Bilateral Exertion Research Articles

Neuromuscular Characteristics of Unilateral and Bilateral Maximal Voluntary Isometric Contractions following ACL Reconstruction.

Despite the advancement of diagnostic surgical techniques in anterior cruciate ligament (ACL) reconstruction and rehabilitation protocols following ACL injury, only half of the athletes return to sports at a competitive level. A major concern is neuromechanical dysfunction, which occurs with injuries persisting in operated and non-operated legs following ACL rehabilitation. One of the criteria for a safe return to sports participation is based on the maximal voluntary isometric contraction (MVIC) performed unilaterally and a comparison between the 'healthy knee' and the 'operated knee'. The present study aimed to investigate MVIC in athletes following ACL rehabilitation during open kinetic chain exercise performed unilaterally and bilateral exercises. Twenty subjects participated in the present investigation: 10 male athletes of regional-national level (skiers, rugby, soccer, and volleyball players) who were previously operated on one knee and received a complete rehabilitation protocol (for 6-9 months) were included in the ACL group (age: 23.4 ± 2.11 years; stature: 182.0 ± 9.9 cm; body mass: 78.6 ± 9.9 kg; body mass index: 23.7 ± 1.9 kg/m2), and 10 healthy male athletes formed the control group (CG: age: 24.0 ± 3.4 years; stature: 180.3 ± 10.7 cm; body mass: 74.9 ± 13.5 kg; body mass index: 22.8 ± 2.7 kg/m2). MVICs synchronised with electromyographic (EMG) activity (recorded on the vastus lateralis, vastus medialis, and biceps femoris muscles) were performed during unilateral and bilateral exertions. The rate of force development (RFD) and co-activation index (CI) were also calculated. The differences in the MVIC and RFD between the two legs within each group were not significant (p > 0.05). Vastus lateralis EMG activity during MVIC and biceps femoris EMG activity during RFD were significantly higher in the operated leg than those in the non-operated leg when exertion was performed bilaterally (p < 0.05). The CI was higher in the operated leg than that in the non-operated leg when exertion was performed bilaterally (p < 0.05). Vice versa, vastus medialis EMG activity during RFD was significantly higher in the right leg than that in the left leg when exertion was performed bilaterally (p < 0.05) in the CG. MVICs performed bilaterally represent a reliability modality for highlighting neuromechanical asymmetries. This bilateral exercise should be included in the criteria for a safe return to sports following ACL reconstruction.

Female maximal push/pull strength capabilities by humeral abduction angle in bilateral exertions

Pushing or pulling an object constitutes the majority of manual materials handling tasks. Anthropometric differences between workers alter pushing and pulling strategies at fixed heights, potentially modifying exposures and causing overexertion. Capability normalized to arm posture rather than work height remains unknown, and the purpose of this research was to quantify maximal pushing and pulling strength and upper extremity joint moments using fixed humeral abduction angles and a flexed arm position. Twenty university-aged females completed maximal pushes and pulls at 0°, 30°, 60°, 90°, and 120° of humeral abduction with an elbow to 90°. Abduction angle was the largest modifier of both push and pull force generating capability (p < 0.01), with increasing abduction reducing force capability by up to 30%. While push exertions on average exceeded pulls by 17 N (p < 0.01), the difference is much smaller than previous reports. Ergonomists should consider humeral angle of the worker rather than work heights, as individuals working in increased abduction decreases capability and increases potential overexertion injury risk.

Altered force perception in stroke survivors with spastic hemiplegia.

To investigate the effect of spasticity and involuntary synergistic activation on force perception during voluntary activation of spastic paretic muscles. Eleven stroke subjects with spastic hemiparesis performed various isometric elbow-flexion force-matching tasks. Subjects were instructed to generate a target reference force with visual feedback using one arm (impaired or non-impaired) and then to produce a force with the other arm to match the magnitude of the reference force without visual feedback. The reference arm was at rest in unilateral exertion trials and maintained contraction in bilateral exertion trials during the matching force-production period. Both force and effort mismatches occurred in most conditions, and there were asymmetries in force perception. When the non-impaired arm was the matching arm, effort and force overestimation occurred, but effort was matched better than force. When the impaired arm was the matching arm, force underestimation and effort overestimation occurred, but force was matched better than effort. No difference in matching performance was found between unilateral and bilateral exertion-matching tasks. Overall, both force and effort misperceptions occur in stroke survivors with spasticity. Spasticity and spastic synergistic activation probably contribute to force and effort misperception during voluntary activation in chronic stroke.

