Torque steadiness was determined as the standard deviation (SD) and coefficient of variation (CV) of the torque trace in the ISO and RFE condition during activation-matching (20% and 60% integrated EMG) and torque-matching (20% and 60% maximal voluntary contraction) experiments. Two-tailed, paired t tests were used, within subjects, to determine the presence of RFE/activation reduction (AR) and whether there was a difference in torque steadiness between ISO and RFE conditions.

RESULTS

During the maximal and submaximal conditions, there was 5%-9% RFE with a 9%-11% AR (p < 0.05), respectively, with no difference in antagonist coactivation between RFE and ISO (p > 0.05). There were no differences in SD and CV of the torque trace for the 20% and 60% activation-matching or the 60% and maximal torque-matching trials in either the RFE or ISO condition (p > 0.05). During the 20% torque-matching trial, there were ∼37% higher values for SD and CV in the RFE as compared with the ISO condition (p < 0.05). A significant moderate-to-strong negative relationship was identified between the reduction in torque steadiness following active lengthening and the accompanying AR (p < 0.05).

CONCLUSION

It appears that while the RFE-associated AR provides some improved neuromuscular economy, this comes at the cost of increased torque fluctuations in the isometric steady-state following active lengthening during submaximal contractions.

Predictors of residual force enhancement in voluntary contractions of elbow flexors

BACKGROUND

The steady-state increase in muscle force generating potential following a lengthening contraction is called residual force enhancement (RFE). In this study, we aimed to test for differences in torque, electromyographic activity (EMG), and the associated neuromuscular efficiency (NME) between isometric voluntary contractions of elbow flexors preceded and not preceded by a lengthening contraction. The dependence of such differences on (i) stretch amplitude, (ii) the region of the force-length (FxL) relationship where contraction occurs, and (iii) the individual's ability to produce (negative) work during the stretch was investigated.

METHODS

Sixteen healthy adults participated in the study. Elbow flexor torque, angle, and biceps brachii EMG for purely isometric contractions (reference contractions) and for isometric contractions preceded by active stretches of 20° and 40° were measured at the ascending, plateau, and descending regions of subject-specific FxL curves. All contractions were performed in an isokinetic dynamometer. Two-factor (stretch × FxL region) repeated measures analysis of variance ANOVAs was used to analyze the effect of active stretch on EMG, torque, and NME across conditions. The relationships between mechanical work during stretch-calculated as the torque-angular displacement integral-and the changes in EMG, torque, and NME were analyzed using Pearson correlation.

RESULTS

In general, torque, EMG, and NME following active stretches differed from the values observed for the purely isometric reference contractions. While although the detailed effects of active stretch on torque and EMG differed between regions of the FxL relationship, NME increased by about 19% for all muscle lengths. Up to 30% of the interindividual variability in torque generating potential change in response to active stretching was accounted for by differences in (negative) work capacity between subjects.

CONCLUSION

Our results suggest that (i) RFE contributes to "flatten" the elbow flexor torque-angle relationship, favoring torque production at lengths where the purely isometric torques are reduced substantially, and (ii) RFE contributes to a reduction in energy cost of torque production during isometric contractions for the entire operating range.

The effect of muscle length on transcranial magnetic stimulation-induced relaxation rate in the plantar flexors

Transcranial magnetic stimulation (TMS) of the motor cortex during a maximal voluntary contraction (MVC) permits functionally relevant measurements of muscle group relaxation rate (i.e., when muscles are actively contracting under voluntary control). This study's purpose was twofold: (1) to explore the impact of muscle length on TMS‐induced plantar flexor relaxation rate; and (2) to incorporate ultrasonography to measure relaxation‐induced lengthening of medial gastrocnemius (MG) fascicles and displacement of the muscle–tendon junction (MTJ). Eleven males (24.8 ± 7.0 years) performed 21 brief isometric plantar flexor MVCs. Trials were block‐randomized every three MVCs among 20° dorsiflexion (DF), a neutral ankle position, and 30° plantar flexion (PF). During each MVC, TMS was delivered and ultrasound video recordings captured MG fascicles or MTJ length changes. Peak relaxation rate was calculated as the steepest slope of the TMS‐induced drop in plantar flexor torque or the rate of length change for MG fascicles and MTJ. Torque relaxation rate was slower for PF (−804 ± 162 Nm·s⁻¹) than neutral and DF (−1896 ± 298 and −2008 ± 692 Nm·s⁻¹, respectively). Similarly, MG fascicle relaxation rate was slower for PF (−2.80 ± 1.10 cm·s⁻¹) than neutral and DF (−5.35 ± 1.10 and −4.81 ± 1.87 cm·s⁻¹, respectively). MTJ displacement rate showed a similar trend (P = 0.06), with 3.89 ± 1.93 cm·s⁻¹ for PF compared to rates of 6.87 ± 1.55 and 6.36 ± 2.97 cm·s⁻¹ for neutral and DF, respectively. These findings indicate muscle length affects the torque relaxation rate recorded after TMS during an MVC. Comparable results were obtained from muscle fascicles, indicating ultrasound imaging is suitable for measuring evoked contractile properties during voluntary contraction.

