Muscle Activation Strategies in Multiple Muscle Systems - Research

The relevance of these factors to muscle activation strategy in multiple muscle systems is ... nective tissues, tendon, and adjacent joints to simple motor.
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Muscle Activation Strategies in Multiple Muscle Systems MINORU SHINOHARA School of Applied Physiology, Georgia Institute of Technology, Atlanta, GA; Rehabilitation R&D Center of Excellence, Atlanta VA Medical Center, Decatur, GA ABSTRACT SHINOHARA, M. Muscle Activation Strategies in Multiple Muscle Systems. Med. Sci. Sports Exerc., Vol. 41, No. 1, pp. 181–183, 2009. Multiple neural and mechanical factors need to be taken into account to understand muscle activation strategies during human movement. These factors include lateral force transmission via connective tissue, summation of forces across a joint, tendon compliance that influences muscle shortening/lengthening, and the activity of biarticular muscles that influences activation strategies in monoarticular muscles. The relevance of these factors to muscle activation strategy in multiple muscle systems is introduced. Key Words: LATERAL FORCE TRANSMISSION, FORCE FLUCTUATIONS, TENDON COMPLIANCE, POSTURE CONTROL, COSINE TUNING, BIARTICULAR MUSCLE

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search designs purposefully neglect potential influences arising from tendon, connective tissues, neighboring muscles, and adjacent joints to focus on specific neuromechanical pathways or processes. Contributions of these neglected factors, however, need to be clarified in applying the knowledge obtained from these simplified research designs for elucidating more realistic neuromechanical mechanisms underlying motor performance in humans who use multiple muscles concurrently. Recently, unique insights and appropriate applications of new technology and analysis techniques advanced the understanding of the contribution of neighboring muscles, connective tissues, tendon, and adjacent joints to simple motor performance. The following symposium articles (5,7–9) shed light on these issues by synthesizing relevant studies by the authors. The primary topics that are reviewed are as follows: the amount of lateral force transmission to a neighboring muscle via connective tissues during electrically evoked contractions (1 in Fig. 1) (8); modulations of muscle activation strategy in individual muscles that lead to increased fluctuations in joint torque during steady contractions (2 in Fig. 1) (9); the influence of tendon compliance on paradoxical shortening and lengthening of muscles during postural control (3 in Fig. 1) (5); and the influence of biarticular muscle activity and force direction across joints on monoarticular muscle activity (4 in Fig. 1) (7). Contraction of a muscle may mechanically influence neighboring muscles via connective tissues. One of the mechanical consequences may be a nonlinear summation of force via lateral transmission (1 in Fig. 1). Sandercock and Maas (8) review recent studies that quantified the nonlinear summation of forces between cat ankle extensors during

uman movement is caused by net joint torque that results from the summation of forces from multiple muscles. The performance of human movement is therefore largely determined by the activation strategies and the contractile properties of the muscles involved. For example, adaptations in motor performance, such as impaired and improved ability of an individual to control force and posture with age or disease and practice/training, respectively, are attributable in part to alterations in muscle activation strategies (3,4,12). In scientific studies, researchers often use simplified research designs to understand the basic neuromechanical mechanisms for motor performance. For instance, contractile properties of muscle have been assessed for isolated single muscle fibers without connective tissues and tendon (2,11); the influence of muscle activation strategy on motor performance has been examined in single muscles that produce majority of torque about a joint (e.g., first dorsal interosseus muscle) (4,10); and the association between muscle activation strategy and motor performance has been tested by focusing on single joints (1,6). These previous re-

Address for correspondence: Minoru Shinohara, Ph.D., F.A.C.S.M., School of Applied Physiology, Georgia Institute of Technology, 281 Ferst Drive, Atlanta, GA 30332-0356; E-mail: [email protected] Submitted for publication December 2007. Accepted for publication March 2008. 0195-9131/09/4101-0181/0 MEDICINE & SCIENCE IN SPORTS & EXERCISEÒ Copyright Ó 2008 by the American College of Sports Medicine DOI: 10.1249/MSS.0b013e318183c0b2

