Express arm Responses Appear Bilaterally on Upper-limb Muscles in Arm Choice Reaching Task

When required, humans can generate very short latency reaches towards a visual target, like catching a phone falling off a desk. During such rapid reaches, express arm responses are the first wave of upper limb muscle recruitment, occurring within ~80-100 ms of target appearance. There is accumulating evidence that express arm responses arise from signaling along the tecto-reticulo-spinal tract, but the involvement of the reticulo-spinal tract has not been well-studied. Since the reticulospinal tract projects bilaterally, we studied whether express arm responses would be expressed bilaterally. Human participants (n = 14; 7 female) performed visually guided reaches in a modified emerging target paradigm where either arm could be used to intercept a target once it emerged below a barrier. We recorded electromyographic activity bilaterally from the pectoralis major muscle. Our analysis focused on target locations where participants reached with the right arm on some trials, and the left arm on others. In support of the involvement of the reticulospinal tract, the express arm response persisted bilaterally regardless of which arm reached to the target. While the latency of the express arm response was the same on the reaching vs non-reaching arm, the response magnitude was slightly larger on the reaching arm, in part due to anticipatory muscle recruitment related to arm choice. Our results support the involvement of the reticulo-spinal tract in mediating the express arm response, and we surmise that the increased magnitude on the arm chosen to move arises from convergence of cortically derived signals with the largely independent express arm response.

Significance of Lower Body Postures in Chair Design

Objective This study examined a system-level perspective to investigate the changes in the whole trunk and head postures while sitting with various lower extremity postures. Background Sitting biomechanics has focused mainly on the lumbar region only, whereas the anatomy literature has suggested various links from the head and lower extremity. Method Seventeen male participants were seated in six lower extremity postures, and the trunk kinematics and muscle activity measures were captured for 5 s. Results Changes in the trunk-thigh angle and the knee angle affected the trunk and head postures and muscle recruitment patterns significantly, indicating significant interactions between the lower extremity and trunk while sitting. Specifically, the larger trunk-thigh angle (T135°) showed more neutral lumbar lordosis (4.0° on average), smaller pelvic flexion (1.8°), smaller head flexion (3.3°), and a less rounded shoulder (1.7°) than the smaller one (T90°). The smaller knee angle (K45°) revealed a more neutral lumbar lordosis (6.9°), smaller pelvic flexion (9.2°), smaller head flexion (2.6°), and less rounded shoulder (2.4°) than the larger condition (K180°). The more neutral posture suggested by the kinematic measures confirmed significantly less muscular recruitment in the trunk extensors, except for a significant antagonistic co-contraction. Conclusion The lower and upper back postures were more neutral, and back muscle recruitment was lower with a larger trunk-thigh angle and a smaller knee angle, but at the cost of antagonistic co-contraction. Application The costs and benefits of each lower extremity posture can be used to design an ergonomic chair and develop an improved sitting strategy.

Co-contraction training: myoelectric activity and recruitment strategies in an unorthodox resistance training technique.

Objective To elucidate the muscle recruitment patterns and interindividual variability during co-contraction training sessions for lower limbs. Methods Ten active male young adults underwent two days of tests, in which they performed, for each leg, a maximal isometric voluntary contraction protocol followed by a co-contraction training set. We acquired myoelectric (EMG) activity from the sartorius, biceps femoris long and short heads, semitendinosus, semimembranosus, rectus femoris, vastus lateralis and vastus medialis and tensor facia latae during both protocols. We used iterative HLM analyses and bootstrap ANOVAs to explain within and between participant variances. Results On average, participants started recruiting 36% of their maximum EMG amplitude, showing decays of 0.41% per repetition and increasing 7.45% from day 1 to day 2. Participants who started with higher recruitment showed greater decays over repetitions and vice-versa. The training stimulated similarly the ratio of participants’ flexors and extensors. However, participants demonstrated different average muscle recruitment patterns with some individuals modifying, largely, their recruitment over repetitions/days. Between and within-variability in recruitment pattern was maintained throughout repetitions and days. We found no consistent similarity in terms of pairs of participants as to find common types of recruitment. Conclusion Co-contraction training seems to be effective to recruit thigh muscles of both legs along an entire set of repetitions and days. Despite the accounted variations in intramuscular recruitment, co-contraction training evokes similar muscular in flexor’s and extensor’s recruitment among participants.

High contrast, moving targets in an emerging target paradigm promote fast visuomotor responses during visually guided reaching

Humans have a remarkable capacity to rapidly interact with the surrounding environment, often by transforming visual input into motor output on a moment-to-moment basis. But what visual features promote rapid reaching? High contrast, fast-moving targets elicit strong responses in the superior colliculus (SC), a structure associated with express saccades and implicated in rapid electromyographic (EMG) responses on upper limb muscles. To test the influence of stimulus properties on rapid reaches, we had human subjects perform visually guided reaches to moving targets varied by speed (experiment 1) or speed and contrast (experiment 2), in an emerging target paradigm which has recently been shown to robustly elicit fast visuomotor responses. Our analysis focused on stimulus-locked responses (SLRs) on upper limb muscles. SLRs appear within <100 ms of target presentation, and as the first wave of muscle recruitment, they have been hypothesized to arise from the SC. Across 32 subjects studied in both experiments, 97% expressed SLRs in the emerging target paradigm, whereas only 69% expressed SLRs in an immediate response paradigm towards static targets. Faster moving targets (experiment 1) evoked large magnitude SLRs, while high contrast fast moving targets (experiment 2) evoked short latency, large magnitude SLRs. In some instances, SLR magnitude exceeded the magnitude of movement aligned activity. Both large magnitude and short latency SLRs were correlated with short latency reach reaction times. Our results support the hypothesis that, in scenarios requiring expedited responses, a subcortical pathway originating in the SC elicits the earliest wave of muscle recruitment, expediting reaction times.

