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Sofia I. Lampropoulou and Alexander V. Nowicky

The way psychometric and neurophysiological measurements of fatigue are connected is not well understood. Thus, the time course of perceived effort changes due to fatigue, as well as the peripheral and central neurophysiological changes accompanying fatigue, were evaluated. Twelve healthy participants (35 ± 9 years old) undertook 10 min intermittent isometric fatiguing exercise of elbow flexors at 50% of maximum voluntary contraction (MVC). Perceived effort ratings, using the 0–10 numeric rating scale (NRS), were recorded at midrange of MVC. Single pulse TMS of the left motor cortex and electrical stimulation over the biceps muscle was used for the assessment of voluntary activation and peripheral fatigue. The fatiguing exercise caused a 44% reduction in the MVC (p < .001) accompanied by an 18% nonsignificant reduction of the biceps MEP amplitude. The resting twitch force decreased (p < .001) while the superimposed twitches increased (p < .001) causing a decrease (19%) of the voluntary activation (p < .001). The perceived effort ratings increased by 1 point at 30%, by 2 points at 50% MVC respectively on the NRS (p < .001) and were accompanied by an increase in mean biceps EMG. A substantial role of the perceived effort in the voluntary motor control system was revealed.

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Boris l. Prilutsky

The purpose of this paper is three-fold: (a) to summarize available data on coordination of major two- and one-joint muscles in multijoint tasks and identify basic features of muscle coordination, (b) to demonstrate that there may exist an optimization criterion that predicts essential features of electromyographic activity of individual muscles in a variety of tasks, and (c) to address the functional consequences of the observed muscle coordination and underlying mechanisms of its control. The analysis of the literature revealed that basic features of muscle coordination are similar among different voluntary motor tasks and reflex responses. It is demonstrated that these basic features of coordination of one- and two-joint muscles in two-dimensional tasks are qualitatively predicted by minimizing the sum of muscle stresses cubed. Functional consequences of the observed coordination of one- and two-joint muscles are (a) reduction of muscle force as well as stress, mechanical and metabolic energy expenditure, muscle fatigue, and perceived effort; (b) a spring-like behavior of a multi-joint limb during maintenance of an equilibrium posture; and (c) energy transfer between joints via two-joint muscles. A conceptual scheme of connections between motoneuron pools of one- and two-joint muscles, which accounts for the observed muscle coordination, is proposed. An important part of this scheme is the force-dependent inhibition and excitation from two-joint to one-joint synergists and antagonists, respectively.

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Boris I. Prilutsky

In this response, the major criticisms of the target article are addressed. Terminology from the target article that may have caused some confusion is clarified. In particular, the tasks that have the basic features of muscle coordination, as identified in the target article, have been limited in scope. Anew metabolic optimization criterion suggested by Alexander (2000) is examined for its ability to predict muscle coordination in walking. Issues concerning the validation of muscle force predictions, the rules of muscle coordination, and the role of directional constraints in coordination of two-joint muscles are discussed. It is shown in particular that even in one-joint systems, the forces predicted by the criterion of Crowninshield and Brand (1981) depend upon the muscle moment arms and the physiological cross-sectional areas in much more complex ways than either previously assumed in the target article, or incorrectly derived by Herzog and Ait-Haddou (2000). It is concluded that the criterion of Crowninshield and Brand qualitatively predicts the basic coordination features of the major one- and two-joint muscles in a number of highly skilled, repetitive motor tasks performed by humans under predictable conditions and little demands on stability and accuracy. A possible functional significance of such muscle coordination may be the minimization of perceived effort, muscle fatigue, and/or energy expenditure.

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Daniel Viggiani and Jack P. Callaghan

A prolonged standing exposure can identify asymptomatic adults who have a higher risk of developing clinical low back pain later in life. Hip abductor cocontraction differences can predict low back pain development during standing exposures. This study’s purpose was to determine if hip abductor strength, fatigability, and recovery during prolonged standing were related to standing-induced low back pain. Forty young, asymptomatic adults (50% female) performed two 2-hour standing sessions; a fatiguing hip abductor exercise was performed prior to 1 of the 2 standing sessions. Hip abductor strength and surface electromyography of gluteus medius and tensor fascia latae were measured. Self-reported low back pain differentiated low back pain developing (PD) and nonpain developing (NPD) groups. The PD group hip abductors fatigued before the NPD group, with similar perceived effort and force losses. Mean power frequency decreases with fatigue were similar between pain groups for all muscles measured after the fatiguing exercise. Unlike NPDs, PDs did not recover force losses after 120 minutes of standing. Hip abductor fatigability may be related to the development of low back pain in this population.

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Matt Greig

of the forward kicking motion, and greater reliance on thigh rotation during the forward kicking motion were observed. These changes suggest greater mechanical effort, or at least perceived effort, to maintain performance in the fatigued state. Kicking is a primary mechanism for muscle strain injury

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Yumeng Li, Rumit S. Kakar, Marika A. Walker, Yang-Chieh Fu, Timothy S. Oswald, Cathleen N. Brown and Kathy J. Simpson

and pelvis is the same in the static calibration. Participants performed a 5-minute jogging warm-up on the treadmill at a self-selected pace. Then, if they were an SF-AIS participant, the performer ran while the speed was increased until reaching the test speed, that is, “high” perceived effort (≥ 14

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Graeme G. Sorbie, Fergal M. Grace, Yaodong Gu, Julien S. Baker and Ukadike C. Ugbolue

, Martin BJ , Chaffin DB . Predictors of perceived effort in the shoulder during load transfer tasks . Ergonomics . 2007 ; 50 ( 7 ): 1004 – 1016 . PubMed doi: 10.1080 17510819 10.1080/00140130701295947 3. Horton J , Lindsay D , Macintosh B . Abdominal muscle activation of elite male

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Thomas J. Birk and Marianne Mossing

The purpose of this study was to determine whether heart rate and ventilation can be predicted from RPE. Also, this study determined whether breathing or heart rate sensations caused perceived effort or strain (degree of perceived central strain). Eight ambulatory active teenagers (ages 13-16 years, M= 14.75) with spastic cerebral palsy performed a discontinuous maximum bicycle ergometer protocol. Rate of perceived exertion (RPE), heart rate (HR) (V5 lead), and V̇E (Wright respirometer) were recorded each minute. Results indicated that Minutes 1 and 2 of Stage 1 were significantly correlated for RPE and ventilation, and RPE and heart rate were significantly correlated for Minutes 3 and 4 of Stages 1 and 2. Low standard error of estimates values were also evident for each of these minutes wherein significant correlations resulted. Initially, pulmonary strain was perceived as greater than HR for a given RPE value. However, by the conclusion of the second stage, breathing was slightly less than cardiac strain. Results suggest that RPE can be used to predict heart rate after the 2nd minute of discontinuous exercise training or testing. The accurate estimation of ventilation for the initial minutes of exercise may be limited since some anxiety, secondary to hyperventilation, could result. Also, breathing is perceived as a greater strain than heart rate initially but appears to equal and be slightly lower at the conclusion of aerobic exercise.