Bilateral motor tasks involve more brain regions and higher neural activation than unilateral tasks: an fMRI study.

Movements that involve simultaneous coordination of muscles of the right and left lower limbs form a large part of our daily activities (e.g., standing, rising from a chair). This study used functional magnetic resonance imaging to determine which brain areas are used to control coordinated lower-limb movements, specifically comparing regions that are activated during bilateral exertions to those performed unilaterally. Plantarflexor exertions were produced at a target force level of 15% of the participants' maximum voluntary contraction, in three conditions, with their right (dominant) foot, with their left foot, and with both feet simultaneously. A voxel-wise analysis determined which regions were active in the bilateral, but not in the unilateral conditions. In addition, a region of interest (ROI) approach was used to determine differences in the percent signal change (PSC) between the conditions within motor areas. The voxel-wise analysis showed a large number of regions (cortical, subcortical, and cerebellar) that were active during the bilateral condition, but not during either unilateral condition. The ROI analysis showed several motor regions with higher activation in the bilateral condition than unilateral conditions; further, the magnitude of bilateral PSC was more than the sum of the two unilateral conditions in several of these regions. We postulate that the greater levels of activation during bilateral exertions may arise from interhemispheric inhibition, as well as from the greater need for motor coordination (e.g., synchronizing the two limbs to activate together) and visual processing (e.g., monitoring of two visual stimuli).

STRUCTURE OF DIFFERENT INDICATORS FOR EVALUATING ISOMETRIC LEG EXTENSORS EXPLOSIVE FORCE IN TOP LEVEL ATHLETES

SUMMARY The aim of this work was to establish factore structure of different indicators force-time curve for evaluating the leg extensors explosiveness regarding different sports and both genders. The research included 378 examinees divided into 8 groups based on gender and training process distinctiveness they have been subjected to. To evaluate contractile characteristics of leg extensors, standardized equipment was used and standardized “seating leg press” test. The isometric force-time characteristics of the leg extensors were evaluated using the 15 variables during unilateral (dominant and nondominant leg) and bilateral exertions measured at 100 and 50% of the maximal force and at 100, 180 and 250 ms from the beginning of the muscle contraction. The results obtained in this study show that the measured characteristics of the leg extensors explosive force obtained in the bi and unilateral exertions, in regard to the various sports groups, have different structures as a function separate sets of factors influenced by different mechanisms than on training in various sports disciplines.

Effect of bilateral versus unilateral exertion tests on maximum voluntary activity and within-participant reproducibility in the shoulder

Normalized surface EMG requires known muscle activity from a well defined reference contraction, often a maximum voluntary isometric exertion (MVIE). One factor that may impact the activity obtained during a MVIE is the use of a bilateral or unilateral version of the test. The objective of this work was to investigate how performing MVIE tests unilaterally or bilaterally affects maximum voluntary electrical activity (MVE) for selected shoulder muscles. Further, we examined if within-participant reproducibility could be improved with the use of bilateral or unilateral MVIE tests. Sixteen participants completed five repetitions of a palm press, empty can, and external rotation MVIE tests both bilaterally and unilaterally. Muscle activity was recorded from seven muscles acting at the glenohumeral joint during the performance of each test. Repeated measures ANOVAs were used to determine within-participant differences on muscle specific MVEs and hand force measures between the two versions of the test. Gender was compared as a between participants factor. Within-participant reproducibility in muscle activity between bilateral and unilateral MVIEs was compared using coefficients of variation. Performing a bilateral empty can MVIE test elicited significantly higher MVEs for the upper trapezius (14%) and supraspinatus muscles (15%) than a unilateral test in this study population. MVEs during the palm press and external rotation were not affected by performing the tests bilaterally or unilaterally. Reliability was similar between unilateral and bilateral exertions across MVIE tests. Hence, we recommend that the empty can MVIE test be performed bilaterally, while either bilateral or unilateral configurations are equivalent during the palm press and external rotation MVIE tests.