Residual Force Enhancement in Humans: A Systematic Review

A systematic literature search was conducted to review the evidence of residual force enhancement (RFE) in vivo human muscle. The search, adhered to the PRISMA statement, of CINAHL, EBSCO, Embase, MEDLINE, and Scopus (inception-July 2017) was conducted. Full-text English articles that assessed at least 1 measure of RFE in vivo voluntarily contracted human skeletal muscle were selected. The methodologies of included articles were assessed against the Downs and Black checklist. Twenty-four studies were included (N = 424). Pooled Downs and Black scores ranked "fair" ([Formula: see text] [2.26]). RFE was observed in all muscles tested. Joint range of motion varied from 15° to 60°. Contraction intensities ranged from 10% to >95% maximum. Although transient force enhancement during the stretch phase may change with angular velocity, RFE in the subsequent isometric phase is independent of velocity. The magnitude of RFE was influenced by smaller stretch amplitudes and greatest at joint angles indicative of longer muscle lengths. Contraction and activation intensity influenced RFE, particularly during the initial isometric contraction phase of a poststretch isometric contraction. RFE resulted in increased torque production, reduced muscular activation, and enhanced torque production when the neuromuscular system is weakened seen in an aged population.

Influence of Joint Angle on Residual Force Enhancement in Human Plantar Flexors

Compared to pure isometric contractions, isometric muscle force at a given length is larger when the eccentric contraction is conducted before the isometric contraction. This phenomenon is widely known as residual force enhancement, and has been confirmed consistently in isolated muscle experiments. The purpose of this study was to confirm whether residual force enhancement also occurs in human plantar flexors and to examine its joint angle dependence. Eleven men participated in this study. Isometric joint torque was measured in a Control trial (pure isometric contraction) and Residual force enhancement (RFE) trial (isometric contraction after eccentric contraction) at plantar flexion 0° (Short condition) and dorsiflexion 15° (Long condition). Fascicle length and pennation angle of the medial gastrocnemius were measured simultaneously to evaluate the influence of architectural parameters on isometric joint torque. Isometric joint torque observed in the Short condition was not significantly different between the Control and RFE trials (Control: 42.9 ± 8.0 Nm, RFE: 45.1 ± 8.4 Nm) (p = 0.200). In contrast, significant differences in isometric joint torque were observed in the Long condition between Control and RFE trials (Control: 40.5 ± 9.3 Nm, RFE: 47.1 ± 10.5 Nm) (p = 0.001). Fascicle length and pennation angle were not different between Control and RFE trials in the Short and Long conditions. Isometric joint torque was larger when eccentric contraction was conducted before isometric contraction while architectural differences were not observed, indicating that residual force enhancement occurs in human plantar flexors. However, the influence of residual force enhancement may be limited in dorsiflexed positions because the magnitude of residual force enhancement is considered to be prominent in the descending limb (long muscle length condition) and small in the ascending limb (short muscle length condition) where human plantar flexors operate in plantar flexed positions.

