Synergistic Muscle Activation Research Articles

Task-related changes in intracortical inhibition assessed with paired- and triple-pulse transcranial magnetic stimulation.

Recent research has demonstrated a task-related modulation of postsynaptic intracortical inhibition within primary motor cortex for tasks requiring isolated (abduction) or synergistic (precision grip) muscle activation. The current study sought to investigate task-related changes in pre- and postsynaptic intracortical inhibition in motor cortex. In 13 young adults (22.5 ± 3.5 yr), paired-pulse transcranial magnetic stimulation (TMS) was used to measure short (SICI)- and long-interval intracortical inhibition (LICI) (i.e., postsynaptic motor cortex inhibition) in first dorsal interosseous muscle, and triple-pulse TMS was used to investigate changes in SICI-LICI interactions (i.e., presynaptic motor cortex inhibition). These measurements were obtained at rest and during muscle activation involving isolated abduction of the index finger and during a precision grip using the index finger and thumb. SICI was reduced during abduction and precision grip compared with rest, with greater reductions during precision grip. The modulation of LICI during muscle activation depended on the interstimulus interval (ISI; 100 and 150 ms) but was not different between abduction and precision grip. For triple-pulse TMS, SICI was reduced in the presence of LICI at both ISIs in resting muscle (reflecting presynaptic motor cortex inhibition) but was only modulated at the 150-ms ISI during index finger abduction. Results suggest that synergistic contractions are accompanied by greater reductions in postsynaptic motor cortex inhibition than isolated contractions, but the contribution of presynaptic mechanisms to this disinhibition is limited. Furthermore, timing-dependent variations in LICI provide additional evidence that measurements using different ISIs may not represent activation of the same cortical process.

Task constraints and minimization of muscle effort result in a small number of muscle synergies during gait.

Finding muscle activity generating a given motion is a redundant problem, since there are many more muscles than degrees of freedom. The control strategies determining muscle recruitment from a redundant set are still poorly understood. One theory of motor control suggests that motion is produced through activating a small number of muscle synergies, i.e., muscle groups that are activated in a fixed ratio by a single input signal. Because of the reduced number of input signals, synergy-based control is low dimensional. But a major criticism on the theory of synergy-based control of muscles is that muscle synergies might reflect task constraints rather than a neural control strategy. Another theory of motor control suggests that muscles are recruited by optimizing performance. Optimization of performance has been widely used to calculate muscle recruitment underlying a given motion while assuming independent recruitment of muscles. If synergies indeed determine muscle recruitment underlying a given motion, optimization approaches that do not model synergy-based control could result in muscle activations that do not show the synergistic muscle action observed through electromyography (EMG). If, however, synergistic muscle action results from performance optimization and task constraints (joint kinematics and external forces), such optimization approaches are expected to result in low-dimensional synergistic muscle activations that are similar to EMG-based synergies. We calculated muscle recruitment underlying experimentally measured gait patterns by optimizing performance assuming independent recruitment of muscles. We found that the muscle activations calculated without any reference to synergies can be accurately explained by on average four synergies. These synergies are similar to EMG-based synergies. We therefore conclude that task constraints and performance optimization explain synergistic muscle recruitment from a redundant set of muscles.

Immediate effects and short-term retention of multi-modal instruction compared to written only on muscle activity during the prone horizontal abduction exercise in individuals with shoulder pain

In rehabilitation, exercise instructions are multi-modal and can include a focus of increasing mean activity of a target muscle and inhibiting aberrant synergistic muscle activity, particularly during shoulder exercises, such as the prone horizontal abduction (PHA). The objective was to compare the immediate effects and short-term retention of multi-modal exercise instruction by a physical therapist written only instruction on normalized mean upper and lower trapezius muscle activity during three phases (concentric/isometric/eccentric) versus of an isotonic PHA exercise between participants with and without shoulder pain. Surface electromyography (EMG) was recorded from fourteen healthy participants and twelve participants with shoulder pain during the PHA exercise under two conditions: (1) written only instructions and (2) multi-modal instruction. Retention of multi-modal instruction on muscle activity was assessed one week later. Results demonstrate 12.8–16.0% increase in lower trapezius muscle activity during the concentric and isometric phases with multi-modal instructions in both groups. Inhibition of the upper trapezius did not occur in either group. Facilitation effects were maintained in short-term follow-up. Findings suggest that regardless of shoulder pain, multi-modal instruction by a physical therapist facilitates greater neuromuscular activity of a targeted muscle compared to written instructions alone and these effects are retained.

