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Purpose: This study aimed to verify whether the slowing of muscle contraction quickness, typically observed in states of fatigue, may worsen force control by decreasing the rate with which force fluctuations are modulated. Therefore, we investigated the relationship between rate of force development (RFD), and force fluctuations' magnitude (Coefficient of variation, CoV) and complexity (Approximate Entropy, ApEn; Detrended fluctuation analysis, DFAα). Methods: Fourteen participants performed intermittent ballistic isometric contractions of the plantar dorsiflexors at 70% of maximal voluntary force until task failure (under 60% twice). Results: Indices of RFD (RFDpeak, RFD50, RFD100, and RFD150) decreased over time by approximately 46, 32, 44, and 39%, respectively (p all ≤ 0.007). DFAα increased by 10% (p < 0.001), and CoV increased by 15% (p < 0.001), indicating decreased force complexity along with increased force fluctuations, respectively. ApEn decreased by just over a quarter (28%, p < 0.001). The linear hierarchical models showed negative associations between RFDpeak and DFAα (β = − 3.6 10–4, p < 0.001), CoV (β = − 1.8 10–3, p < 0.001), while ApEn showed a positive association (β = 8.2 × 10–5, p < 0.001). Conclusion: The results suggest that exercise-induced reductions in contraction speed, lead to smoother force complexity and diminished force control due to slower adjustments around the target force. The fatigued state resulted in worsened force producing capacity and overall force control. [ABSTRACT FROM AUTHOR]

The neural drive to the muscle is the primary determinant of the rate of force development (RFD) in the first 50 ms of a rapid contraction. It is still unproven if repetitive rapid contractions specifically impair the net neural drive to the muscles. To isolate the fatiguing effect of contraction rapidity, 17 male adult volunteers performed 100 burst‐like (i.e., brief force pulses) isometric contractions of the knee extensors. The response to electrically‐evoked single and octet femoral nerve stimulation was measured with high‐density surface electromyography (HD‐sEMG) from the vastus lateralis and medialis muscles. Root mean square (RMS) of each channel of HD‐sEMG was normalized to the corresponding M‐wave peak‐to‐peak amplitude, while muscle fiber conduction velocity (MFCV) was normalized to M‐wave conduction velocity to compensate for changes in sarcolemma properties. Voluntary RFD 0–50 ms decreased (d = −0.56, p < 0.001) while time to peak force (d = 0.90, p < 0.001) and time to RFDpeak increased (d = 0.56, p = 0.034). Relative RMS (d = −1.10, p = 0.006) and MFCV (d = −0.53, p = 0.007) also decreased in the first 50 ms of voluntary contractions. Evoked octet RFD 0–50 ms (d = 0.60, p = 0.020), M‐wave amplitude (d = 0.77, p = 0.009) and conduction velocity (d = 1.75, p < 0.001) all increased. Neural efficacy, i.e., voluntary/octet force ratio, largely decreased (d = −1.50, p < 0.001). We isolated the fatiguing impact of contraction rapidity and found that the decrement in RFD, particularly when calculated in the first 50 ms of muscle contraction, can mainly be explained by a decrease in the net neural drive. [ABSTRACT FROM AUTHOR]

Walking and running are based on rapid burst‐like muscle contractions. Burst‐like contractions generate a Gaussian‐shaped force profile, in which neuromuscular determinants have never been assessed. We investigated the neural and contractile determinants of the rate of force development (RFD) in burst‐like isometric knee extensions. Together with maximal voluntary force (MVF), voluntary and electrically evoked (8 stimuli at 300 Hz, octets) forces were measured in the first 50, 100, and 150 ms of burst‐like quadriceps contractions in 24 adults. High‐density surface electromyography (HDsEMG) was adopted to measure the root mean square (RMS) and muscle fiber conduction velocity (MFCV) from the vastus lateralis and medialis. The determinants of voluntary force at 50, 100, and 150 ms were assessed by stepwise multiple regression analysis. Force at 50 ms was explained by RMS (R2 = 0.361); force at 100 ms was explained by octet (R2 = 0.646); force at 150 ms was explained by MVF (R2 = 0.711) and octet (R2 = 0.061). Peak RFD (which occurred at 60 ± 10 ms from contraction onset) was explained by MVF (R2 = 0.518) and by RMS50 (R2 = 0.074). MFCV did not emerge as a determinant of RFD. Muscle excitation was the sole determinant of early RFD (50 ms), while contractile characteristics were more relevant for late RFD (≥100 ms). As peak RFD is mostly determined by MVF, it may not be more informative than MVF itself. Therefore, a time‐locked analysis of RFD provides more insights into the neuromuscular characteristics of explosive contractions. [ABSTRACT FROM AUTHOR]

Physical exercise has an impact in biasing attention to positive or negative emotional stimuli. While attentional shift to emotions varies with age, evidence is lacking on the effect of prolonged endurance exercise on age-related attentional bias to emotions. This study aims at filling this knowledge gap, by applying a dot-probe task to measure attentional bias to emotions before and after a half-marathon in healthy participants of different ages (age range 21–65 years). State anxiety, positive and negative affect were also assessed. Younger adults showed attentional bias towards anger and away from sadness after the race, supporting the hypothesis of the congruency between the high-arousing task and the associated emotion (anger) in the modulation of attention. Conversely, older adults showed a bias away from anger, likely representing an attempt to maintain an optimal emotional level after the competition. This study sheds new light on how age impacts on emotional mechanisms involved in prolonged endurance exercise and suggests that regulatory processes in response to stress may be involved differently, depending on age. [ABSTRACT FROM AUTHOR]