Agonist Electromyographic Research Articles

Coactivation does not contribute to fatigue-induced decreases in torque during reciprocal, isokinetic muscle actions

BACKGROUND: Studies of coactivation have typically utilized single movement isometric or isokinetic fatiguing muscle actions. OBJECTIVE: The purpose of the current study was to examine coactivation of the biceps brachii (BB) and triceps brachii (TB) in response to a maximal, reciprocal, isokinetic fatiguing task of the forearm flexors and extensors at slow (60∘/s) and moderate (180∘/s) isokinetic velocities in men. METHODS: Ten men (mean ± SD: age = 21.6 ± 1.3 years) completed 50 consecutive, maximal, reciprocal, isokinetic muscle actions of the right forearm flexors and extensors at 60 and 180∘/s. The amplitude (AMP) and mean power frequency (MPF) contents of the electromyographic (EMG) and mechanomyographic (MMG) signals from the BB and TB were recorded simultaneously throughout the fatiguing task. Repeated measures ANOVAs with Tukey post hocs were used to determine mean differences for the torque and neuromuscular parameters across repetitions. RESULTS: The torque analyses indicated greater fatigability at 180∘/s, compared to 60∘/s (p= 0.02). There were no significant changes in EMG AMP for either muscle during flexion or extension at 60∘/s (p> 0.05). At 180∘/s, there were significant increases in agonist EMG AMP (p= 0.01 to 0.004), however, no changes in antagonist EMG AMP (p> 0.05). For EMG MPF, there were significant decreases during flexion and extension (p< 0.001 to p= 0.02) at both velocities, collapsed across muscle. There were no significant (p> 0.05) changes across repetition for MMG AMP or MPF. CONCLUSIONS: This study indicated velocity-specific responses to fatigue, with a greater magnitude of fatigability at 180∘/s. Furthermore, despite increases in EMG AMP of the agonist muscles at 180∘/s only, it was not sufficient to alter the ratio of coactivation, likely due to common neural drive between muscles. Thus, the decreases in torque in the present study were not attributable to increases in coactivation.

Characteristics of rectus femoris activation and rectus femoris-hamstrings coactivation during force-matching isometric knee extension in subacute stroke.

The spectral properties of surface electromyographic (EMG) signal in the rectus femoris (RF) and the coactivation in the medial hamstrings (MH) were investigated in 45 stroke subjects (22 ± 12days post-onset) and 30 age-matched healthy controls who performed unilateral knee extensions at maximum effort (100% MVC) and during 5-s force-matching tasks (10, 30, 50% MVC). The spectral properties were obtained through a power spectrum analysis based on Fast Fourier Transform. The coactivation was measured as the MH amplitude (%max) and MH/RF amplitude ratio. Force variability was expressed as the coefficient of variation. Both knee extensors and flexors were weaker in the paretic leg than the non-paretic and control legs (p < 0.001). A significantly higher relative power in the 5-13 and 13-30Hz bands was found in the paretic than the non-paretic leg across all force levels (p ≤ 0.001) without changes in the 30-60 and 60-100Hz bands or the mean and median frequencies. Regarding the antagonist coactivation, MH amplitude in the paretic leg was higher than in the non-paretic leg (submaximal levels, p < 0.0001) and the control leg (all force levels, p = 0.0005) with no differences between legs in the MH/RF ratio. The steadiness of the knee extension force was not related to the spectral properties of the agonist EMG or antagonistic coactivation. Greater coactivation was associated with weaker paretic knee flexors (p ≤ 0.0002). The overall results suggest variably altered agonist activation and antagonistic coactivation over the range of isometric knee extension contractions in subacute stroke.

The Effects of Accentuated Eccentric Loading on Mechanical Variables and Agonist Electromyography during the Bench Press.

We compared the effects of accentuated eccentric loading (AEL) on mechanical variables and agonist muscle activation using low (30% 1-repetition maximum (1RM)) and high (80% 1RM) upward-phase loading with AEL (100% 1RM during downward phase) to traditional loading schemes (T) in the bench press. Twelve resistance-trained men (26 ± 6 years; 1RM: 134 ± 33 kg) performed sets of two repetitions with three-minute intervals using loading schemes of 30AEL, 30T, 80AEL, and 80T. AEL was applied using weight releasers while force plates and a 3D motion-analysis system were used to measure mechanical variables. Electromyographic activity of the pectoralis major and triceps brachii muscles was also recorded. The greater downward-phase loads experienced during the AEL conditions allowed greater overall mean vertical forces (mean difference (Diff): 118 N, p < 0.001), greater work (Diff: 43 J, p < 0.001), and greater pectoralis major muscle activation (Diff: 27 µV, p = 0.002) compared to the corresponding traditional loading schemes. However, there was little evidence of potentiation of the mechanical variables or muscle activity during the subsequent upward phases caused by the AEL schemes. It is possible that the use of weight releasers may disrupt lifting technique particularly during low AEL schemes thereby diminishing any benefits.

