Normalized maximal rate of torque development during voluntary and stimulated static contraction in human tibialis anterior: Influence of age.

The risk of falling in older adults has been related, among other factors, to the reduction of the rate of torque development (RTD) with age. It is well known that both structural/peripheral and neural factors can influence the RTD. The purpose of this study was to compare the normalized RTD in young and older participants obtained during a) rapid voluntary tension production and b) neuromuscular electrical stimulation. The tibialis anterior of 19 young subjects (10 males and 9 females; age 21-33 years old) and 19 older participants (10 males and 9 females; age 65-80 years old) was studied. The subjects performed a series of maximal isometric explosive dorsiflexions and underwent trains of supra-maximal electrical stimulations (35 Hz) on the tibialis anterior motor point. Muscle shortening was indirectly measured using a laser (surface mechanomyogram, MMG). Both torque and MMG were normalized to their maximum value. Using a 20 ms sliding window on the normalized torque signal, the normalized maximum RTD was calculated for both voluntary and stimulated contractions. Active stiffness of the muscle- tendon unit was calculated as the area of the normalized torque with respect to the normalized MMG. Normalized maximum RTD was found significantly lower in older adults during voluntary activity (young: 751.9 ± 216.3%/s and old: 513.9 ± 173.9%/s; P < .001), and higher during stimulated contractions (young: 753.1 ± 225.9%/s and old: 890.1 ± 221.3%/s; P = .009). Interestingly, active stiffness was also higher in older adults (young: 3524.6 ± 984.6‰ and old 4144.6 ± 816.6‰; P = .041) and significantly correlated to the normalized maximum RTD during stimulated contractions. This dichotomy suggests that modifications in the structural/peripheral muscle properties are not sufficient to counteract the age-related decrease in neural drive to the muscle during voluntary isometric contractions in aged participants.

Influence of age on motor control accuracy during static ramp contractions.

We investigated the influence of the ageing process on the performance of the motor control system accuracy during a challenging motor task throughout the analysis of force output oscillations. The force signal of the first dorsal interosseous during linearly varying static contraction, 0-100-0% of the maximal volitional abduction in 15 s, was studied in 11 young and older adults. The relative error between the target and the actual force as well as several parameters of the force oscillations (corrections) were estimated. To understand the experimental results, we analyzed the force output generated by a set of computational simulations of a pool of motor units controlled by a proportional-integral-derivative system. Compared to young adults the older subjects presented larger errors and a lower number of corrections with longer duration and larger relative amplitude. The motor control system modelling varied the error update frequency (UF) of the controller (from 1 to 2.5 Hz) as well as the range of contraction time (CT) of the recruited motor unit (30-90 ms and 60-120 ms reflecting young and old ranges, respectively). The simulation generated force profiles with parameters similar to experimental recordings in young (UF = 1.5; CT 30-90 ms) and older (UF = 1; CT 60-120 ms) adults. Interestingly, the results of the simulations suggested that the improvement in the error update frequency of the controller was not able to compensate for the contractile changes in the motor unit twitches. In conclusion, the peripheral contractile changes with age can influence motor unit control strategies and represent a crucial phenomenon in the generation of larger force oscillations in older adults.

Extra-torque of human tibialis anterior during electrical stimulation with linearly varying frequency and amplitude trains.

This work aimed to characterise the whole human muscle input/output law during electrical stimulation with triangular varying frequency and amplitude trains through combined analysis of torque, mechanomyogram (MMG) and electromyogram (EMG). The tibialis anterior (TA) of ten subjects (age 23-35 years) was investigated during static contraction obtained through neuromuscular electrical stimulation. After potentiation, TA underwent two 15s stimulation patterns: (a) frequency triangle (FT): 2 > 35 > 2 Hz at Vmax (amplitude providing full motor unit recruitment); (b) amplitude triangle (AT): Vmin > Vmax > Vmin (Vmin providing TA least mechanical response) at 35 Hz. 2 > 35 Hz or Vmin > Vmax as well as 35 > 2 Hz or Vmax > Vmin were defined as up-going ramp (UGR) and down-going ramp (DGR), respectively. TA torque, MMG and EMG were detected by a load cell, an optical laser distance sensor and a probe with two silver bar electrodes, respectively. For both FT and AT, only the two mechanical signals resulted always larger in DGR than in UGR, during AT extra-torque and extra-MMG were present even in the first 1/3 of the amplitude range where EMG data presented no significant differences between DGR and UGR. Our data suggest that extra-torque and extra-displacement are evident for both FT and AT, being mainly attributed to an intrinsic muscle property.