PERCEIVED EFFORT IN UNILATERAL AND BILATERAL ISOMETRIC EXERTIONS

Subjects produce larger unilateral forces than bilateral forces for the same perceived submaximal effort. This submaximal bilateral deficit has been attributed to physiological and perceptual components. PURPOSE To examine the perceived effort of unilateral and bilateral exertions when absolute force was held constant. METHODS Forty-two healthy subjects (22.8±3.5 yrs. 171.0±26.4 cm, 69.4±10.7 kg) without upper extremity injury performed three 3-second trials each of bilateral (B), unilateral dominant (UD) and non-dominant (UN) elbow flexion at 100%, 80%, 60%, 40%, and 20% of maximal effort. Subjects matched force production to a target force using visual feedback from a realtime display. Subjects indicated perceived effort for that trial using a standardized measurement scale. RESULTS The mean coefficient of variation of force for maximal effort trials ranged from 4.8–5.8% and 1.6–2.3% for submaximal effort trials. The bilateral index at 100% effort (−8.6±5.6%) was significantly less than zero but was small (< 1%) for submaximal trials. Unilaterally, subjects matched the target force of 80% maximum effort most closely (79.6±10.9% and 79.0±7.9%, UD and UN, respectively). At the 60% target force subjects slightly overestimated the effort they were producing in UD and UN (63.7±9.2% and 62.1±8.8%, respectively). This overestimation increased at the 40% and 20% target forces in UD and UN (47.1±9.6%, 46.5±7.6%, 27.8±11.5%, and 26.0±11.3%, respectively). The perception of effort in bilateral trials was consistently and significantly (p < 0.01) higher than recorded in unilateral trials. Perceived effort at 80% and 60% target force was 7.9–9.6% higher in bilateral trials than unilateral trials (87.2±8.1% and 71.6±9.1%, respectively). The elevated perception of effort in bilateral trials was slightly less at the 40% and 20% target forces but still ranged from 5.5–7.2% higher than in unilateral trials (52.6±9.7% and 33.3±10.4%, respectively). CONCLUSION Force production was perceived to be more strenuous when performed bilaterally suggesting that the submaximal bilateral deficit contains a perceptual component.

Reduced Neural Drive in Bilateral Exertions: A Performance-Limiting Factor?

Activity of the motor cortex in one hemisphere reduces the maximum motor outflow of homologous parts of the opposite hemisphere, causing a reduction in the maximum force a muscle can exert when the contralateral homologous muscle is activated concurrently. The purpose of this study was to establish whether this bilateral deficit is large enough to explain limitations in performance in bilateral exertions. Voluntary force production and neural drive during unilateral and bilateral exertions were compared in three experiments, consisting of unilateral maximum contractions, synchronous bilateral contractions, and asynchronous bilateral contractions of finger flexors and knee extensors. Maximum voluntary force was overall about 7% lower in bilateral knee extension as compared with unilateral knee extension (P < 0.001). In finger flexion, a bilateral voluntary force deficit of as much as 20% was found ( P= 0.001). Corresponding deficits in agonist EMG activity were also significant and on average found to be of similar size, though the magnitude of the bilateral deficit in EMG was not consistently related to the magnitude of the bilateral force deficit. In knee extension, a deficit in voluntary activation of 4% (P = 0.003) was demonstrated by means of superimposed tetanic stimulation. The magnitude of this deficit was correlated to the magnitude of the voluntary force deficit (r = 0.80, P= 0.002). The maximum rate of force development in bilateral knee extensions was 13% lower than in a unilateral knee extension (P = 0.002). These results suggest that deficits in bilateral force production are large enough to constitute an important performance-limiting factor. Furthermore, the data suggest that a reduced neural drive underlies this bilateral deficit.

Lateral specificity in resistance training: the effect of bilateral and unilateral training

Maximal voluntary strength of simultaneous bilateral exertion has been shown to be small compared to the sum of the unilateral exertions. Three experiments were conducted to determine the effects of bilateral and unilateral resistance training on this bilateral deficit and to compare these in hands, arms, and legs. In each experiment, the subjects were divided into three groups: unilateral training group, bilateral training group, and control group. The subjects of the training group performed maximal isometric handgrip training in experiment I, and maximal isokinetic arm and leg extension training in experiments II and III. In each experiment, the subjects of the training group continued one of these resistance training exercises three times a week, for 6 weeks. The increase in handgrip strength of the bilateral training group produced in the bilateral condition [5.1 (SEM 2.4)%, after 3 weeks, 6.4 (SEM 2.3) %, after 6 weeks] was significantly greater compared with the control group [-1.1 (SEM 1.0) %, after 3 weeks, -1.5 (SEM 1.1) %, after 6 weeks. The increase in leg extension power of the bilateral training group produced in the bilateral condition [16.1 (SEM 9.6) %, after 3 weeks, 24.1 (SEM 7.4) %, after 6 weeks] was significantly greater compared with the unilateral training group [-5.0 (SEM 3.4) %, after 3 weeks, -3.4 (SEM 4.2) %, after 6 weeks] and the control group [-4.3 (SEM 2.5) %, after 3 weeks, 1.5 (SEM 5.5) %, after 6 weeks]. The increase in handgrip strength of the unilateral training group produced in the unilateral condition [7.3 (SEM 1.7) %, after 3 weeks] was significantly greater compared with the control group [-0.9 (SEM 1.8) %, after 3 weeks]. The increase in arm extension power of the unilateral training group produced in the unilateral condition [7.2 (SEM 1.8) %, after 6 weeks] was significantly greater compared with the bilateral training group [-3.0 (SEM 2.3) %, after 6 weeks] and the control group [-2.1 (SEM 2.6) %, after 6 weeks]. Bilateral indexes (BI) were shifted in a positive direction by bilateral training and tended to shift in a negative direction by unilateral training. With regard to the magnitude of change in BI, there were no significant differences among handgrip, arm extension, and leg extension training. It is suggested that there is lateral specificity in resistance training and that there is no difference among body parts in the modification of bilateral deficit by lateral training.

