This study investigated the effect of muscle pain on muscle activation strategies during dynamic exercises. Ten healthy volunteers performed cyclic elbow flexion/extension movements at maximum speed for 2 min after injection of (1) hypertonic (painful) saline in the biceps brachii, (2) hypertonic saline in both biceps brachii and triceps brachii, and (3) isotonic (nonpainful) saline in the biceps brachii muscle. Surface electromyographic (EMG) signals were collected from the upper trapezius, biceps brachii, triceps brachii, and brachioradialis muscles (to estimate EMG amplitude) and with an electrode arrays from biceps brachii (to estimate muscle fiber conduction velocity [CV]). In all conditions, the acceleration of the movement decreased throughout the exercise, and kinematic parameters were not altered by pain. With respect to the control condition, pain induced a decrease of the biceps brachii (mean +/- SE, -23+/-4%) and brachioradialis (-10+/-0.4%) integrated EMG (IEMG) in the beginning of the exercise, and an increase (45+/-3.5%) of the upper trapezius IEMG at all time points during the exercise. The biceps brachii IEMG decreased over time during the nonpainful exercises (-11+/-0.6%) while it remained constant in the painful condition. Biceps brachii CV decreased during painful conditions (-12.8+/-2.2%) while it remained constant during the nonpainful condition. In conclusion, muscle pain changes the motor control strategy to sustain the required dynamic task both in the relative contribution between synergistic muscles and in the motor unit activation within the painful muscle. Such a changed motor strategy may be highly relevant in models of occupational musculoskeletal pain conditions.

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The effects of muscle pain on movement can easily be observed in daily life routines. However, the influence of muscle pain on motor control strategies has not been fully clarified. In this human experimental study it was hypothesized that muscle pain affects the motor control of elbow flexion movements, in different combinations of range of motion and target size, by decreased agonistic muscle activity and increased antagonistic muscle activity with consequent implications on kinematic parameters. The effects of experimentally induced muscle pain on movement strategy for: (1) small and large range of motion (ROM) elbow flexion movements towards a wide target, (2) large ROM flexion movements towards a narrow and wide target, and (3) subsequent coactivation of agonistic and antagonistic muscles to elbow flexion were assessed. Muscle pain induced by injections of hypertonic saline (1 ml, 5.8%) in either m. biceps brachii or m. triceps brachii caused similar effects on the movements. For low accurate movements the initial (100 ms) integrated electromyographic (EMG) activity of m. biceps brachii was decreased during muscle pain. In contrast, integrated EMG of the entire m. biceps brachii burst was decreased by muscle pain only for small ROM at a low accuracy, which also showed decreased EMG activity of m. triceps brachii and m. brachioradialis, together with increased activity of m. trapezius. Finally, high accurate movements and post-movement coactivation were generally not modulated by muscle pain. In summary, the present study shows that acute muscle pain can perturb the motor control strategy, which might be highly important in occupational settings where such a change may need compensatory actions from other muscles and thereby eventually contribute to the development of musculoskeletal pain problems.

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This study assessed interactions between mild/moderate muscle pain and inertial load on the control of human elbow-flexion movements. It is hypothesized that high inertial load combined with moderate muscle pain intensity affect the motor control more than for low inertial-load combined with low-intensity pain. Fifteen subjects performed horizontal pointing movements (70 degree range) under three load conditions: 0, 4, and 10 kg. Pain was induced by injection of 0.5 ml and 1.5 ml hypertonic saline into the biceps muscle. Subjects scored the muscle pain intensity on a visual analogue scale (VAS). Elbow joint position, VAS, and the electromyograms (EMG, m. biceps brachii, m. triceps brachii, m. brachioradialis, and m. trapezius) were recorded. Mild and moderate muscle pain attenuated acceleration profiles [6.1(0.9)%], effective movement amplitude [3.2 (0.7)%], peak velocity [5.8 (0.9)%] and prolonged the reaction time [21 (5)%]. No interaction between muscle pain intensity and inertial load was found for the kinematic parameters. EMG profiles from m. biceps brachii, m. triceps brachii, and m. brachioradialis were similarly attenuated [10.2 (0.80)%] by mild and moderate muscle pain in all inertial load conditions. For high inertial load, the initial agonist EMG burst activity was more attenuated [50 (5.3)%] by moderate muscle pain compared with mild muscle pain [34 (4.2)%]. These data suggest that for high effort-demanding tasks muscle pain differently affects the motor planning according to the pain-intensity level. Perturbations of motor planning lead to changes on movement strategies, which might be a potential cause of musculoskeletal problems.

