Douglas A Keen

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• Keen, D. A., Konhilas, J., & Constantopoulos, E. (2015). The Impact of Post-exercise Hydration on Rehydration and Exercise Performance. Journal of internation sprorts nutrition.
• Keen, D., Fuglevand, A. J., Dutoit, A. P., Johns, R. K., & Keen, D. A. (2006). Evaluation of plateau-potential-mediated 'warm up' in human motor units. The Journal of physiology, 571(Pt 3).
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  Spinal motor neurones can exhibit sustained depolarization in the absence of maintained synaptic or injected current. This phenomenon, referred to as a plateau potential, is due to the activation of monoamine-dependent persistent inward currents. Accordingly, activation of a plateau potential should result in a decrease in the excitatory synaptic drive required to activate a motor unit. This, in turn, has been suggested to cause a progressive decline in the muscle force at which motor units are recruited during repeated voluntary contractions. Such a progressive decrease in threshold force associated with preceding activation of a plateau potential is referred to as 'warm up'. Furthermore, activation of a plateau potential is thought to manifest itself as a decrease in the derecruitment force compared to recruitment force. Multiple muscles, however, can contribute to the detected force and their relative contributions may vary over time, which could confound measures of recruitment and derecruitment force. Therefore, the purpose of this study was to compare the recruitment and derecruitment forces of single motor units in the human extensor digitorum and tibialis anterior during repetitive triangular-force contractions in which the contributions of other muscles had been minimized. In both muscles, we found that the recruitment thresholds of single motor units were unchanged during repeated contractions, and that the derecruitment force was consistently greater than the recruitment force. These results suggest either that plateau potentials were not engaged (or were rapidly extinguished) under these experimental conditions or that changes in recruitment and derecruitment force are not suitable criteria for detecting them.
• Keen, D., Fuglevand, A. J., & Keen, D. A. (2003). Re-evaluation of muscle wisdom in the human adductor pollicis using physiological rates of stimulation. The Journal of physiology, 549(Pt 3).
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  Motor unit discharge rates decline by about 50 % over 60 s of a sustained maximum voluntary contraction (MVC). It has been suggested that this decline in discharge rate serves to maintain force by protecting against conduction failure and by optimizing the input to motor units as their contractile properties change. This hypothesis, known as muscle wisdom, is based in part on studies in which muscle force was shown to decline more rapidly when stimulation was maintained at a high rate than when stimulus rate was reduced over time. The stimulus rates used in those studies, however, were higher than those normally encountered during MVCs. The purpose of this study was to compare force loss under constant and declining stimulus rate conditions using rates similar to those that occur during voluntary effort. Isometric force and surface EMG signals were recorded from human adductor pollicis muscles in response to supramaximal stimuli delivered to the ulnar nerve at the elbow. Three fatigue protocols, each 60 s in duration, were carried out on separate days on each of 10 subjects: (1) continuous stimulation at 30 Hz, (2) stimulation at progressively decreasing rates from 30 to 15 Hz and (3) sustained MVC. The relative force-time integral (endurance index) was significantly smaller for the sustained MVC (0.75 +/- 0.08) and decreasing stimulus rate conditions (0.76 +/- 0.16) compared to the condition in which stimulus rate was maintained at 30 Hz (0.90 +/- 0.13). These findings suggest that decreases in discharge rate may contribute to force decline during a sustained MVC.