Motor modules of human locomotion: influence of EMG averaging, concatenation, and number of step cycles.

Locomotion can be investigated by factorization of electromyographic (EMG) signals, e.g., with non-negative matrix factorization (NMF). This approach is a convenient concise representation of muscle activities as distributed in motor modules, activated in specific gait phases. For applying NMF, the EMG signals are analyzed either as single trials, or as averaged EMG, or as concatenated EMG (data structure). The aim of this study is to investigate the influence of the data structure on the extracted motor modules. Twelve healthy men walked at their preferred speed on a treadmill while surface EMG signals were recorded for 60s from 10 lower limb muscles. Motor modules representing relative weightings of synergistic muscle activations were extracted by NMF from 40 step cycles separately (EMGSNG), from averaging 2, 3, 5, 10, 20, and 40 consecutive cycles (EMGAVR), and from the concatenation of the same sets of consecutive cycles (EMGCNC). Five motor modules were sufficient to reconstruct the original EMG datasets (reconstruction quality >90%), regardless of the type of data structure used. However, EMGCNC was associated with a slightly reduced reconstruction quality with respect to EMGAVR. Most motor modules were similar when extracted from different data structures (similarity >0.85). However, the quality of the reconstructed 40-step EMGCNC datasets when using the muscle weightings from EMGAVR was low (reconstruction quality ~40%). On the other hand, the use of weightings from EMGCNC for reconstructing this long period of locomotion provided higher quality, especially using 20 concatenated steps (reconstruction quality ~80%). Although EMGSNG and EMGAVR showed a higher reconstruction quality for short signal intervals, these data structures did not account for step-to-step variability. The results of this study provide practical guidelines on the methodological aspects of synergistic muscle activation extraction from EMG during locomotion.

Anodal Transcranial Direct Current Stimulation Alters Elbow Flexor Muscle Recruitment Strategies

Transcranial direct current stimulation (tDCS) is known to reliably alter motor cortical excitability in a polarity dependent fashion such that anodal stimulation increases cortical excitability and cathodal stimulation inhibits cortical excitability. However, the effect of tDCS on agonist and antagonist volitional muscle activation is currently not known. This study investigated the effect of motor cortical anodal tDCS on EMG/force relationships of biceps brachii (agonist) and triceps brachii (antagonist) using surface electromyography (EMG). Eighteen neurologically intact adults (9 tDCS and 9 controls) participated in this study. EMG/force relationships were established by having subjects perform submaximal isometric contractions at several force levels (12.5%, 25%, 37.5%, and 50% of maximum). Results showed that anodal tDCS significantly affected the EMG/force relationship of the biceps brachii muscle. Specifically, anodal tDCS increased the magnitude of biceps brachii activation at 37.5% and 50% of maximum. Anodal tDCS also resulted in an increase in the peak force and EMG values during maximal contractions as compared to the control condition. EMG analyses of other elbow muscles indicated that the increase in biceps brachii activation after anodal tDCS was not related to alterations in synergistic or antagonistic muscle activity. Our results indicate that anodal tDCS significantly affects the voluntary EMG/force relationship of the agonist muscles without altering the coactivation of the antagonistic muscles. The most likely explanation for the observed greater EMG per unit force after anodal tDCS appears to be related to alterations in motor unit recruitment strategies.