Low Gain Servo Control During the Kohnstamm Phenomenon Reveals Dissociation Between Low-Level Control Mechanisms for Involuntary vs. Voluntary Arm Movements.

The Kohnstamm phenomenon is a prolonged involuntary aftercontraction following a sustained voluntary isometric muscle contraction. The control principles of the Kohnstamm have been investigated using mechanical perturbations, but previous studies could not dissociate sensorimotor responses to perturbation from effects of gravity. We induced a horizontal, gravity-independent Kohnstamm movement around the shoulder joint, and applied resistive or assistive torques of 0.5 Nm after 20° angular displacement. A No perturbation control condition was included. Further, participants made velocity-matched voluntary movements, with or without similar perturbations, yielding a 2 × 3 factorial design. Resistive perturbations produced an increase in agonist electromyography (EMG), in both Kohnstamm and voluntary movements, while assistive perturbations produced a decrease. While overall Kohnstamm EMGs were greater than voluntary EMGs, the EMG responses to perturbation, when expressed as a percentage of unperturbed EMG activity, were significantly smaller during Kohnstamm movements than during voluntary movements. The results suggest that the Kohnstamm aftercontraction involves a central drive, coupled with low-gain servo control by a negative feedback loop between afferent input and a central motor command. The combination of strong efferent drive with low reflex gain may characterize involuntary control of postural muscles. Our results question traditional accounts involving purely reflexive mechanisms of postural maintenance. They also question existing high-gain, peripheral accounts of the Kohnstamm phenomenon, as well as accounts involving a central adaptation interacting with muscle receptors via a positive force feedback loop.

History dependence of the EMG-torque relationship

The influence of active lengthening (residual force enhancement: RFE) and shortening (force depression: FD) on the electromyography (EMG)-torque relationship was investigated by matching torque and activation at 20%, 40%, 60%, 80% and 100% maximal voluntary contraction (MVC). Sixteen males performed lengthening and shortening contractions of the dorsiflexors over 25° into an isometric steady-state. There was 5% greater torque, with no change in agonist EMG during the RFE condition as compared to the isometric condition. Sub-maximally, in the force enhanced state, there was less agonist EMG during the torque clamp at all intensities relative to isometric, and greater torque during the activation clamps relative to isometric was observed across all intensities except 20% MVC. During the FD state compared to isometric, there was less torque produced during MVC (∼15%) with no change in agonist EMG. Sub-maximally, in the FD state, there was greater agonist EMG during the torque clamp and less torque during the activation clamp relative to the isometric condition across all intensities. The EMG-torque relationship was bilinear for all contraction types but was shifted to the left and right for FD and RFE, respectively as compared with isometric, indicating altered neuromuscular activation strategies in the history-dependent states of RFE and FD.

A potential mechanism by which torque output is preserved in cerebral palsy during fatiguing contractions of the knee extensors.

The purpose of this study was to compare agonist and antagonist electromyography (EMG) during an isokinetic fatigue protocol in subjects with cerebral palsy (CP) and typical development (TD). Nine individuals with CP and 11 with TD completed 50 repetitions of maximum concentric knee extensions (KE) and flexions (KF) at 60°/second. Rate of decline in peak torque for KE was significantly less in CP compared with TD. Rate of decline in agonist EMG was not significantly different between groups, but the rate of decline in antagonist EMG was significantly greater in CP. There were no differences between groups for KF. Declining agonist EMG occurred in parallel with declining antagonist hamstring activity in CP, decreasing the relative opposing force and resulting in a lesser decline in net torque. This finding illustrates a potential mechanism by which net torque is preserved in those with CP who are inherently weaker.

Task-specific neural adaptations to isoinertial resistance training.

