Does quadriceps neuromuscular activation capability explain walking speed in older men and women?

Comparison of strategies for assessment of rate of torque development in older and younger adults

There is increasing appreciation of the role of rate of torque development (RTD) in physical function of older adults (OAs). This study compared various RTD strategies and electromyography (EMG) in the knee extensors and focused on discriminating groups with potential limitations in voluntary activation (VA) and associations of different RTD indices with functional tests that may be affected by VA in OAs. Neuromuscular function was assessed in 20 younger adults (YAs, 22.0 ± 1.7 years) and 50 OAs (74.4 ± 7.0 years). Isometric ballistic and peak torque during maximal voluntary contractions (pkTMVC), doublet stimulation and surface EMG were assessed and used to calculate VA during pkTMVC and RTD and rate of EMG rise during ballistic contractions. Select mobility tests (e.g., gait speed, 5× chair rise) were also assessed in the OAs. Voluntary RTD and RTD normalized to pkTMVC, doublet torque, and peak doublet RTD were compared. Rate of EMG rise and voluntary RTD normalized to pkTMVC did not differ between OAs and YAs, nor were they associated with functional test scores. Voluntary RTD indices normalized to stimulated torque parameters were significantly associated with VA (r = 0.319-0.459), and both indices were significantly lower in OAs vs YAs (all p < 0.020). These RTD indices showed significant association with the majority of mobility tests, but there was no clear advantage among them. Thus, voluntary RTD normalized to pkTMVC was ill-suited for use in OAs, while results suggests that voluntary RTD normalized to stimulated torque parameters may be useful for identifying central mechanisms of RTD impairment in OAs.Clinical trial registration number NCT02505529; date of registration 07/22/2015.

Reductions in Motor Unit Firing are Associated with Clinically Meaningful Leg Extensor Weakness in Older Adults

Weakness, one of the key characteristics of sarcopenia, is a significant risk factor for functional limitations and disability in older adults. It has long been suspected that reductions in motor unit firing rates (MUFRs) are one of the mechanistic causes of age-related weakness. However, prior work has not investigated the extent to which MUFR is associated with clinically meaningful weakness in older adults. Forty-three community-dwelling older adults (mean: 75.4 ± 7.4 years; 46.5% female) and 24 young adults (mean: 22.0 ± 1.8 years; 58.3% female) performed torque matching tasks at varying submaximal intensities with their non-dominant leg extensors. Decomposed surface electromyographic recordings were used to quantify MUFRs from the vastus lateralis muscle. Computational modeling was subsequently used to independently predict how slowed MUFRs would negatively impact strength in older adults. Bivariate correlations between MUFRs and indices of lean mass, voluntary activation, and physical function/mobility were also assessed in older adults. Weak older adults (n = 14) exhibited an approximate 1.5 and 3 Hz reduction in MUFR relative to non-weak older adults (n = 29) at 50% and 80% MVC, respectively. Older adults also exhibited an approximate 3 Hz reduction in MUFR relative to young adults at 80% MVC only. Our model predicted that a 3 Hz reduction in MUFR results in a strength decrement of 11-26%. Additionally, significant correlations were found between slower MUFRs and poorer neuromuscular quality, voluntary activation, chair rise time performance, and stair climb power (r's = 0.31 to 0.43). These findings provide evidence that slowed MUFRs are mechanistically linked with clinically meaningful leg extensor weakness in older adults.

Age-related changes in gait biomechanics and their impact on the metabolic cost of walking: Report from a National Institute on Aging workshop

Changes in old age that contribute to the complex issue of an increased metabolic cost of walking (mass-specific energy cost per unit distance traveled) in older adults appear to center at least in part on changes in gait biomechanics. However, age-related changes in energy metabolism, neuromuscular function and connective tissue properties also likely contribute to this problem, of which the consequences are poor mobility and increased risk of inactivity-related disease and disability. The U.S. National Institute on Aging convened a workshop in September 2021 with an interdisciplinary group of scientists to address the gaps in research related to the mechanisms and consequences of changes in mobility in old age. The goal of the workshop was to identify promising ways to move the field forward toward improving gait performance, decreasing energy cost, and enhancing mobility for older adults. This report summarizes the workshop and brings multidisciplinary insight into the known and potential causes and consequences of age-related changes in gait biomechanics. We highlight how gait mechanics and energy cost change with aging, the potential neuromuscular mechanisms and role of connective tissue in these changes, and cutting-edge interventions and technologies that may be used to measure and improve gait and mobility in older adults. Key gaps in the literature that warrant targeted research in the future are identified and discussed.