Latent Myofascial Trigger Points Are Associated With an Increased Intramuscular Electromyographic Activity During Synergistic Muscle Activation

The aim of this study was to evaluate intramuscular muscle activity from a latent myofascial trigger point (MTP) in a synergistic muscle during isometric muscle contraction. Intramuscular activity was recorded with an intramuscular electromyographic (EMG) needle inserted into a latent MTP or a non-MTP in the upper trapezius at rest and during isometric shoulder abduction at 90° performed at 25% of maximum voluntary contraction in 15 healthy subjects. Surface EMG activities were recorded from the middle deltoid muscle and the upper, middle, and lower parts of the trapezius muscle. Maximal pain intensity and referred pain induced by EMG needle insertion and maximal pain intensity during contraction were recorded on a visual analog scale. The results showed that higher visual analog scale scores were observed following needle insertion and during muscle contraction for latent MTPs than non-MTPs (P < .01). The intramuscular EMG activity in the upper trapezius muscle was significantly higher at rest and during shoulder abduction at latent MTPs compared with non-MTPs (P < .001). This study provides evidence that latent MTPs are associated with increased intramuscular, but not surface, EMG amplitude of synergist activation. The increased amplitude of synergistic muscle activation may result in incoherent muscle activation pattern of synergists inducing spatial development of new MTPs and the progress to active MTPs. PerspectiveThis article presents evidence of increased intramuscular, but not surface, muscle activity of latent MTPs during synergistic muscle activation. This incoherent muscle activation pattern may overload muscle fibers in synergists during muscle contraction and may contribute to spatial pain propagation.

Does the latissimus dorsi tendon transfer for massive rotator cuff tears remain active postoperatively and restore active external rotation?

The purpose of this study is to evaluate the muscle activity with surface electromyography (EMG) and the clinical outcome of the latissimus dorsi transfer. It remains unclear whether the clinical results of the latissimus dorsi transfer for massive posterosuperior rotator cuff tears are achieved either by active muscle contractions or by a passive tenodesis effect of the transfer. Eight patients were evaluated preoperatively and at 1year (SD, 0.1) after the latissimus dorsi transfer. Clinical evaluation of outcomes included active range of motion, Constant score, and visual analog scale (VAS) for pain and activities of daily living (ADL). Muscle activity was recorded with EMG during directional isometric abduction and adduction tasks. The external rotation in adduction improved from 23° to 51° (P=.03). The external rotation in abduction improved from 10° to 70° (P=.02). The mean Constant score improved from 39 to 62 postoperatively (P=.01). The VAS for pain at rest improved from 3.3 preoperatively to 0.1 (P=.02). The VAS for ADL improved from 4.9 to 2.3 (P=.05). The transferred latissimus dorsi remained active in all cases, as reflected by increased latissimus dorsi EMG activity during abduction tasks. In addition, the latissimus dorsi EMG activity shifted from preoperative antagonistic co-activation in adduction to synergistic activation in abduction. The latissimus dorsi has synergistic muscle activity after transfer. Apart from a tenodesis effect, directional muscle activity seems relevant for improved clinical outcome and pain relief. A specific gain was observed for external rotation in elevated arm positions, a motion essential for ADL tasks.

Motor unit properties from three synergistic muscles during ramp isometric elbow extensions

Many tasks require synergistic activation of muscles that possess different architectural, mechanical, and neural control properties. However, investigations of the motor unit (MU) mechanisms which modulate force are mostly restricted to individual muscles and low forces. To explore the pattern of MU recruitment and discharge behavior among three elbow extensors (lateral and long heads of the triceps brachii, and anconeus) during ramp isometric contractions, recruitment thresholds of 77 MUs in five young men were determined and corresponding MU discharge rates were tracked in 1-s epochs over forces ranging from 0 to 75% of maximal voluntary isometric force (MVC). Across all forces, MUs in the lateral head discharged at higher rates than those in the anconeus (p<0.001, Δ=0.23). When all MUs were considered, recruitment thresholds in the long head of the triceps brachii were higher than the lateral head (p<0.05,Δ=0.70) with a trend (p=0.08,Δ=0.48) for higher recruitment thresholds in the long head compared with the anconeus. Together, these data indicate a potential mechanical disadvantage of the long head of the triceps brachii at 0° shoulder flexion. However, among low-threshold MUs (<10% MVC), recruitment thresholds were lower in the anconeus than in both heads of the triceps brachii consistent with the expected twitch contractile and fibertype differences among these muscles. These findings illustrate the importance of considering synergistic relations among muscles used for a coordinated task, and the sensitivity of synergies to muscle architectural, mechanical, and possibly specific synaptic input factors.