This study aimed to delineate the contribution of adaptations in agonist, antagonist, and stabilizer muscle activation to changes in isometric and isoinertial lifting strength after short-term isoinertial resistance training (RT). Following familiarization, 45 men (23.2 ± 2.8 years) performed maximal isometric and isoinertial strength tests of the elbow flexors of their dominant arms before and after 3 weeks of isoinertial RT. During these tasks, surface electromyography (EMG) amplitude was recorded from the agonist (biceps brachii short and long heads), antagonist (triceps brachii lateral head), and stabilizer (anterior deltoid, pectoralis major) muscles and normalized to either Mmax (agonists) or to maximum EMG during relevant reference tasks (antagonist, stabilizers). After training, there was more than a twofold greater increase in training task-specific isoinertial than isometric strength (17% vs 7%). There were also task-specific adaptations in agonist EMG, with greater increases during the isoinertial than isometric strength task [analysis of variance (ANOVA), training × task, P = 0.005]. A novel finding of this study was that training increased stabilizer muscle activation during all the elbow flexion strength tasks (P < 0.001), although these were not task-specific training effects. RT elicited specific neural adaptations to the training task that appeared to explain the greater increase in isoinertial than isometric strength.

Motor 'surround inhibition' is not correlated with activity in surround muscles.

Surround inhibition (SI) is a neural process that has been extensively investigated in the sensory system and has been recently probed in the motor system. Muscle-specific modulation of corticospinal excitability at the onset of an isolated finger movement has been assumed to reflect the presence of SI in the motor system. This study attempted to characterise this phenomenon in a large cohort of normal volunteers and investigate its relationship with muscle activity in the hand. Corticospinal excitability of the pathways projecting to three hand muscles [first dorsal interosseus (FDI), abductor pollicis brevis (APB) and abductor digiti minimi (ADM)] and electromyographic (EMG) activity of the same muscles were assessed in 31 healthy volunteers during an isolated index finger movement. In the agonist FDI muscle both corticospinal excitability and EMG activity were found to be increased at the onset of the movement (P < 0.001 and P < 0.001, respectively). On the contrary, in the surround ADM, there was dissociation between the corticospinal excitability (decreased: P < 0.001) and EMG activity (increased: P < 0.001). Cross-correlation analysis of the EMG activity showed that neuronal signals driving the agonist and surround muscles are not synchronised when SI is present. The results suggest a distinctive origin of the neuronal signals driving the agonist and surround muscles. In addition, they indicate that cortical output might be simultaneously modulated by voluntary and non-voluntary activity, generated in cortical and subcortical structures, respectively.

Neuromuscular differences between boys with and without intellectual disability during squat jump

The purpose of this study was to identify the differences in vertical squat jump (SJ) between volunteers with and without intellectual disability (ID). Thirteen boys with ID (average intelligence quotient, estimated by Wisk III test: 55.6 ± 11.2) and 13 peers without disabilities performed maximal SJ on a force platform. Kinematic data were captured using a six-camera 3D motion analysis system and electromyographic (EMG) activity was recorded using surface electrodes. Unpaired T-test determined the statistical difference between the two groups. The obtained results indicated that the group with ID, jumped lower, developed lower vertical ground reaction forces, knee power output, knee angular velocity, and take-off velocity, and showed longer propulsion duration, decreased mean to maximum agonist EMG activity and higher antagonist/agonist activity ratio. The deficit in the SJ observed in individuals with ID was attributed to a deficit in the examined mechanical and neuromuscular parameters, and especially to the agonist and antagonist co-contraction.

Magnified visual feedback exacerbates positional variability in older adults due to altered modulation of the primary agonist muscle

The purpose of this study was to determine whether magnified visual feedback during position-holding contractions exacerbates the age-associated differences in motor output variability due to changes in the neural activation of the agonist muscle in the upper and lower limb. Twelve young (18-35years) and ten older adults (65-85years) were instructed to accurately match a target position at 5° of index finger abduction and ankle dorsiflexion while lifting 10% of their 1 repetition maximum (1RM) load. Position was maintained at three different visual angles (0.1°, 1°, and 4°) that varied across trials. Each trial lasted 25s and visual feedback of position was removed from 15 to 25s. Positional error was quantified as the root mean square error (RMSE) of the subject's performance from the target. Positional variability was quantified as the standard deviation of the position data. The neural activation of the first dorsal interosseus and tibialis anterior was measured with surface electromyography (EMG). Older adults were less accurate compared with young adults and the RMSE decreased significantly with an increase in visual gain. As expected, and independent of limb, older adults exhibited significantly greater positional variability compared with young adults that was exacerbated with magnification of visual feedback (1° and 4°). This increase in variability at the highest magnification of visual feedback was predicted by a decrease in power from 12 to 30Hz of the agonist EMG signal. These findings demonstrate that motor control in older adults is impaired by magnified visual feedback during positional tasks.