Determining the cortical, spinal and muscular adaptations to strength-training in older adults: A systematic review and meta-analysis

There are observable decreases in muscle strength as a result of ageing that occur from the age of 40, which are thought to occur as a result of changes within the neuromuscular system. Strength-training in older adults is a suitable intervention that may counteract the age-related loss in force production. The neuromuscular adaptations (i.e., cortical, spinal and muscular) to strength-training in older adults are largely equivocal and a systematic review with meta-analysis will serve to clarify the present circumstances regarding the benefits of strength-training in older adults. 20 studies entered the meta-analysis and were analysed using a random-effects model. A best evidence synthesis that included 36 studies was performed for variables that had insufficient data for meta-analysis. One study entered both. There was strong evidence that strength-training increases maximal force production, rate of force development and muscle activation in older adults. There was limited evidence for strength-training to improve voluntary-activation, the volitional-wave and spinal excitability, but strong evidence for increased muscle mass. The findings suggest that strength-training performed between 2 and 12 weeks increases strength, rate of force development and muscle activation, which likely improves motoneurone excitability by increased motor unit recruitment and improved discharge rates.

Lower limb muscle quality and phase angle contribute to the reduced walking speed among older adults

AIM

This cross-sectional study aimed to determine the predictors of a walking speed <1 m/s. Factors associated with the health of muscle cells, including the phase angle analyzed using bioimpedance analysis, answers to the mini-nutritional assessment-short form, albumin levels, leg muscle quality and vitamin D levels, were assessed.

METHODS

The study participants were outpatients from the Locomotor Frailty Outpatient Clinic which was established at our center in 2016 (231 women, 76.5 ± 7.6 years; 137 men, 78.0 ± 6.3 years). Participants were classified into two groups, with 1 m/s walking speed as the cutoff value. Binominal logistic regression analysis was performed with walking speed as the dependent variable, and age, leg muscle quality, mini-nutritional assessment-short form answers, albumin levels, vitamin D levels and the phase angle as explanatory variables. Leg muscle quality was defined as the mean value of both lower limbs' isometric knee extensor strength, divided by the mean value of both lower limbs' muscle mass through bioimpedance analysis.

RESULTS

The explanatory variables associated with decreased walking speed among women were leg muscle quality (P = 0.004, odds ratio: 0.57) and phase angle (P = 0.017, odds ratio: 0.42). Men were also more likely to have leg muscle quality (P = 0.004, odds ratio: 0.43).

CONCLUSIONS

Leg muscle quality and the phase angle of lower extremities were independently associated with low walking speeds (<1 m/s) among older outpatients. Geriatr Gerontol Int 2022; ••: ••-••.

Serum levels of C-terminal telopeptide (CTX) are associated with muscle function in community-dwelling older adults

BACKGROUND

Markers of bone metabolism have been associated with muscle mass and function. Whether serum cross-linked C-terminal telopeptides of type I collagen (CTX) is also associated with these measures in older adults remains unknown.

METHODS

In community-dwelling older adults at high risk of falls and fractures, serum CTX (biochemical immunoassays) was used as the exposure, while appendicular lean mass (dual-energy x-ray absorptiometry) and muscle function (grip strength [hydraulic dynamometer], short physical performance battery [SPPB], gait speed, sit to stand, balance, Timed Up and Go [TUG]) were used as outcomes. Potential covariates including demographic, lifestyle and clinical factors were considered in statistical models. Areas under the ROC curves were calculated for significant outcomes.

RESULTS

299 older adults (median age: 79 years, IQR: 73, 84; 75.6% women) were included. In multivariable models, CTX was negatively associated with SPPB (β = 0.95, 95% CI: 0.92, 0.98) and balance (β = 0.92, 0.86, 0.99) scores, and positively associated with sit to stand (β = 1.02, 95% CI: 1.00, 1.05) and TUG (β = 1.03, 95% CI: 1.00, 1.05). Trend line for gait speed (β = 0.99, 95% CI: 0.98, 1.01) was in the hypothesized direction but did not reach significance. AUC curves showed low diagnostic power (<0.7) of CTX in identifying poor muscle function (SPPB: 0.63; sit to stand: 0.64; TUG: 0.61).