Emerging evidence that exercise-induced improvements in muscular strength are partly due to adaptations in the brain

When people perform exercise against heavy loads, their capacity to produce force with the relevant muscles increases. It is well established that these improvements in strength are partly because of adaptive changes within the trained muscles, such as increased muscle fibre diameter. However, it has been recognized for some time that central nervous system (CNS) adaptations also contribute to enhanced strength, especially in the early phase of training. The exact form of these CNS changes has been the subject of considerable attention over the last 10 years, because non-invasive methods of probing the connections between the brain and muscles, such as transcranial magnetic stimulation (TMS), have become widely available. Nevertheless, much of the available evidence regarding the neural mechanisms of strength gains obtained from TMS studies is conflicting and difficult to interpret (Carroll et al. 2011). In the current issue of Acta Physiologica, Weier et al. (2012) report a large increase in the size of responses to TMS and associated reductions in the efficacy of inhibitory circuits within the motor cortex. The data add to recent evidence that strength training can induce the adaptation in the motor cortex that contributes to enhanced performance. The main novel aspect of the study by Weier et al. (2012) was the use of a paired-pulse TMS paradigm to assess the responsiveness of intracortical inhibitory interneurons. In this method, a TMS pulse that is subthreshold for eliciting a motor-evoked response (MEP) is used to condition the MEP produced by a subsequent (2-3ms later) test TMS pulse (Kujirai et al. 1993). The first pulse activates GABAergic inhibitory interneurons in the motor cortex (Ilic et al. 2002), which attenuate the excitatory response to the second pulse. The proportional reduction in the test pulse response when compared to control is taken as a measure of short-interval intracortical inhibition (SICI). The observation that SICI was released in the Weier et al. (2012) study implies a reduction in the responsiveness of the intracortical inhibitory interneurons as a consequence of strength training or a reduction in the synaptic efficacy of their synapses. In either case, the results indicate that strength training affects the cortical circuitry, which is an advance on previous studies that measured MEP size alone, because MEP amplitude can be modulated at both cortical and subcortical sites. Moreover, previous evidence on the effect of strength training on MEP size is mixed. In different studies, MEPs have been reported to increase, decrease or remain unchanged [see (Carroll et al. 2011) for review]. The large increase in MEP size reported by Weier et al. is striking and begs speculation as to why it differs from previous results. A likely possibility is that the nature of the training task was more complex that many previous types of training studied with TMS. The task (squat lift) required synergistic activation of multiple multi-articular muscles in a dynamic, multi-joint action and was paced to an external signal. These features may have provided a stimulus for reorganization within the motor cortex that exceeded that provided by previous studies that typically involved relatively constrained, single-joint tasks and thereby resulted in larger changes in the response to TMS. While the results of Weier et al. (2012) indicate that strength training causes adaptations within motor cortex, they do not necessarily imply that the adaptations directly contribute to improved strength. However, Hortobagyi et al. (2009) recently provided evidence for such a causal link, by demonstrating that strength gains were attenuated when the motor cortex was repetitively stimulated to interfere with post-training cortical processing. This type of repetitive TMS protocol has been shown previously to prevent the consolidation of newly acquired motor skill (Muellbacher et al. 2002). Furthermore, Selvanayagam et al. (2011) reported that the direction of mechanical twitches evoked by TMS shifted towards the direction of motion involved in a strength training session, which implies that corticospinal connections that favour the production of force produced during training are reinforced. Taken together, it seems likely that repetitive muscle actions against high loads alter motor cortical connectivity to facilitate the specific patterns of muscle activity required to produce maximal force in the training context. The results of Weier et al. (2012) are based on a relatively small sample and do not identify which additional areas in the CNS, besides the intracortical inhibitory interneurons, contribute to the large increases in MEP size. Nevertheless, they provide additional support for the view that improved strength relies at least partly on adaptations in motor cortex. The important challenge that remains is to characterize the links between specific training characteristics, neural adaptations and functional improvements. There is no conflict of interest to report.

