Comparison Between Isometric and Concentric Motor Fatigability in Persons With Multiple Sclerosis and Healthy Controls - exploring central and peripheral contributions of motor fatigability.

BACKGROUND: Motor fatigability (i.e. contraction-induced reduction in muscle strength) from a concentric task associate stronger to walking and perception of fatigue in persons with multiple sclerosis (pwMS), compared with an isometric task. However, the central and peripheral contributions of motor fatigability between these tasks have not been investigated. OBJECTIVE: Compare the central and peripheral contributions of motor fatigability in the knee extensors in a sustained isometric fatigability protocol versus a concentric fatigability protocol and in pwMS versus healthy controls (HCs). METHODS: Participants (n=31 pwMS; n=15 HCs) underwent neuromuscular testing before and immediately after two knee extensor fatigability tasks (sustained isometric and concentric) in an isokinetic dynamometer. Neuromuscular testing of fatigability consisted of maximal voluntary contraction, voluntary activation (central/neural contributor), and resting twitch (peripheral/muscular contributor) determined by the interpolated twitch technique. RESULTS: Sustained isometric and concentric fatigability protocols resulted in motor fatigability for both pwMS and HCs, with no between-protocols differences for either group. Regression analysis showed that motor fatigability variance in pwMS was mainly attributed to central fatigability in the sustained isometric protocol and to both central and peripheral fatigability in the concentric protocol. In HCs, the variance in sustained isometric and concentric fatigability were attributed to both peripheral and central fatigability. CONCLUSION: Central and peripheral contributions of motor fatigability differed between sustained isometric and concentric protocols as well as between pwMS and HCs. These between-protocol differences in pwMS provide a neuromuscular dimension to the reported difference in the strength of associations of concentric and isometric tasks to walking and perception of fatigue in pwMS.

Unprompted vigorous physical activity is associated with higher levels of subsequent sedentary behaviour in participants with low cardiorespiratory fitness: a cross-sectional study.

Performing physical activity is considered health promoting but may induce a need for subsequent rest periods. This study aimed to determine the within-day interactions between vigorous physical activity (VPA) and sedentary behaviour (SB) in participants with low cardiorespiratory fitness. We tested the hypothesis that VPA is associated with a temporary subsequent increase in SB. One week of accelerometer data containing a minimum of one 10-min bout of VPA from 62 participants with low cardiorespiratory fitness (31-50 years old) were obtained from the MILE study. A comparison of SB was made between days with a bout of VPA and days without (control). Due to a positive association between VPA and number and duration of sedentary bouts, the time accumulated in both uninterrupted and total sedentary bouts were 27 (95% CI, 10-45) min and 29 (95% CI, 9-50) min higher on VPA days compared to control days (P < 0.05). Our results indicate that in participants with low cardiovascular fitness, unprompted VPA is positively associated with an increase in subsequent sedentary time. We propose that such VPA-associated sedentary time may be viewed as part of a healthy activity pattern.

Neural drive increases following resistance training in patients with multiple sclerosis.

The present study tested the hypothesis that lower body progressive resistance training (PRT) increases the neural drive expressed as surface electromyographical (EMG) activity in patients with multiple sclerosis (MS). The study was a randomised controlled trial (RCT) including a 12-week follow up period. Thirty-eight MS patients were randomized to an exercise group (n = 19) or a control group (n = 19). During the intervention period, the exercise group performed a 12-week supervised lower body PRT program, whereas the control group maintained their usual daily activity level. After the 12 week intervention period, the exercise group were encouraged to continue training on their own for a 12-week follow up period, while the control group completed the 12-week supervised PRT program. Surface EMG was recorded from vastus lateralis, rectus femoris and semitendinosus during maximal isometric knee extension and knee flexion, before and after the intervention and at follow up. From the recordings, the area under the rectified, low-pass filtered EMG signal (integrated EMG, iEMG) was calculated. Muscle strength was expressed as the angular impulse (AI) and was measured during the same period as the iEMG. After PRT significant improvements for iEMG of vastus lateralis and rectus femoris during maximal knee extension and for semitendinosus during maximal knee flexion and for AI during both maximal knee extension and flexion were found in the exercise group, when compared to the control group. When compared to the post values, all effects, except for AI during knee flexion, were maintained at follow up in the exercise group. When the control group was exposed to PRT, a similar pattern of improvements were found, albeit not all improvements were significant. In conclusion twelve weeks of intense PRT of the lower extremities improved the neural drive expressed as maximal surface EMG activity in patients with MS, with effects persisting 12 weeks after the intervention. The study was registered at clinicalTrials.gov, Protocol no. NCT00381576.

Evaluation of the postural stability of elderly persons using time domain signal analysis.

A force platform is widely used in the evaluation of postural stability in man. Although an abundance of parameters are typically retrieved from force platform data, no uniform analysis of the data has been carried out. In general, the signal analysis does not analyze the underlying postural event, i.e., whether the signal consists of several small corrections or large excursions. In the present work, we studied the postural stability of 4589 elderly persons from Iceland on a force platform under visual and non-visual conditions during stance on a solid surface. We analyzed the internal relationship between frequently used time domain variables. In addition, we conducted a factor analysis using a subset of selected variables. Factor analysis yielded three components that can be considered different strategies for maintaining posture. In one control strategy, long swaying amplitues were pronounced, allowing the person to react when postural confidence limits are reached. In a second strategy, a high oscillation rate about the stationary point was the dominant characteristic of maintaining postural control. The third strategy appears to involve a short critical time period during which an open loop control changes into a closed loop that very rapidly controls excessive postural oscillations. The findings suggest that conventional parameters such as swaying velocity and amplitude alone do not provide sufficient information regarding a person's ability to maintain an upright stance.