High contrast, moving targets in an emerging target paradigm promote fast visuomotor responses during visually guided reaching

Humans have a remarkable capacity to rapidly interact with the surrounding environment, often by transforming visual input into motor output on a moment-to-moment basis. But what visual features promote the shortest-latency reach responses? To address this question, we had human subjects perform visually guided reaches to moving targets varied by speed (experiment 1), or speed and contrast (experiment 2) in an emerging target paradigm, which has recently been shown to robustly elicit fast visuomotor responses. Our analysis focused on stimulus-locked responses (SLRs) on upper limb muscles. SLRs represent the first wave of muscle recruitment tied to visual target onset, appearing within <100 ms. Across 32 subjects studied in both experiments, 97% expressed SLRs in the emerging target paradigm. In comparison, 69% of these subjects expressed SLRs in a visually-guided reach paradigm. Within the emerging target paradigm, we found that target speed impacted SLR magnitude, whereas target contrast impacted SLR latency and magnitude. Thus, high contrast, faster-moving targets in the emerging target paradigm robustly recruited the circuitry mediating the most rapid visuomotor transformations for reaching, and such responses were associated with shorter latency RTs. Our results support the hypothesis that a subcortical pathway originating in the superior colliculus may be involved in the earliest wave of muscle recruitment following visual stimulus presentation. In scenarios requiring expedited responses, cortical areas may serve to prime this subcortical pathway, and elaborate subsequent phases of muscle recruitment following the SLR.Significance StatementHumans have a remarkable capacity, when necessary, to rapidly transform vision into action. But how does the brain do this? Here, by studying human subjects reaching to suddenly-appearing targets, we find that the earliest visually-guided actions are produced in response to high-contrast, moving targets. A millisecond-resolution examination of upper limb muscle recruitment shows that motor output can begin within less than 100 ms of target presentation. We surmise that this earliest recruitment arises from a phylogenetically-conserved brainstem circuit originating in the superior colliculus. Rather than being directly involved in the earliest phase of visuomotor actions, cortical areas may prime this brainstem circuit to produce initial muscle recruitment, and then elaborate subsequent phases of recruitment when time is of the essence.

Comparison of Electromyographic Activity of Anterior Tibial Muscles and Long Fibular in Athletes With and Without Chronic Ankle Instability

Chronic ankle instability (ICT) is common in jumping sports, may alter muscle recruitment, result in functional limitations and recurrence of sprains in this joint. The purpose of the study was to compare the muscle recruitment of the anterior tibial and long fibular muscles of athletes with and without chronic ankle instability by means of surface electromyography. Thirty-four athletes were recruited, divided into instability group (GI: n=14) and control group (GC: n=20), of both sexes and from different sport modalities, aged between 18 and 27 years old, history of ankle sprain in the last 12 months and functional limitation established by the Cumberland Ankle Instability (CAIT) questionnaire, the recruitment of the anterior tibial and long fibular muscles was analyzed by surface electromyography during the lunge exercise. The results for GI and GC were, respectively: age 21.3±2.88 and 22.4±3.25, height 1.77±0.10 and 1.74±0.08, CAIT 17[12.2-19] and 29.5 [27-30] (p<0.001). The anterior tibial and long fibular muscle recruitment in GI was 111.1[62.5-165.4] and 68.2±29, respectively and in GC 106.8[79.8-230.5] and 54.4±26.4, without significant difference. ICT did not interfere in the recruitment of the anterior tibial and long fibular muscles during the lunge exercise in athletes. Keywords: Ankle. Sprains and Strains. Health Evaluation. ResumoA instabilidade crônica do tornozelo (ICT) é comum em esportes de saltos, pode alterar o recrutamento muscular, resultar em limitações funcionais e recidivas de entorses nesta articulação. O objeto do estudo foi comparar o recrutamento muscular dos músculos tibial anterior e fibular longo de atletas com e sem instabilidade crônica de tornozelo por meio da eletromiografia de superfície. Foram recrutados 34 atletas, divididos em grupo instabilidade (GI: n=14) e grupo controle (GC: n=20), de ambos os sexos e de diferentes modalidades esportivas, com idade entre 18 e 27 anos, história de entorse de tornozelo nos últimos 12 meses e limitação funcional estabelecida pelo questionário Cumberland Ankle Instability (CAIT), o recrutamento dos músculos tibial anterior e fibular longo foi analisado pela eletromiografia de superfície durante o exercício de agachamento afundo. Os resultados para GI e GC foram, respectivamente: idade 21,3±2,88 e 22,4±3,25, altura 1,77±0,10 e 1,74±0,08, CAIT 17[12,2–19] e 29,5 [27-30] (p<0,001). O recrutamento muscular do tibial anterior e fibular longo no GI foi 111,1[62,5-165,4] e 68,2±29, respectivamente e no GC 106,8[79,8-230,5] e 54,4±26,4, sem diferença significativa. A ICT não interferiu no recrutamento dos músculos tibial anterior e fibular longo durante a realização do exercício de agachamento afundo em atletas. Palavras-chave: Tornozelo. Entorses e Distensões. Avaliação em Saúde