Alteration of synergistic muscle activity following neuromuscular electrical stimulation of one muscle

The aim of the study was to determine muscle activation of the m. triceps surae during maximal voluntary contractions (MVCs) following neuromuscular electrical stimulation (NMES) of the m. gastrocnemius lateralis (GL). The participants (n = 10) performed three MVC during pretest, posttest, and recovery, respectively. Subsequent to the pretest, the GL was stimulated by NMES. During MVC, force and surface electromyography (EMG) of the GL, m. gastrocnemius medialis (GM), and m. soleus (SOL) were measured. NMES of GL induced no significant decline (3%) in force. EMG activity of the GL decreased significantly to 81% (P < 0.05), whereas EMG activity of the synergistic SOL increased to 112% (P < 0.01). The GM (103%, P = 1.00) remained unaltered. Decreased EMG activity in the GL was most likely caused by failure of the electrical propagation at its muscle fiber membrane. The decline of EMG activity in GL was compensated by increased EMG activity of SOL during MVC. It is suggested that these compensatory effects are caused by central contributions induced by NMES.

Alterations in Synergistic Muscle Activation Impact Fluctuations in Net Force

Net muscle force recorded during voluntary contractions with multiple agonist muscles is the summated result of individual muscle forces. The purpose of this article is to synthesize recent findings from several studies on the modulations of muscle activity and force fluctuations during steady voluntary contractions with multiple agonist muscles in humans. During a sustained low-force contraction with the knee extensor muscles, fluctuations in knee extension force changed concurrently with the involuntary alternate muscle activity between the rectus femoris muscle and the vasti muscles. After prolonged bed rest, the amount of change in the fluctuations in net force was associated with the amount of change in the EMG distribution among individual muscles within the knee extensor group and the ankle extensor group. As an adaptation to bed rest, an increase in the fluctuations in plantarflexion force accompanied an increase in the relative contribution of the medial gastrocnemius muscle to net force. In contrast, an increased contribution of the medial gastrocnemius by an acute increase in muscle length reduced force fluctuations. The discrepancy in the changes in force fluctuations due to an increased contribution of the medial gastrocnemius between two interventions (bed rest and muscle length change) is associated with increased low-frequency power in the EMG of the medial gastrocnemius after bed rest. In summary, the fluctuations in net force were influenced by the alteration in muscle activity among agonist muscles in terms of relative contributions to net force and frequency characteristics.

Changes in Motor Unit Firing Rate in Synergist Muscles Cannot Explain the Maintenance of Force During Constant Force Painful Contractions

The firing rate of low threshold motor units is decreased in constant force contractions during experimental pain. However, as firing rate is a determinant of force, it is unclear how force is maintained. Increased synergist muscle activity may compensate. This was investigated by evaluation of motor unit firing rate in synergist ankle plantar flexor muscles (triceps surae). Single motor unit action potentials were recorded in medial gastrocnemius and soleus muscles with fine wire electrodes in 10 subjects. Gross muscle activity was estimated from surface electromyographic (EMG) recordings. Bolus injections of 5% hypertonic saline were injected into lateral gastrocnemius to induce pain (low intensity, 0.5 mL; high intensity, 1.5 mL). Subjects gently plantar-flexed the ankle to recruit 1 to 4 motor units and performed 3 20-second contractions to this target before, during, and after pain. Firing rate decreased approximately 12% in synergist heads of triceps surae during pain and recovered after pain. Despite reduced firing rate, root-mean-square surface EMG amplitude did not change. The effect of nociceptor stimulation is not restricted to painful muscles but reduces motor unit firing in synergist muscles. Changes in synergist muscles cannot explain the maintenance of muscle force. Maintenance of surface EMG amplitude suggests recruitment of additional motor units. This study showed that activity of synergist muscles can be affected by muscle pain. However, the changes in activity of synergist muscles may not compensate for changes in the painful muscle. This finding provides evidence of more widespread effects of pain on muscle control.