Muscle fatigue increases the amplitude of fluctuations of tangential forces during isometric contractions

The orientation of the resultant force during a given contraction depends on the architecture of the muscles involved, and cooperation among muscles may induce force fluctuations in multiple directions. A novel setup for recording three-dimensional forces was developed to assess the effects of muscle fatigue on submaximal contractions. Fifteen subjects performed isometric dorsiflexions, elbow flexions, knee extensions, plantarflexions, and trunk extensions (12s, 2.5–80% of maximal voluntary force) before and after a fatigue protocol. Electromyography (EMG) was recorded from relevant muscles. The standard deviation of force in the three directions and the agonist EMG amplitude increased with target force (p<.045), while the coefficient of variation of force and total excursions of the center of pressure showed maximal values at low target forces, reaching a plateau at moderate forces. Fatigue induced higher force fluctuations in task-related and tangential directions and increased agonist EMG amplitude for all muscle groups (p<.04). Fluctuations of tangential forces were greater during knee extensions compared with other muscle groups (p<.007). The present data demonstrate that fatigue increases the amplitude of force fluctuations in task-related and tangential forces. Moreover, alternation of activity between multiple synergist muscles is associated with increased fluctuations of tangential forces, especially during fatigue.

The role of co-activation in strength and force modulation in the elbow of children with unilateral cerebral palsy

To study the role of coactivation in strength and force modulation in the elbow joint of children and adolescents with cerebral palsy (CP), we investigated the affected and contralateral arm of 21 persons (age 8–18) with spastic unilateral CP in three tasks: maximal voluntary isokinetic concentric contraction and passive isokinetic movement during elbow flexion and extension, and sub-maximal isometric force tracing during elbow flexion. Elbow flexion–extension torque and surface electromyography (EMG) of the biceps brachii (BB) and triceps brachii (TB) muscles were recorded. During the maximal contractions, the affected arm was weaker, had decreased agonist and similar antagonist EMG amplitudes, and thus increased antagonist co-activation (% of maximal activity as agonist) during both elbow flexion and extension, with higher coactivation levels of the TB than the BB. During passive elbow extension, the BB of the affected arm showed increased resistance torque and indication of reflex, and thus spastic, activity. No difference between the two arms was found in the ability to modulate force, despite increased TB coactivation in the affected arm. The results indicate that coactivation plays a minor role in muscle weakness in CP, and does not limit force modulation. Moreover, spasticity seems particularly to increase coactivation in the muscle antagonistic to the spastic one, possibly in order to increase stability.

Effects of Intensive Physical Rehabilitation on Neuromuscular Adaptations in Adults with Poststroke Hemiparesis

Hemiparesis-disability and muscle weakness of 1 side of the body-is a common consequence of stroke. High-intensity strength training may be beneficial to regain function, but strength coaches in the field of rehabilitation need evidence-based guidelines. The purpose of this study was to evaluate the effect of intensive physical rehabilitation on neuromuscular and functional adaptations in outpatients suffering from hemiparesis after stroke. A within-subject repeated-measures design with the paretic leg as the experimental leg and the nonparetic leg as the control leg was used. Eleven outpatients with hemiparesis after stroke participated in 12 weeks of intensive physical rehabilitation comprising unilateral high-intensity strength training with near-maximal loads (4-12 repetition maximum) and body weight supported treadmill training. At baseline and 12-week follow-up, the patients went through testing consisting of isokinetic muscle strength, neuromuscular activation measured with electromyography (EMG), electrically evoked muscle twitch contractile properties, and gait performance (10-m Walk Test and 6-min Walk Test). After the 12-week conditioning program, knee extensor and flexor strength increased during all contraction modes and velocities in the paretic leg. Significant increases were observed for agonist EMG amplitude at slow concentric and slow eccentric contraction. Twitch torque increased, whereas twitch time-to-peak tension remained unchanged. By contrast, no significant changes were observed in the nonparetic control leg. Gait performance increased 52-68%. In conclusion, intensive physical rehabilitation after stroke leads to clinically relevant neuromuscular improvements, leading to increased voluntary strength during a wide range of contraction modes and velocities, and improved gait velocity. Strength training coaches working in the field of rehabilitation can use this knowledge to safely and efficiently add high-intensity strength training to existing rehabilitation paradigms.