Force Depression in Plantar Flexors Exists Equally in Plantar Flexed and Dorsiflexed Regions

Isometric muscle force attained during isometric contractions decreases after active shortening compared to that attained during purely isometric contractions. This phenomenon is called residual force depression. The aim of this study was to examine whether residual force depression occurs in human plantar flexors in both plantar flexed and dorsiflexed region. In addition, the magnitude of fascicle shortening was evaluated because not only muscle force but also fascicle shortening during active shortening are considered to affect force depression. Eleven male subjects were recruited. All muscle contractions were evoked by muscle belly-electrical stimulation. In the reference trials, isometric plantar flexion (PF) was performed at 0° and 15° of PF. In the residual force depression trials, the following two contractions were conducted: (1) muscles were activated isometrically at 15° of dorsiflexion, then actively shortened to 0° of PF (long condition) and (2) muscles were activated isometrically at 0° of PF, then actively shortened to 15° of PF (short condition). Isometric joint torque obtained 4.9 s after the onset of contraction was compared between the reference and residual force depression trials at the same joint angle to calculate the magnitude of residual force depression. At the same time point, fascicle length and pennation angle were obtained from ultrasonographic images to examine whether the muscle architecture affected residual force depression. As a result, residual force depression was confirmed in both the long and short length conditions (long: 87.1 ± 9.1%, short: 92.1 ± 7.8%) while the magnitude was not different (p = 0.182). The fascicle length and pennation angle were not different between the reference and residual force depression trials (p = 0.291–0.906). These results indicate that residual force depression occurs in the physiological range of motion in the human plantar flexors, and this phenomenon is not related to muscle architecture. In addition, joint angle dependence of the residual force depression was not observed between long and short muscle length conditions.

Effect of Preactivation on Torque Enhancement by the Stretch-Shortening Cycle in Knee Extensors

The stretch-shortening cycle is one of the most interesting topics in the field of sport sciences, because the performance of human movement is enhanced by the stretch-shortening cycle (eccentric contraction). The purpose of the present study was to examine whether the influence of preactivation on the torque enhancement by stretch-shortening cycle in knee extensors. Twelve men participated in this study. The following three conditions were conducted for knee extensors: (1) concentric contraction without preactivation (CON), (2) concentric contraction with eccentric preactivation (ECC), and (3) concentric contraction with isometric preactivation (ISO). Muscle contractions were evoked by electrical stimulation to discard the influence of neural activity. The range of motion of the knee joint was set from 80 to 140 degrees (full extension = 180 degrees). Angular velocities of the concentric and eccentric contractions were set at 180 and 90 degrees/s, respectively. In the concentric contraction phase, joint torques were recorded at 85, 95, and 105 degrees, and they were compared among the three conditions. In the early phase (85 degrees) of concentric contraction, the joint torque was larger in the ECC and ISO conditions than in the CON condition. However, these clear differences disappeared in the later phase (105 degrees) of concentric contraction. The results showed that joint torque was clearly different among the three conditions in the early phase whereas this difference disappeared in the later phase. Thus, preactivation, which is prominent in the early phase of contractions, plays an important role in torque enhancement by the stretch-shortening cycle in knee extensors.

Influence of preactivation on fascicle behavior during eccentric contraction

BACKGROUND

Because muscle fascicle behavior affects to the force-generating capability, understanding of muscle fascicle length changes during dynamic movements is important. Preactivation may affect the muscle fascicle length changes, especially in the case of eccentric contraction. However, its influence has not been clarified. To this end, muscle fascicle behavior during eccentric contraction was compared between preactivation and no-preactivation conditions.

RESULTS

Seven healthy men (24.6 ± 2.2 years, 169 ± 2 cm, 68.0 ± 5.1 kg) participated in this study. An eccentric knee extension controlled by a Biodex system was adopted as the testing motion. Muscle fascicle behavior of vastus lateralis during eccentric knee extensions was compared following two conditions. In preactivation condition, isometric preactivation was conducted before initiating eccentric contraction. On the other hand, in no-preactivation condition, muscle contraction was initiated immediately after initiating the knee angle change induced by a dynamometer. The muscle fascicle length at the onset of eccentric contraction was significantly shorter in preactivation condition than in no-preactivation condition (Cohen's d = 0.98, p < 0.001) although that at the end of eccentric contraction was not different (Cohen's d = 0.08, p = 0.844). The muscle fascicle was elongated throughout the eccentric contraction phase in preactivation condition. On the other hand, muscle fascicle was shortened in the first part, and then elongated in the latter part of the eccentric contraction phase in no-preactivation condition.

CONCLUSIONS

The muscle fascicle is shortened even during eccentric contraction phase. However, this shortening is disappeared when preactivation is conducted.