CONCLUSION

In older adults, higher CTX levels were associated with poorer lower-limb muscle function (but showed poor diagnostic power for these measures). These clinical data build on the biomedical link between bone and muscle.

Does skeletal muscle morphology or functional performance better explain variance in fast gait speed in older adults?

BACKGROUND

Fast gait speed is being increasingly recognized as an important clinical tool in older adults. However, the underlying muscular and functional contributors to fast gait speed performance remain poorly understood.

AIM

We sought to determine predictors of fast gait speed in older adults. We hypothesized that lower-extremity skeletal muscle size and quality would be strong predictors.

METHODS

Ninety community-dwelling older adults (33 men, 57 women; mean ± SD age = 74 ± 6 years) participated. B-mode ultrasonography was used to capture images of the vastus lateralis, rectus femoris, and gastrocnemius in the transverse plane. Each participant performed 30-second chair stand, heel-rise, functional reach, and grip strength tests. Fast gait speed was measured using the NIH Toolbox 4-Meter Walk Test. ImageJ software was used to quantify cross-sectional area (CSA), subcutaneous tissue thickness, and echo intensity. Two separate stepwise regression analyses were performed, one using muscle morphology variables as independent variables, and another including the functional outcomes.

RESULTS

The ultrasound variables exhibited weak-to-moderate correlations with fast gait speed (|r| range = 0.168-0.416). The initial regression analysis indicated that the combination of medial gastrocnemius CSA and subcutaneous tissue thickness explained 22.8% of the variance in fast gait speed. The secondary analysis indicated that 30-second chair stand, heel-rise, and grip strength performance explained 45.5% of the variance.

CONCLUSION

While medial gastrocnemius morphology is important, measures of upper and lower-extremity muscle function are better predictors of fast gait speed. These results highlight a dissociation between skeletal muscle morphology and fast gait speed.

Relationships between hamstring morphological characteristics and postural balance in elderly men

OBJECTIVES

The link between hamstring morphology and postural balance performance in older adults is not well understood. This study aimed to examine the relationships between hamstring morphological characteristics of muscle size (cross-sectional area [CSA]) and quality (echo intensity [EI]) and postural balance with the eyes open and closed in elderly men.

METHODS

Nineteen healthy elderly men (age= 73±4 years) participated in this study. Muscle CSA and EI were determined from ultrasound scans of the hamstrings. Postural balance was assessed with the eyes open and closed using a commercially designed balance testing device, which provides a measurement of static stability based on the sway index.

RESULTS

The sway index with eyes closed was significantly related to muscle EI (r=0.474; P=0.040) but not CSA (r=0.021; P=0.932). The sway index with eyes open was not related to muscle CSA (r= -0.036; P=0.883) or EI (r=-0.079; P=0.747).

CONCLUSIONS

The significant relationship observed between the sway index with eyes closed and muscle EI suggests that hamstring muscle quality may be a characteristic relevant to postural balance in the absence of visual feedback. These findings may provide important insight regarding the morphological mechanisms involved in maintaining balance and in the development of proper training programs aimed at improving postural stability in older individuals.

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.

Regression analysis of gait parameters and mobility measures in a healthy cohort for subject-specific normative values

Background

Deviation in gait performance from normative data of healthy cohorts is used to quantify gait ability. However, normative data is influenced by anthropometry and such differences among subjects impede accurate assessment. De-correlation of anthropometry from gait parameters and mobility measures is therefore desirable.

Methods

87 (42 male) healthy subjects varying form 21 to 84 years of age were assessed on gait parameters (cadence, ankle velocity, stride time, stride length) and mobility measures (the 3-meter/7-meter Timed Up-and-Go, 10-meter Walk Test). Multiple linear regression models were derived for each gait parameter and mobility measure, with anthropometric measurements (age, height, body mass, gender) and self-selected walking speed as independent variables. The resulting models were used to normalize the gait parameters and mobility measures. The normalization’s capability in de-correlating data and reducing data dispersion were evaluated.

