The patterns of EMG response emphasize the basis of neuromuscular fatigue and task dependency. Additionally, our data suggest that the EMG MNF should be used when monitoring the progression of local muscle fatigue.

The purpose of this study was to examine the strength and electromyographic (EMG) responses in exercised and nonexercised limbs after concentric (CON) vs. eccentric (ECC) exercise of the forearm flexors. Twenty-five men (mean ± SD age, 23.6 ± 3.8 years; height, 179.7 ± 6.6 cm; body weight, 87.4 ± 14.6 kg) performed 6 sets of 10 maximal CON isokinetic (CON exercise) or ECC isokinetic (ECC exercise) muscle actions of the dominant (DOM) forearm flexors on 2 separate randomly ordered visits. Each subject performed isometric maximal voluntary contractions (MVCs) of both the DOM and nondominant (NONDOM) forearm flexors before (PRE) and immediately after (POST) the exercise interventions. The DOM limb was the only limb exercised for both interventions. A bipolar EMG signal was detected from the biceps brachii during each MVC. The results showed that there were significant 17 and 21% decreases in maximal strength after the CON exercise and ECC exercise, respectively. When collapsed across exercise conditions, strength for the DOM and NONDOM limbs significantly decreased 36 and 4% after exercise, respectively. Accompanied with the strength losses, normalized EMG amplitude for the DOM and NONDOM limbs also reduced 21 and 7%, respectively. These findings suggested that the CON exercise and ECC exercise interventions caused similar strength losses for the exercised arm. There was also a strength loss in the contralateral nonexercised arm that was likely because of neural factors.

The purpose of this study was to examine the acute effects of concentric (CON) vs. eccentric (ECC) exercise on isometric strength, force steadiness, and surface electromyographic (EMG) responses of the forearm flexors. Fifteen resistance-trained men (mean ± SD: age = 23.7 ± 3.5 years; height = 178.9 ± 4.7 cm; body weight = 86.2 ± 9.8 kg) performed 6 sets of 10 maximal CON isokinetic or ECC isokinetic muscle actions using the dominant forearm flexors on 2 separate experimental visits. Before and immediately after the exercise interventions, isometric strength testing and submaximal trapezoid isometric contractions were performed, with bipolar EMG signals detected from the biceps brachii. The coefficient of variation of the force output from the mid 8-second portion of each submaximal trapezoid isometric contraction was calculated to assess force steadiness. In addition, the EMG signal was selected from the same portion as the force signal. The results showed that both CON and ECC caused similar isometric strength losses, but ECC caused a greater loss of force steadiness than CON. In addition, EMG amplitude increased similarly after both exercise interventions, but the magnitude of the increase in EMG mean frequency after ECC tended to be smaller, when compared with that after CON. These findings suggested that, even for resistance-trained individuals that are more resistant to ECC exercise-induced muscle damage than untrained individuals, their ability to maintain a steady submaximal force can be impaired after a bout of ECC exercise.

Background Approximately 35% of individuals > 70 years have mobility limitations. Historically, it was posited lean mass and muscle strength were major contributors to mobility limitations, but recent findings indicate lean mass and muscle strength only moderately explain mobility limitations. One likely reason is that lean mass and muscle strength do not necessarily incorporate measures globally reflective of motor function (defined as the ability to learn, or to demonstrate, the skillful and efficient assumption, maintenance, modification, and control of voluntary postures and movement patterns). In this study we determined the relative contribution of lean mass, muscle strength, and the four square step test, as an index of lower extremity motor function, in explaining between-participant variance in mobility tasks. Methods In community-dwelling older adults ( N = 89; 67% women; mean 74.9 ± 6.7 years), we quantified grip and leg extension strength, total and regional lean mass, and time to complete the four square step test. Mobility was assessed via 6-min walk gait speed, stair climb power, 5x-chair rise time, and time to complete a complex functional task. Multifactorial linear regression modeling was used to determine the relative contribution (via semi-partial r 2 ) for indices of lean mass, indices of muscle strength, and the four square step test. Results When aggregated by sex, the four square step test explained 17–34% of the variance for all mobility tasks ( p <  0.01). Muscle strength explained ~ 12% and ~ 7% of the variance in 6-min walk gait speed and 5x-chair rise time, respectively ( p <  0.02). Lean mass explained 32% and ~ 4% of the variance in stair climb power and complex functional task time, respectively (p <  0.02). When disaggregated by sex, lean mass was a stronger predictor of mobility in men. Conclusion The four square step test is uniquely associated with multiple measures of mobility in older adults, suggesting lower extremity motor function is an important factor for mobility performance. Trial registration NCT02505529 –2015/07/22.

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