In numerous motor tasks, muscles around a joint act coactively to generate opposite torques. A variety of indexes based on electromyography signals have been presented in the literature to quantify muscle coactivation. However, it is not known how to estimate it reliably using such indexes. The goal of this study was to test the reliability of the estimation of muscle coactivation using electromyography. Isometric coactivation was obtained at various muscle activation levels. For this task, any coactivation measurement/index should present the maximal score (100% of coactivation). Two coactivation indexes were applied. In the first, the antagonistic muscle activity (the lower electromyographic signal between two muscles that generate opposite joint torques) is divided by the mean between the agonistic and antagonistic muscle activations. In the second, the ratio between antagonistic and agonistic muscle activation is calculated. Moreover, we computed these indexes considering different electromyographic amplitude normalization procedures. It was found that the first algorithm, with all signals normalized by their respective maximal voluntary coactivation, generates the index closest to the true value (100%), reaching 92 ± 6%. In contrast, the coactivation index value was 82 ± 12% when the second algorithm was applied and the electromyographic signal was not normalized (P < 0.04). The new finding of the present study is that muscle coactivation is more reliably estimated if the EMG signals are normalized by their respective maximal voluntary contraction obtained during maximal coactivation prior to dividing the antagonistic muscle activity by the mean between the agonistic and antagonistic muscle activations.

This study aimed to investigate the activation of lower limb muscles during barefoot and shod running with forefoot or rearfoot footfall patterns. Nine habitually shod runners were asked to run straight for 20 m at self-selected speed. Ground reaction forces and thigh and shank muscle surface electromyographic (EMG) were recorded. EMG outcomes (EMG intensity [iEMG], latency between muscle activation and ground reaction force, latency between muscle pairs and co-activation index between muscle pairs) were compared across condition (shod and barefoot), running cycle epochs (pre-strike, strike, propulsion) and footfall (rearfoot and forefoot) by ANOVA. Condition affected iEMG at pre-strike epoch. Forefoot and rearfoot strike patterns induced different EMG activation time patterns affecting co-activation index for pairs of thigh and shank muscles. All these timing changes suggest that wearing shoes or not is less important for muscle activation than the way runners strike the foot on the ground. In conclusion, the guidance for changing external forces applied on lower limbs should be pointed to the question of rearfoot or forefoot footfall patterns.

Aims: (Stroke patients often present sensory-motor alterations and less aerobic capacity. Joint position sense, which is crucial for balance and gait control, is also affected in stroke patients). To compare the effect of two exercise training protocols (walking in deep water and on a treadmill) on the knee position sense of stroke patients. Methods: This study was designed as a randomized controlled clinical trial. Twelve adults, who suffered a stroke at least one year prior to the start of the study, were randomly assigned to one of two groups: 1) pool group submitted to aerobic deep water walking training; and 2) the treadmill group which was submitted to aerobic walk on a treadmill. Measurements: The position sense, absolute error and variable error, of the knee joint was evaluated prior to and after nine weeks of aerobic training. Results: The pool group presented smaller absolute (13.9 o versus 6.1 o ; p < 0.05) and variable (9.2 o versus 3.9 o ; p < 0.05) errors after nine-weeks gait training than the treadmill group. Conclusions: Nineweek aerobic exercise intervention in aquatic environment improved precision in the position sense of the knee joint of stroke patients, suggesting a possible application in a rehabilitation program.