Superficial Cooling Inhibits Force Loss in Damaged Muscle

The purpose of the study was to determine the effect of muscle damage with and without superficial cooling on force and neural activation of the triceps surae muscles. Seven men performed maximal plantarflexion contractions with and without superficial cooling over the medial gastrocnemius, before, immediate after, and 2 days after transcutaneous electrical stimulation for the medial gastrocnemius, respectively. Transcutaneous electrical stimulation was used to induce muscle damage. The normalized value, which was expressed as percentages of the corresponding relative values obtained before transcutaneous electrical stimulation to after transcutaneous electrical stimulation, of peak torque and integrated electromyogram for the soleus were significantly greater with than without superficial cooling. There was a significant correlation in normalized integrated electromyogram between the medial gastrocnemius and soleus. We conclude that superficial cooling appears to have reduced the magnitude of force loss during maximal voluntary contraction following damage to one of the muscles and the synergistic muscle activation may have contributed to the better force maintenance. The implications of this study are that TES enables more selective damage of muscle than standard protocols, and that superficial cooling over a damaged muscle may have an important role in the acute treatment of muscle injuries.

ISOMETRIC SQUAT FORCE OUTPUT AND MUSCLE ACTIVITY IN STABLE AND UNSTABLE CONDITIONS

The purpose of this study was to assess the effect of stable vs. unstable conditions on force output and muscle activity during an isometric squat. Nine men involved in recreational resistance training participated in the investigation by completing a single testing session. Within this session subjects performed isometric squats either while standing directly on the force plate (stable condition, S) or while standing on inflatable balls placed on top of the force plate (unstable condition, U). Electromyography (EMG) was recorded during both conditions from the vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), and medial gastrocnemius (G) muscles. Results indicated peak force (PF) and rate of force development (RFD) were significantly lower, 45.6% and 40.5% respectively, in the U vs. S condition (p < 0.05). Average integrated EMG values for the VL and VM were significantly higher in the S vs. U condition. VL and VM muscle activity was 37.3% and 34.4% less in U in comparison to S. No significant differences were observed in muscle activity of the BF or G between U and S. The primary finding in this investigation is that isometric squatting in an unstable condition significantly reduces peak force, rate of force development, and agonist muscle activity with no change in antagonist or synergist muscle activity. In terms of providing a stimulus for strength gain no discernable benefit of performing a resistance exercise in an unstable condition was observed in the current study.

Synergistic muscle activation during maximum voluntary contractions in children with and without spastic cerebral palsy

We examined muscle recruitment patterns in children with cerebral palsy (CP) and comparison children without CP under conditions of maximum voluntary contractions. Three groups of children participated in the study: (1) 12 children with diplegic CP (eight males, four females; age range 4–10y, mean age 7y [SD 2y 4mo]); (2) six children with hemiplegic CP (four males, two females; age range 5–10y, mean age 7y 4mo [SD 2y]); and (3) 13 comparison children with normal motor function (seven males, six females; age range 4–11y, mean age 7y 2mo, [SD 2y]). The children with CP were classified according to the Gross Motor Function Classification System: eight were Level I, five were Level II, four were Level III, and one was Level I V. Surface electromyography was recorded from four proximal and distal lower extremity (LE) muscles. Children with CP more frequently activated a muscle other than the intended prime mover first, compared with the comparison children, especially when the prime mover was a distal muscle. For example, during ankle plantar flexion, when the lateral gastrocnemius muscle was the prime mover, children with hemiplegia showed preactivation of the tibialis anterior muscle and children with diplegia showed medial hamstring coactivation. In conclusion, children with CP showed considerable differences to the comparison children in how LE muscles were voluntarily activated. Greater understanding of muscle recruitment patterns under a variety of tasks may provide new directions for motor control retraining or other forms of intervention.