Results

Gait parameters were predominantly influenced by height and walking speed, while mobility measures were affected by age and walking speed. Normalization de-correlated data from anthropometric measurements from |r_s| < 0.74 to |r_s| < 0.23, and reduced data dispersion by up to 69%.

Conclusion

Normalization of gait parameters and mobility measures through linear regression models augment the capability to compare subjects with varying anthropometric measurements.

Effects of Three Types of Exercise Interventions on Healthy Old Adults' Gait Speed: A Systematic Review and Meta-Analysis

BACKGROUND

Habitual walking speed predicts many clinical conditions later in life, but it declines with age. However, which particular exercise intervention can minimize the age-related gait speed loss is unclear.

PURPOSE

Our objective was to determine the effects of strength, power, coordination, and multimodal exercise training on healthy old adults' habitual and fast gait speed.

METHODS

We performed a computerized systematic literature search in PubMed and Web of Knowledge from January 1984 up to December 2014. Search terms included 'Resistance training', 'power training', 'coordination training', 'multimodal training', and 'gait speed (outcome term). Inclusion criteria were articles available in full text, publication period over past 30 years, human species, journal articles, clinical trials, randomized controlled trials, English as publication language, and subject age ≥65 years. The methodological quality of all eligible intervention studies was assessed using the Physiotherapy Evidence Database (PEDro) scale. We computed weighted average standardized mean differences of the intervention-induced adaptations in gait speed using a random-effects model and tested for overall and individual intervention effects relative to no-exercise controls.

RESULTS

A total of 42 studies (mean PEDro score of 5.0 ± 1.2) were included in the analyses (2495 healthy old adults; age 74.2 years [64.4-82.7]; body mass 69.9 ± 4.9 kg, height 1.64 ± 0.05 m, body mass index 26.4 ± 1.9 kg/m2, and gait speed 1.22 ± 0.18 m/s). The search identified only one power training study, therefore the subsequent analyses focused only on the effects of resistance, coordination, and multimodal training on gait speed. The three types of intervention improved gait speed in the three experimental groups combined (n = 1297) by 0.10 m/s (±0.12) or 8.4% (±9.7), with a large effect size (ES) of 0.84. Resistance (24 studies; n = 613; 0.11 m/s; 9.3%; ES: 0.84), coordination (eight studies, n = 198; 0.09 m/s; 7.6%; ES: 0.76), and multimodal training (19 studies; n = 486; 0.09 m/s; 8.4%, ES: 0.86) increased gait speed statistically and similarly.

CONCLUSIONS

Commonly used exercise interventions can functionally and clinically increase habitual and fast gait speed and help slow the loss of gait speed or delay its onset.

Weaker Seniors Exhibit Motor Cortex Hypoexcitability and Impairments in Voluntary Activation

BACKGROUND

Weakness predisposes seniors to a fourfold increase in functional limitations. The potential for age-related degradation in nervous system function to contribute to weakness and physical disability has garnered much interest of late. In this study, we tested the hypothesis that weaker seniors have impairments in voluntary (neural) activation and increased indices of GABAergic inhibition of the motor cortex, assessed using transcranial magnetic stimulation.

METHODS

Young adults (N = 46; 21.2±0.5 years) and seniors (N = 42; 70.7±0.9 years) had their wrist flexion strength quantified along with voluntary activation capacity (by comparing voluntary and electrically evoked forces). Single-pulse transcranial magnetic stimulation was used to measure motor-evoked potential amplitude and silent period duration during isometric contractions at 15% and 30% of maximum strength. Paired-pulse transcranial magnetic stimulation was used to measure intracortical facilitation and short-interval and long-interval intracortical inhibition. The primary analysis compared seniors to young adults. The secondary analysis compared stronger seniors (top two tertiles) to weaker seniors (bottom tertile) based on strength relative to body weight.

RESULTS

The most novel findings were that weaker seniors exhibited: (i) a 20% deficit in voluntary activation; (ii) ~20% smaller motor-evoked potentials during the 30% contraction task; and (iii) nearly twofold higher levels of long-interval intracortical inhibition under resting conditions.

CONCLUSIONS

These findings indicate that weaker seniors exhibit significant impairments in voluntary activation, and that this impairment may be mechanistically associated with increased GABAergic inhibition of the motor cortex.