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FIGURE 1—Schematic diagram of the activation of muscles and transmission of force in a multiple agonist muscle system. The arrows indicate the primary direction of force and joint torque. Numbers indicate the order of the symposium articles. 1. The amount of lateral force transmission to a neighboring muscle via connective tissues (8). 2. Modulations of muscle activation strategy in individual muscles that lead to increased fluctuations in joint torque (9). 3. The influence of tendon compliance on paradoxical shortening and lengthening of muscles (5). 4. The influence of biarticular muscle activity and force direction across joints on monoarticular muscle activity (7).

electrically evoked contractions. They conclude that the nonlinear summation of forces between neighboring muscles is very small if acting at physiological lengths and relative positions. The role of connective tissues appears to be substantial only in nonphysiological conditions (e.g., when tendon is cut). Their conclusion suggests that individual agonist muscles can be regarded as independent actuators in physiological conditions. Accordingly, the fluctuations in joint torque during voluntary contractions are regarded as the summation of fluctuations in individual muscle forces (2 in Fig. 1). Theoretically speaking, fluctuations in joint torque or net force will be augmented if the waveforms of force fluctuations in individual muscle forces are similar. Shinohara et al. (9) review their recent studies that identified unique modulations of muscle activation strategies during steady voluntary contractions due to sustained contraction, disuse, and change in muscle length. They conclude that selective

changes in muscle activity (low-frequency modulation and/ or increased magnitude) in a specific muscle among agonist muscles lead to increases in the fluctuations in net force during steady contractions. Force produced by the activation of muscles is transmitted to tendon before it causes joint torque. Traditionally, the potential influence of the compliance of Achilles tendon has not been properly taken into account in discussing the activation strategies of calf muscles in controlling human posture. During forward body sway, with a traditional view, calf muscles would be stretched and the mechanoreflex response would enhance muscle activity and thus shorten the calf muscles to produce extension torque about the ankle joint. Loram et al. (5) review their series of studies that contradicted with these ideas. Using their automatic analysis technique of ultrasound images, their studies have demonstrated that calf muscles actively interact with the compliant Achilles tendon—the muscles shorten during forward sway and lengthen during backward sway (3 in Fig. 1). Because the stretch reflex mechanisms for postural control do not match these observations, the authors question the postural role of the calf muscle spindles. When researchers refer to mechanical consequence of muscle activity, they often focus on force components that are directly associated with the production of torque about a single joint of interest. In addition, activation of a biarticular muscle produces unintended torque about a joint in another end of the muscle. Nozaki (7) reviews recent studies that comprehensively investigated the influences of muscle activity and force direction in biarticular muscles on monoarticular muscles (4 in Fig. 1). He proposes that these associations are explained by a novel ‘‘cosine-tuning’’ model. This idea suggests that the level of muscle activity in a monoarticular muscle is influenced by the torque about an adjacent joint. The conclusions drawn from these reviews are novel and involve significant information for further understandings of muscle activation strategies in multiple muscle systems in humans. Because the conclusions are based on studies with limited experimental settings, further investigations may be necessary in applying them to other muscle groups or conditions. Nonetheless, it is no doubt that the reviewed issues have significant implications for a variety of fields including physical training and inactivity, muscle injury, rehabilitation, sports performance, postural control, fatigue, and aging. The author is supported in part by an NIH grant NS052480. The contents of this article do not constitute endorsement by ACSM.

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4. Kornatz KW, Christou EA, Enoka RM. Practice reduces motor unit discharge variability in a hand muscle and improves manual dexterity in old adults. J Appl Physiol. 2005;98:2072–80. 5. Loram ID, Maganaris CN, Lakie M. Paradoxical muscle movement during postural control. Med Sci Sports Exerc. 2009;41(1):198–204. 6. Maffiuletti NA, Pensini M, Martin A. Activation of human plantar flexor muscles increases after electromyostimulation training. J Appl Physiol. 2002;92:1383–92. 7. Nozaki D. Torque interaction among adjacent joints due to the action of biarticular muscles. Med Sci Sports Exerc. 2009;41(1):205–9. 8. Sandercock TG, Maas H. Force summation between muscles: are muscles independent actuators? Med Sci Sports Exerc. 2009; 41(1):184–90.

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MUSCLE ACTIVATION STRATEGIES IN MULTIPLE MUSCLE SYSTEMS

Medicine & Science in Sports & Exercised

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