Reproducibility of subjectively graded voluntary isometric muscle strength in unilateral and simultaneous bilateral exertion.

Reproduction error of voluntary isometric muscle strength that was graded subjectively by the subject was investigated under unilateral and bilateral conditions. Six kinds of tasks, i.e. elbow flexion, elbow extension, hand grip, second digit abduction, knee extension and leg extension were employed. It was clear that absolute error (AE) increased with increase of exerted muscle strength in both unilateral and bilateral conditions. Mean of AE in each task was 4-5% of maximal muscle strength in the task in unilateral conditions. Algebraic or constant error (CE) decreased with increase of exerted strength. Demands for smaller strength tended to overshoot and larger ones tended to undershoot. In bilateral conditions when the different levels of strength for each limb were required to be exerted, AE on the weaker side tended to be larger than that in unilateral conditions. Thus a possibility that magnitude of AE might be affected by amount of attention was suggested. The subjectively graded criterion strength for a specific required level, when the strength was expressed as relative to the maximal strength, was almost the same in every task within a subject although it was different between subjects. It is suggested that each subject might have an awareness of relative magnitude of exerted strength.

Influence of simultaneous bilateral exertion on muscle strength during voluntary submaximal isometric contraction

The influence of simultaneous bilateral exertion on muscle strength was tested under the conditions in which the same or different levels of strength were exerted by the right and left arm (or hand). Isometric muscle strength of elbow flexion, elbow extension and hand grip was studied. Subjects voluntarily exerted 25%, 50%, and 75% of maximal strength based on their subjective judgement without the feedback of the strength actually exerted. Involuntary decrements of muscle strength were caused by the bilateral exertion. Muscle strength of both sides decreased under the condition where the same level of strength was exerted by the right and the left arm (or hand). When different levels of strength were exerted by each arm (or hand), the strength of the weaker side considerably decreased, while the strength of the stronger side did not decrease.

Decrease in human voluntary isometric arm strength induced by simultaneous bilateral exertion.

Human voluntary isometric maximum muscle strength during simultaneous bilateral versus separate unilateral elbow extension and flexion was measured. The following results were obtained. (1) Strength was reduced under the condition of simultaneous bilateral exertion of contralateral corresponding muscles as compared to the unilateral condition. (2) The decrease ratio of strength was 24.6 and 18.8% for the right and left for extension and 6.3 and 7.6% for the right and left for flexion. The extension-flexion difference was statistically significant but right-left difference was not significant. (3) Integrated EMG showed the tendency comparable to the strength and high correlation was found between strength and integrated EMG. (4) Decrease ratio was reduced when extension of one arm and flexion of the other arm was simultaneously performed. (5) Possible mechanisms subserving these findings were postulated and discussed.

Decrease in grip strength induced by simultaneous bilateral exertion with reference to finger strength.

Voluntary maximum isometric grip strength was measured under the conditions of simultaneous bilateral and separate unilateral exertion on 16 male and 8 female subjects. Strengths of each finger and surface electromyograms of finger flexors in the forearm were recorded concurrently with grip strength recording. The Following results were obtained: (1) Grip strength and integrated EMG of both arms were significantly reduced by simultaneous bilateral exertion. (2) The decrease ratio in strength was 5–14%. (3) There was an obvious linear relationship between grip strength and integrated EMG both for original values and for decrease ratios. (4) Parallelism between strength and electrical activity of muscle suggests that neurophysiological mechanisms subserve this strength decrease. (5) The middle finger occupied the largest share of grip strength and the ring, index and little finger followed in decreasing order. (6) The proportion occupied by each finger in decrease ratio of grip strength was largest in the c...