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PURPOSE: With aging, the decline in preferred walking speed (PWS), influenced by the increased energy cost of walking (CoW), is a key predictor of morbidity. However, the determinants associated with PWS and CoW remain poorly understood, especially after 80 years old. The aim of the study was to characterize the amplitude and mechanisms of age-related decline in CoW and PWS in old (OM) and very old (VOM) men. METHODS: Thirty-nine young men (YM, 22.1 ± 3.4 years), 34 OM (71.7 ± 4.1 years) and 23 VOM (85.8 ± 2.7 years) performed aerobic, neuromuscular, and gait assessments. Net CoW was measured on a treadmill. Physical activity (PA) was evaluated by questionnaire and accelerometry. RESULTS: Net CoW was 32% (p < 0.001), 19% (p < 0.01), 26% (p < 0.001) higher in VOM compared to OM for 1.11, 1.67 m.s-1 and PWS. Net CoW was also 27% (p < 0.001), 26% (p < 0.01), and 29% (p < 0.001) higher in OM compared to YM at these speeds. Linear regression stratified by age showed that net CoW at PWS was associated to step frequency (r = 0.79; p < 0.001) for OM and to both CV of stride mean time (r = 0.48; p < 0.05) and maximal strength of knee extensors (r = -0.54; p < 0.05) for VOM. The same analysis revealed that PWS was correlated with net CoW (r = -0.56; p < 0.05) and PA (r = 0.47; p < 0.05) in VOM. CONCLUSIONS: The progressive increase in net CoW with age was associated with gait and neuromuscular impairments, particularly after the age of 80. This increase in net CoW was related to a decrease in PWS in VOM, suggesting an adaptation of PWS to compensate for the increase in energy demand. Maintaining a high level of PA may potentially delay the age-related decline in PWS despite an age-related increase in net CoW.

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PURPOSE: This study aimed to compare performance and fatigability between young (n = 13; 18-30 yr), old (n = 13; 60-80 yr), and very old (n = 12; >80 yr) men during a single-joint isometric (ISO) and concentric (CON) task performed on an isokinetic dynamometer and a cycling (BIKE) task. METHODS: Participants randomly performed incremental tasks consisting of stages of 75 contractions (i.e., 120 s, 0.8 s on/0.8 s off) for ISO and CON and 120 s at 37.5 rpm (similar duty cycle) for BIKE. Increments were set as a percentage of body weight. Knee extensor maximal force, voluntary activation, and twitch amplitude were measured at baseline, after each stage, and at task failure (five out of eight contractions below the target force or 6 s in a row at a cadence <37.5 rpm). RESULTS: Compared with young men, performance (number of stages) was 24% and 40% lower in old and very old men in ISO, 54% and 59% lower in CON, and 36% and 60% lower in BIKE (all P < 0.05). Performance of old and very old differed only in BIKE (P < 0.01). For the last common stages performed, compared with young, force loss was greater for very old men in ISO and for old and very old men in BIKE (all P < 0.05). Overall, for the last common stage performed and task failure, old and very old men presented similar force loss, alterations in voluntary activation, and twitch amplitude. CONCLUSIONS: Our findings reveal that, with workloads relative to body weight, differences in performance between old and very old men could only be observed during BIKE (i.e., the more ecologically valid task). Results from isometric or concentric conditions might not be transferable to dynamic exercise with large muscle masses.

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PURPOSE: The effectiveness of a neuromuscular electrical stimulation (NMES) program is proportional to the level of evoked torque, which can be achieved with either conventional or wide-pulse stimulations. The aim of this study was to compare evoked torque, objective fatigability, and related peripheral and central alterations, as well as changes in central nervous system (CNS) excitability induced by an acute session of conventional versus wide-pulse NMES. METHODS: Seventeen young men underwent three 20-min NMES sessions: conventional (0.2 ms/50 Hz), wide-pulse at 50 Hz (1 ms/50 Hz), and wide-pulse at 100 Hz (1 ms/100 Hz). Neuromuscular measurements (i.e., maximal voluntary contraction, voluntary activation, evoked responses to femoral nerve stimulation, and CNS excitability) were performed on the right quadriceps femoris muscle before and after each NMES session. CNS excitability was measured using transcranial magnetic, thoracic, and transcutaneous spinal cord stimulations. RESULTS: The level of evoked torque was not significantly different between conventional and wide-pulse protocols applied at the maximal tolerable current intensity. All NMES protocols induced objective fatigability (~14% decrease in maximal voluntary contraction torque, p < 0.001) associated with peripheral (decrease in doublet torque and potentiated M-wave amplitude, p = 0.002 and p < 0.001, respectively) but not central (unchanged voluntary activation, p = 0.79) alterations. However, these acute changes did not differ between NMES protocols and none of the NMES protocols modified markers of CNS excitability. CONCLUSION: These results may allow to conjecture that chronic effects and treatment effectiveness could be comparable between conventional and wide-pulse NMES.

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The present study aimed to investigate age-related differences in fatigability induced by an isometric quadriceps intermittent fatiguing test in 30 young (<35 years old, 15 females), 19 old (>60 years old, 9 females) and 30 very old (>80 years old, 15 females) adults. Maximal force loss, contractile function and voluntary activation of the knee extensors were evaluated throughout an isometric fatiguing test using femoral nerve magnetic stimulations. Older adults performed more contractions (index of relative performance) than young (P = 0.046) and very old adults (P = 0.007), without differences between young and very old adults (P = 1.000). Force-time integral (absolute performance) was greater for young and old adults compared to very old adults (P < 0.001), without differences between young and old adults (P = 0.124). At exhaustion, force loss was greater for young (-28 ± 9%) compared to old adults (-19 ± 8%; P = 0.002), but not very old adults (-23 ± 8%; P = 0.106), and no difference between old and very old adults (P = 0.414). Data from the interpolated twitch technique suggested similar alteration in contractile function with age, with no impairments in voluntary activation. Impairments were similar across sexes. This study showed that older adults were less fatigable than young adults during an isometric intermittent fatiguing task of the knee extensors. This greater fatigue resistance was not maintained in very old adults independent of sex. Fatigability at exhaustion was likely due to impairments in contractile function for the three age groups.

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Around one third of intensive care unit (ICU) patients will develop severe neuromuscular alterations, known as intensive care unit-acquired weakness (ICUAW), during their stay. The diagnosis of ICUAW is difficult and often delayed as a result of sedation or delirium. Indeed, the clinical evaluation of both Medical Research Council score and maximal voluntary force (e.g., using handgrip and/or handheld dynamometers), two independent predictors of mortality, can be performed only in awake and cooperative patients. Transcutaneous electrical/magnetic stimulation applied over motor nerves combined with the development of dedicated ergometer have recently been introduced in ICU patients in order to propose an early and non-invasive measurement of evoked force. The aim of this narrative review is to summarize the different tools allowing bedside force evaluation in ICU patients and the related experimental protocols. We suggest that non-invasive electrical and/or magnetic evoked force measurements could be a relevant strategy to characterize muscle weakness in the early phase of ICU and diagnose ICUAW.

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