Age-related maintenance of eccentric strength: a study of temperature dependence

With adult aging, eccentric strength is maintained better than isometric strength leading to a higher ratio of eccentric/isometric force production (ECC/ISO) in older than younger adults. The purpose was to investigate the ECC/ISO during electrical activation of the adductor pollicis during lengthening (20-320° s(-1)) contractions in 24 young (n = 12, ∼24 years) and old (n = 12, ∼72 years) males across muscle temperatures (cold ∼19 °C; normal ∼30 °C; warm ∼35 °C). For isometric force, the old were 20-30 % weaker in the normal and cold conditions (P < 0.05) with no difference for the warm condition compared to young (P > 0.05). Half-relaxation time (HRT) did not differ across age for the normal and warm temperatures (P > 0.05), but it slowed significantly for old in the cold condition compared with young (∼15 %; P < 0.05), as well, there was a 20 and 40 % increase in muscle stiffness for the young and old, respectively. ECC/ISO was 50-60 % greater for the cold condition than the normal and warm conditions. There was no age difference in ECC/ISO across ages for the normal and warm conditions (P > 0.05), but for the cold, the old exhibited a 20-35 % higher ECC/ISO than did the young for velocities above 60° s(-1) (P < 0.05). A contributing factor to the elevated ECC/ISO is an increased proportion of weakly compared to strongly bound crossbridges. These findings highlight the relationship (r = 0.70) between intrinsic muscle contractile speed (HRT) and eccentric strength in old age.

Residual force enhancement following shortening is speed-dependent

The steady-state isometric force following active muscle shortening or lengthening is smaller (force depression; FD) or greater (residual force enhancement; RFE) than a purely isometric contraction at the corresponding length. The mechanisms underlying these phenomena are not explained within the context of the cross-bridge theory and are rarely studied in concert. Previous studies have shown RFE to be speed-independent. In the present study, we investigated if RFE preceded by active shortening is time-dependent by electrically evoking RFE in the human adductor pollicis muscle. The results shown that a slow stretch following FD fully re-established RFE compared to higher speeds of stretch. The mechanism(s) responsible for the recovery of RFE following a preceding shortening contraction (FD) might be associated with the recovery of cross-bridge based force and/or the re-engagement of a passive structural element (titin). Voluntary interaction with one’s environment involves highly coordinated shortening and lengthening muscle contractions. Therefore comprehending these history-dependent muscle properties in the context of movement control is paramount in understanding the behavior of in vivo motor control.

Reduction in single muscle fiber rate of force development with aging is not attenuated in world class older masters athletes

Normal adult aging is associated with impaired muscle contractile function; however, to what extent cross-bridge kinetics are altered in aging muscle is not clear. We used a slacken restretch maneuver on single muscle fiber segments biopsied from the vastus lateralis of young adults (∼23 yr), older nonathlete (NA) adults (∼80 yr), and age-matched world class masters athletes (MA; ∼80 yr) to assess the rate of force redevelopment (ktr) and cross-bridge kinetics. A post hoc analysis was performed, and only the mechanical properties of "slow type" fibers based on unloaded shortening velocity (Vo) measurements are reported. The MA and NA were ∼54 and 43% weaker, respectively, for specific force compared with young. Similarly, when force was normalized to cross-sectional area determined via the fiber shape angularity data, both old groups did not differ, and the MA and NA were ∼43 and 48% weaker, respectively, compared with young (P < 0.05). Vo for both MA and NA old groups was 62 and 46% slower, respectively, compared with young. Both MA and NA adults had approximately two times slower values for ktr compared with young. The slower Vo in both old groups relative to young, coupled with a similarly reduced ktr, suggests impaired cross-bridge kinetics are responsible for impaired single fiber contractile properties with aging. These results challenge the widely accepted resilience of slow type fibers to cellular aging.