Synergistic muscle activation during maximum voluntary activation in children with or without spastic CP

Developmental Medicine & Child NeurologyVolume 48, Issue 10 p. 788-788 Free Access Synergistic muscle activation during maximum voluntary activation in children with or without spastic CP Jean-Pierre Lin, Jean-Pierre LinSearch for more papers by this author Jean-Pierre Lin, Jean-Pierre LinSearch for more papers by this author First published: 13 February 2007 https://doi.org/10.1111/j.1469-8749.2006.tb01224.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References 1 Henneman E, Clamman HP, Gillies JD, Skinner RD. (1974) Rank order of motorneurons within a pool, law of combination. J Neurophysiol 37: 1338– 1349. 2 Evarts EV. (1968) Relation of pyramidal tract activity to force exerted during voluntary movement. J Neurophysiol 31: 14– 27. 3 Lin J-P. (2003) The cerebral palsies: a physiological approach. J Neurol Neurosurg Psychiatry 74 (Suppl. 1): i23– i29. 4 Sutherland DH, Olshen RA, Biden EN, Wyatt MP. (1988) The Development of Mature Walking. Clinics in Developmental Medicine No. 104/5. London : Mac Keith Press. p 154– 162. 5 Leonard CT, Hirschfeld H, Forssberg H. (1991) The development of independent walking in children with cerebral palsy. Dev Med Child Neurol 33: 567– 577. 6 Nashner LM, Shumway A, Marin O. (1983) Stance, posture, and control in selected groups of children with cerebral palsy: deficits in sensory integration and muscular coordination. Exp Brain Res 49: 393– 409. 7 Detrembleur C, Willems P, Plaghki L. (1997) Does walking speed influence the time pattern of muscle activation in normal children Dev Med Child Neurol 39: 803– 807. 8 Lin J-P. (2004) The assessment and management of hypertonus in cerebral palsy: a physiological atlas (road map). In: D Scrutton, D Damiano, M Maystone, editors Management of the Motor Disorders of Children with Cerebral Palsy. Clinics in Developmental Medicine No. 161. London : Mac Keith Press. p 85– 104. Volume48, Issue10October 2006Pages 788-788 ReferencesRelatedInformation

Synergistic muscle activation during maximum voluntary activation in children with or without spastic CP

The traditional model of motor weakness derived from the works of Hughlings Jackson is that of a negative phenomenon following a cortical injury such as stroke, in contrast to the positive phenomena of brisk reflexes and the release of primitive, complex reflexes. The physiology of orderly motor neuron recruitment and de-recruitment elaborated by Henneman, in which the smallest, weakest, and slowest motor neurons are recruited first followed by increasingly large, powerful, and fast motor neurons, provided a physiological explanation for weakness following a cortical insult. This was especially so with the later work of Evarts who demonstrated that the frequency of firing of Betz cells (upper motor neurons) in the cerebral cortex specified the strength of isometric contractions. Maintaining a contraction is underpinned by pools of upper and lower motor neurons being recruited in rotation by relay as it were. Reduced populations and sizes of such upper motor neurons would result in poorly sustained firing of neurons and hence weakness. However, in cerebral palsy (CP), the injury is seldom a focal lesion in the motor cortex but more usually bilateral lesions affecting subcortical white matter. Moreover, the brains of children with CP are immature and still developing. The work of Sutherland demonstrating persistence of the wrap-around electromyogram discharges throughout the gait cycle, and subsequently of Leonard, support a model of ‘developmental co-contraction’ as the default muscle activation pattern out of which, in the uninjured state, refined, selective motor strategies emerge to provide fluency and elegance of movement. As Leonard and colleagues demonstrated, children with CP never shake off this co-contracting pattern which results in clumsy, poorly executed, and energy-intensive movements. According to the landmark study by Tedroff et al. (p 789) children who have a hemiplegia or diplegia frequently select the wrong muscles when attempting to perform a maximum voluntary contraction. This miss-selection often involves an antagonist or remote muscle from the intended joint of action. A consequence of these findings necessitates a revision of the essence of the motor disorders traditionally referred to as ‘spastic’.