Energy metabolism during activity-promoting video games practice in subjects with spinal cord injury: evidences for health promotion.

BACKGROUND: Activity promoting video game (APVG) practice significantly affects energy metabolism through energy expenditure (EE) increase and has been recently included in strategies for health promotion. It is not known if the APVG practice provides similar outcomes in subjects with spinal cord injury (SCI). AIM: Aim of the study was to evaluate cardio-pulmonary and metabolic adaptations during APVG practice and to find whether EE increase above resting condition could suggest the inclusion of this exercise in a more general strategy for health promotion and body weight control in subjects with SCI. DESIGN: Repeated measures study. SETTING: Rehabilitation Institute. POPULATION: Ten male subjects with SCI (lesion levels from C7 to L1) age 26 to 55 years. METHODS: We recorded pulmonary ventilation (VE), oxygen consumption (VO2) for EE esteem and heart rate (HR) at rest and while playing virtual bowling, tennis and boxing games using a portable metabolimeter equipped with ECG electrodes. The standard metabolic equivalent of task (METs) was calculated offline. The metabolic and functional parameters were referred to the 10th minute of each activity. RESULTS: Metabolic and functional parameters increased significantly from rest to bowling, tennis and boxing. METs exceeded in average 3 during boxing. One hour of APVG can increase daily EE by about 6% (bowling), 10% (tennis) and 15% (boxing). CONCLUSION: These considerable results suggest that physical exertion during APVG practice in subjects with SCI could contribute to health promotion as well as caloric balance control, especially when boxing is considered. This can be safely achieved at home with regular activity. CLINICAL REHABILITATION IMPACT: These findings substantiate the potential for novel exercise modalities to counteract deconditioning due to inactivity in subjects with SCI by promoting physical activity through implementation of APVG exercise programs.

Transcutaneous neuromuscular electrical stimulation: influence of electrode positioning and stimulus amplitude settings on muscle response.

The aim of the study was to investigate the influence of two different transcutaneous neuromuscular electrical stimulation procedures on evoked muscle torque and local tissue oxygenation. In the first one (MP mode), the cathode was facing the muscle main motor point and stimulus amplitude was set to the level eliciting the maximal myoelectrical activation according to the amplitude of the evoked electromyogram (EMG); in the second one (RC mode), the electrodes were positioned following common reference charts for electrode placement while stimulus amplitude was set according to subject tolerance. Tibialis Anterior (TA) and Vastus Lateralis (VL) muscles of 10 subjects (28.4 ± 8.2 years) were tested in specific dynamometers to measure the evoked isometric torque. The EMG and near-infrared spectroscopy probes were placed on muscle belly to detect the electrical activity and local metabolic modifications of the stimulated muscle, respectively. The stimulation protocol consisted of a gradually increasing frequency ramp from 2 to 50 Hz in 7.5 s. Compared to RC mode, in MP mode the contractile parameters (peak twitch, tetanic torque, area under the torque build-up) and the metabolic solicitation (oxygen consumption and hyperemia due to metabolites accumulation) resulted significantly higher for both TA and VL muscles. MP mode resulted also to be more comfortable for the subjects. Based on the assumption that proper mechanical and metabolic stimuli are necessary to induce muscle strengthening, our results witness the importance of an optimized, i.e., comfortable and effective, stimulation to promote the aforementioned muscle adaptive modifications.

The use of REE determination in a clinical setting applied to respiratory disease.

The determination of a patient's nutritional condition is an important moment of clinical evaluation in everyday experience. Quite often, it is important to identify symptoms, signs and objective parameters that may reveal a malnourished condition. To satisfy this request, clinicians recur to clinical examination and biochemical tests. It is also possible to use indirect calorimetry. Through the evaluation of oxygen consumption (VO2) and carbon dioxide production (VCO2), indirect calorimetry permits the respiratory quotient (RQ) to be calculated and the exact energy consumption during resting conditions to be determined. This technology, currently used in research departments and in hospital units, is more accurate for the estimation of resting energy expenditure (REE) than the Harris-Benedict equation, especially in the case of patients affected by respiratory diseases or other systemic conditions. The evaluation of REE through indirect calorimetry is a useful monitoring and prognostic index in acutely and chronically ill patients.