Short‐term unilateral resistance training affects the agonist–antagonist but not the force–agonist activation relationship

In this study we investigated the contribution of neural adaptations to strength changes after 4 weeks of unilateral isometric resistance training. Maximal and submaximal isometric knee extension contractions were assessed before and after training. Surface electromyography (EMG) data were collected from the agonist and antagonist muscles and normalized to evoked maximal M-wave and maximal knee flexor EMG, respectively. The interpolated twitch technique (ITT) was also used to determine activation at maximum voluntary force (MVF). MVF increased in the trained (+20%) and untrained (+8%) legs. Agonist EMG at MVF increased in the trained leg (+26%), although activation determined via the ITT was unchanged. In both legs the position of the force-agonist EMG relationship was unchanged, but antagonist coactivation was lower for all levels of agonist activation. Strength gains in the trained leg were due to enhanced agonist activation, whereas decreased coactivation may have affected strength changes in both legs.

Muscle Coactivation Before and After the Impact Phase of Running Following Isokinetic Fatigue

The effects of fatigue on impact loading during running are unclear, with some authors reporting increased impact forces and others reporting decreased forces. To examine the effects of isokinetic fatigue on muscle cocontraction ratios about the knee and ankle during running. Cross-sectional study. Neuromechanics laboratory. Female middle-distance runners (age = 21.3 ± 1.93 years) with at least 5 years of training experience. Participants ran on the treadmill at 3.61 m/s before and immediately after the fatigue protocol, which consisted of consecutive, concentric knee extension-flexion at 120°/s until they could no longer produce 30% of the maximum knee-extension moment achieved in the familiarization session for 3 consecutive repetitions. Electromyographic (EMG) amplitude of the vastus medialis (VM), biceps femoris (BF), gastrocnemius (GAS), and tibialis anterior (TA) was recorded using surface electrodes. Agonist∶antagonist EMG ratios for the knee (VM∶BF) and ankle (GAS∶TA) were calculated for the preactivation (PR), initial loading response (LR(1)), and late loading response (LR(2)) phases of running. Hip-, knee-, and ankle-joint angular displacements at initial foot contact were obtained from 3-dimensional kinematic tracings. Fatigue did not alter the VM∶BF EMG ratio during the PR phase (P > .05), but it increased the ratio during the LR(1) phase (P < .05). The GAS∶TA EMG ratio increased during the LR(1) phase after fatigue (P < .05) but remained unchanged during the PR and LR(2) phrases (P > .05). The increased agonist EMG activation, coupled with reduced antagonist EMG activation after impact, indicates that the acute decrease in muscle strength capacity of the knee extensors and flexors results in altered muscle-activation patterns about the knee and ankle before and after foot impact.

Multiple EMG Activity and Intracortical Inhibition and Facilitation During a Fine Finger Movement Under Pressure

ABSTRACT The 1st purpose of this study was to examine multiple electromyography (EMG) during voluntary hand movements. A secondary purpose was to investigate possible effects of pressure on intracortical inhibition (ICI) and intracortical facilitation (ICF) functions of the motor cortex, using paired-pulse transcranial magnetic stimulation. Twelve participants traced a 15-cm diameter target circle using a small laser pointer attached to the right index finger. After 5 acquisition trials, they performed 3 nonpressure trials followed by 3 pressure trials. The results showed that pressure had effects not only on agonist EMG activity but also on multiple muscles, such as synergist. In addition, a decrease in ICI and an increase in ICF were both observed under pressure for muscles other than the agonist.

Muscle activity differs with load compliance during fatiguing contractions with the knee extensor muscles

The purpose of this study was to determine the influence of load compliance on the time to failure and rate of change in electromyographic (EMG) activity when the knee extensor muscles performed fatiguing contractions against submaximal loads. The low-compliance condition required the subject to exert a force against a rigid restraint (force control), whereas the high-compliance condition involved maintaining the knee joint angle while supporting an equivalent inertial load (position control). Both contractions were sustained for as long as possible. Each subject exerted a similar net torque about the knee joint during the force and position tasks; the target force corresponded to a force at the ankle equal to 20% of the maximal voluntary contraction (MVC) force. Thirteen healthy adults (25 +/- 7 year) participated in the study. MVC forces before the force and position tasks were similar (189 +/- 40 N vs. 179 +/- 43 N, P = 0.4), and the target force was 36 +/- 8 N. The time to task failure was longer for the force task (224 +/- 114 s) than for the position task (110 +/- 36 s, P < 0.05), but MVC force declined to a similar level immediately after task failure for the two tasks (-31 +/- 16%). The briefer time to failure for the position task was accompanied by greater rates of increase in agonist EMG amplitude and the pressor response. Coactivation ratios, in contrast, were similar for the two tasks and did not contribute to task differences in time to failure. These findings indicate that it was more difficult to sustain a submaximal contraction with the knee extensor muscles when the task required position control, despite comparable net muscle torques for the low- and high-compliance tasks.