Synergistic muscle activation during maximum voluntary contractions in children with and without spastic cerebral palsy

We examined muscle recruitment patterns in children with cerebral palsy (CP) and comparison children without CP under conditions of maximum voluntary contractions. Three groups of children participated in the study: (1) 12 children with diplegic CP (eight males, four females; age range 4-10 y, mean age 7 y [SD 2 y 4 mo]); (2) six children with hemiplegic CP (four males, two females; age range 5-10 y, mean age 7 y 4 mo [SD 2 y]); and (3) 13 comparison children with normal motor function (seven males, six females; age range 4-11 y, mean age 7 y 2 mo, [SD 2 y]). The children with CP were classified according to the Gross Motor Function Classification System: eight were Level I, five were Level II, four were Level III, and one was Level IV. Surface electromyography was recorded from four proximal and distal lower extremity (LE) muscles. Children with CP more frequently activated a muscle other than the intended prime mover first, compared with the comparison children, especially when the prime mover was a distal muscle. For example, during ankle plantar flexion, when the lateral gastrocnemius muscle was the prime mover, children with hemiplegia showed preactivation of the tibialis anterior muscle and children with diplegia showed medial hamstring coactivation. In conclusion, children with CP showed considerable differences to the comparison children in how LE muscles were voluntarily activated. Greater understanding of muscle recruitment patterns under a variety of tasks may provide new directions for motor control retraining or other forms of intervention.

Isometric Squat Force Output and Muscle Activity in Stable and Unstable Conditions

The purpose of this study was to assess the effect of stable vs. unstable conditions on force output and muscle activity during an isometric squat. Nine men involved in recreational resistance training participated in the investigation by completing a single testing session. Within this session subjects performed isometric squats either while standing directly on the force plate (stable condition, S) or while standing on inflatable balls placed on top of the force plate (unstable condition, U). Electromyography (EMG) was recorded during both conditions from the vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), and medial gastrocnemius (G) muscles. Results indicated peak force (PF) and rate of force development (RFD) were significantly lower, 45.6% and 40.5% respectively, in the U vs. S condition (p < or = 0.05). Average integrated EMG values for the VL and VM were significantly higher in the S vs. U condition. VL and VM muscle activity was 37.3% and 34.4% less in U in comparison to S. No significant differences were observed in muscle activity of the BF or G between U and S. The primary finding in this investigation is that isometric squatting in an unstable condition significantly reduces peak force, rate of force development, and agonist muscle activity with no change in antagonist or synergist muscle activity. In terms of providing a stimulus for strength gain no discernable benefit of performing a resistance exercise in an unstable condition was observed in the current study.

Lower extremity control and dynamics during backward angular impulse generation in backward translating tasks

Observation of complex whole-body movements suggests that the nervous system coordinates multiple operational subsystems using some type of hierarchical control. When comparing two backward translating tasks performed with and without backward angular impulse, we have learned that task-specific modifications in trunk-leg coordination contribute to the regulation of total-body center of mass (CoM) position relative to the reaction force (RF). In this study, we hypothesized that task-specific differences in trunk-leg coordination would affect the control of the lower extremity joints during the impulse-generation phase of the tasks. Eight highly skilled performers executed a series of backward translating jumps with and without backward rotation (back somersault and back timer, respectively). Sagittal plane kinematics, RFs and electromyograms of lower extremity muscles were acquired during the take-off phase of both tasks. Lower extremity joint kinetics was calculated using inverse dynamics. The results indicate that between-task differences in the relative angles between the lower extremity segments and the net joint forces/RF contributed to significant reductions in knee-extensor net joint moments and increases in hip-extensor net joint moments during the push interval of the back somersault as compared to the back timer. Between-task differences in backward trunk angular velocity also contributed to the re-distribution of work done by the lower extremity net joint moments. Between-task differences in lower extremity joint kinetics were associated with synergistic activation of the bi-articular muscles crossing the knee and hip. These results indicated that task-specific control of CoM relative to the RF in order to regulate the backward angular-impulse-involved modification in the control and dynamics of the knee and hip joints. These results indicate that between-task differences in the control objectives at the total-body level (position of CoM relative to the RF) alters the control and dynamics of the multi-joint lower extremity subsystem.