The stretch-shortening cycle (SSC) revisited: residual force enhancement contributes to increased performance during fast SSCs of human m. adductor pollicis

The stretch-shortening cycle (SSC) occurs in most everyday movements, and is thought to provoke a performance enhancement of the musculoskeletal system. However, mechanisms of this performance enhancement remain a matter of debate. One proposed mechanism is associated with a stretch-induced increase in steady-state force, referred to as residual force enhancement (RFE). As yet, direct evidence relating RFE to increased force/work during SSCs is missing. Therefore, forces of electrically stimulated m. adductor pollicis (n = 14 subjects) were measured during and after pure stretch, pure shortening, and stretch-shortening contractions with varying shortening amplitudes. Active stretch (30°, ω = 161 ± 6°s⁻¹) caused significant RFE (16%, P < 0.01), whereas active shortening (10°, 20°, and 30°; ω = 103 ± 3°s⁻¹, 152 ± 5°s⁻¹, and 170 ± 5°s⁻¹) resulted in significant force depression (9–15%, P < 0.01). In contrast, after SSCs (that is when active stretch preceded active shortening) no force depression was found. Indeed for our specific case in which the shortening amplitude was only 1/3 of the lengthening amplitude, there was a remnant RFE (10%, P < 0.01) following the active shortening. This result indicates that the RFE generated during lengthening affected force depression when active lengthening was followed by active shortening. As conventional explanations, such as the storage and release of elastic energy, cannot explain the enhanced steady-state force after SSCs, it appears that the stretch-induced RFE is not immediately abolished during shortening and contributes to the increased force and work during SSCs.

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.

In vivo measurement of fascicle length and pennation of the human anconeus muscle at several elbow joint angles

Ultrasound imaging has facilitated the reliable measure of the architectural variables fascicle length (LF ) and pennation angle (PA), at rest and during static and dynamic contractions in many human skeletal muscles in vivo. Despite its small size and very modest contribution to elbow extension torque, the anconeus muscle has proven a useful model for the study of neuromuscular function in health and disease. Recent single motor unit (MU) studies in the anconeus have reported discrete and identifiable individual trains of MU potentials from intramuscular electromyography (EMG) recordings during dynamic elbow extensions. It is unknown whether the anconeus has unique architectural features related to alterations in LF and PA throughout the elbow joint range of motion that may help explain these high-quality recordings. Previous anatomical studies have investigated this muscle in cadavers and at mainly one elbow joint angle. The purpose of this study was to measure in vivo PA and LF of the anconeus muscle in a relaxed state at different degrees of elbow flexion using ultrasonography. Ultrasound images were collected from 10 healthy males (25 ± 3 years) at 135°, 120°, 90°, 45°, and 0° of elbow flexion. Average values of LF decreased by 6 mm (10%), 6 mm (12%), and 4 mm (9%) from 135-120°, 120-90°, and 90-45° of elbow flexion, respectively, whereas average PA values increased by 1° (9%), 1° (8%), and 2° (14%) from 135-120°, 120-90°, and 45-0°, respectively. The results indicate that anconeus muscle architecture is dynamic, undergoing moderate changes with elbow joint excursion that are similar to other limb muscles reported elsewhere. The data obtained here are more comprehensive and representative of architectural changes at various elbow joint positions than those data reported in cadaveric studies. Furthermore, the results of this study indicate that despite experiencing similar relative changes in muscle architecture to other skeletal muscles about the elbow joint, the minimal absolute changes in LF of the anconeus likely contribute to the clarity of intramuscular EMG previously reported in this muscle.

Decay of force transients following active stretch is slower in older than young men: support for a structural mechanism contributing to residual force enhancement in old age

Following active lengthening of muscle, force reaches an isometric steady state above that which would be achieved for a purely isometric contraction at the same muscle length. This fundamental property of muscle, termed "residual force enhancement (RFE)," cannot be predicted by the force-length relationship, and is unexplained by the cross-bridge theory of muscle contraction. Recently, we showed that older adults experience higher RFE than young for the ankle dorsiflexors primarily owing to a greater reliance on passive force enhancement (PFE) and similar RFE for the knee extensors but a greater contribution of PFE to total RFE. Natural adult aging may prove a useful model in exploring mechanisms of RFE which may reside in the dissipation of force transients following stretch. A post-hoc analysis was conducted on previously described RFE experiments in young (~26 years) and old (~77 years) men for the dorsiflexors and knee extensors to fit the force following stretch with a biexponential decay. In both muscle groups the decay half-life of the first exponential was two times slower in the older compared with young men. There were significant associations between PFE and the decay in force, suggesting a greater "non-active" contribution to total RFE across muscles in older compared with young men. The greater "non-active" component of RFE in older adults could be due to structural age-related changes causing increased muscle stiffness during and following stretch.

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.