Force variability during isometric wrist flexion in highly skilled and sedentary individuals

The association of expertness in specific motor activities with a higher ability to sustain a constant application of force, regardless of muscle length, has been hypothesized. Ten highly skilled (HS group) young tennis and handball athletes and 10 sedentary (S group) individuals performed maximal and submaximal (5, 10, 20, 50, and 75% of the MVC) isometric wrist flexions on an isokinetic dynamometer (Kin-Com, Chattanooga). The wrist joint was fixed at five different angles (230, 210, 180, 150, and 1300). For each position the percentages of the maximal isometric force were calculated and participants were asked to maintain the respective force level for 5 s. Electromyographic (EMG) activation of the Flexor Carpi Ulnaris and Extensor Digitorum muscles was recorded using bipolar surface electrodes. No significant differences were observed in maximal isometric strength between HS and S groups. Participants of HS group showed significantly (P < 0.05) smaller force coefficient of variability (CV) and SD values at all submaximal levels of MVC at all wrist angles. The CV and SD values remained unaltered regardless of wrist angle. No difference in normalized agonist and antagonist EMG activity was observed between the two groups. It is concluded that long-term practice could be associated with decreased isometric force variability independently from muscular length and coactivation of the antagonist muscles.

Effects of visual gain on force control at the elbow and ankle

Visual feedback is essential when minimizing force fluctuations. Varying degrees of somatotopic organization have been shown in different regions of the brain for the upper and lower extremities, and visual feedback may be processed differently based on the body effector where feedback-based corrections are used. This study compared the effect of changes in visual gain on the control of steady-state force at the elbow and ankle. Ten subjects produced steady-state isometric force to targets at 5 and 40% of their maximum voluntary contraction at seven visual gain levels. Visual gain was used effectively at both joints to reduce variability of the force signal and to improve accuracy, with a greater effect of visual gain at the elbow than the ankle. Visual gain significantly decreased the regularity of force output, and this effect was more pronounced at the elbow than the ankle. There were accompanying changes in the proportion of power in the 0-4, 4-8, and 8-12 Hz frequency bins of the force signal across visual gain at the elbow. Changes in visual gain were accompanied by changes in both agonist and antagonist electromyographic (EMG) activation at the elbow. At the ankle joint, there were only changes in agonist EMG. The results suggest better use of visual information in the control of elbow force than ankle force and this improved control may be related to the changes in the pattern of agonist and antagonist activation.

Effects of Different Accentuated Eccentric Load Levels in Eccentric-Concentric Actions on Acute Neuromuscular, Maximal Force, and Power Responses

This study examined the effects of different dynamic accentuated external resistance load levels during the eccentric(ECC) phase of ECC-concentric (CON) actions on acute neuromuscular, maximal force, and power responses in the bench press exercise in male subjects (age, = 32 +/- 4 years; n = 11). Four maximum strength sessions consisted of 1 repetition maximum (RM) lifts with traditional isoinertial resistances and of 1RM lifts with the different dynamic accentuated external resistance (DAER) loads of 100, 105, 110, and 120% of 1 RM for the ECC phase, whereas 100% of 1RM was constantly used for the CON phase. One explosive strength session consisted of explosive repetitions with the 50, 60, 70, 80, and 90% of 1RM loads for the ECC phase, whereas 50% of 1RM was constantly used for the CON phase. Force, power, and electromyography (EMG) activation of agonist deltoid, pectoralis major, triceps brachii, and antagonist biceps brachii muscles were recorded. In all maximum strength DAER sessions (105/100%, 110/100%, and 120/100%), CON 1RM and CON force reduced (p < 0.05) compared with the control condition (100/100%). ECC muscle activity did not differ in the DAER loading conditions compared with the control loading condition. In the explosive strength session, peak power increased significantly from the 50/50% condition compared to the condition of 77.3 +/- 3.2/50% (p < 0.001) that produced the highest power for each individual. ECC agonist EMG activity increased significantly from the 50% condition to that of the 77.3 +/- 3.2/50% condition (p < 0.05). The present data showed that the different loads used in the DAER bench press action did not enhance maximum concentric strength production, but concentric power output during explosive actions with the individualized optimal load was larger compared with the control condition.