S. Le Bozec

The synergy of elbow extensor muscles during static work in man

SummaryA quantitative comparison of the EMG activity of the three heads of the triceps brachii muscle and the anconeus muscle has been made during static work in man. The subjects maintained different extension torques with arm and forearm in a horizontal plane.The relationships between the electrical activity of the different elbow extensor muscles and the external torque were studied when the elbow was flexed at right angle. The integrated EMGs of the three heads of the triceps muscle increase quadratically with torque. The integrated EMG of the anconeus muscle increases more rapidly than those of the triceps muscle for low values of torque; for higher values of torque, the integrated EMG of the anconeus muscle increases in the same way as for the triceps muscle.These results suggest that:1.In static contractions, the “equivalent muscle” concept can be applied to the extensor muscle group for a large range of external extension torques.2.There is a common motor control for the three heads of the triceps muscle. The influence of the elbow angle on the activity of the different extensor muscles was studied under two conditions:(a)The subjects held a constant external torque at any elbow angle. In this case, the integrated EMG of the three heads of the triceps muscle remains approximately constant. This result suggests that variations in the lever arm of the muscles with the elbow angle compensate for force variations due to the variations in muscle lengths.(b)The force of the triceps muscle was constant with any elbow angle. In this case, there is an important decrease in the integrated EMG which is of the same amount for the three heads of the triceps muscle. This result indicates that the shape of the force-length relationship is approximately the same for the three heads. These results are obtained whatever the position of the arm (horizontal extension-abduction or horizontal extension-adduction or mid position); the length variations of the long head of the triceps muscle resulting from these positions do not influence markedly the activity of the extensor muscles of the elbow within the limits of the tested angular elbow positions.

Effects of seat–thigh contact on kinematics performance in sit-to-stand and trunk flexion tasks

It has been shown that thigh-seat contact-surface influences performance of isometric push-force with upper-limbs. The push-force performance is higher when subjects are seated with partial ischio-femoral / seat contact than when they are seated with full ischio-femoral contact. This was ascribed to greater pelvis and spine mobility induced by the short thigh-seat contact-surface. The present study tested the generalization of this hypothesis during movements involving body segment displacement, namely trunk flexion (TF) and sit-to-stand (STS) motor tasks. Both motor tasks were carried out in similar conditions to those implemented in the isometric push-force tasks, i.e. full ischio-femoral / seat contact (100-IFC) and short ischio-femoral contact (30-IFC, i.e. 30% of full ischio-femoral / seat contact). Results showed that kinematic performances (maximal antero-posterior and vertical center of mass velocity and maximal backward displacement of center of pressure) in both motor tasks were higher in 30-IFC than in 100-IFC. In the sit-to-stand task, time of seat-off is shorter in 30-IFC. As the subjects initial global posture was comparable across the experimental conditions, it can be discarded as a source of performance change. It is discussed that it is the enhanced pelvis mobility induced by the sitting condition which is responsible for the increase of motor performance in both trunk flexion and sit-to-stand tasks. Our results highlight the role of joint mobility in motor performance.

Biomechanical reorganisation of stepping initiation during acute dorsiflexor fatigue

During voluntary step initiation (SI), propulsive forces are generated during anticipatory postural adjustments (APA) which displace the centre-of-gravity (CoG) in the desired direction. These propulsive forces are implemented by ankle synergy, bilateral soleus inhibition followed by activation of tibialis anterior (TA). The aim of this study was to investigate the effect of fatigue applied to ankle dorsiflexors on APA associated with SI and on related motor performance. Eight young healthy participants initiated stepping before and after a protocol designed to generate fatigue in ankle dorsiflexors. Fatigue was induced by series of high-level isometric contractions performed until exhaustion. Results showed that, with fatigue, the level of TA activation during APA, anticipatory postural dynamics (backward centre-of-pressure displacement and forward CoG velocity) and related motor performance (peak of CoG velocity) were attenuated, while APA duration and total SI duration increased. These changes were interpreted as reflecting a protective strategy aiming to preserve the integrity of the fatigued muscles, rather than an impairment associated with muscle weakness.

Effect of lower limb muscle fatigue induced by high-level isometric contractions on postural maintenance and postural adjustments associated with bilateral upper limb push

This study tested the effect of lower limb muscle fatigue induced by series of high-level isometric contractions (IC) on postural adjustments and maintenance of erect posture. Subjects (N = 7) displaced a bar (grasp-bar) forward with both hands at maximal velocity towards a target (‘‘bilateral forward-reach’’ task, BFR), before and after a procedure designed to induce fatigue in dorsal leg muscles. This procedure included IC at 60% of maximum. Postural joint and grasp-bar motion, along with electrical activity of postural and focal muscles were recorded. Integrated electromyographical (EMG) activity per 20 ms period ranging from 400 ms before BFR onset (t0) to 400 ms after t0 was compared before and after the fatiguing procedure. This time-window included ‘‘anticipatory’’, ‘‘on-line’’ and ‘‘corrective’’ postural adjustments, i.e. those postural adjustments occurring before (APAs), during (OPAs) and after (CPAs) BFR, respectively. In contrast to the literature, results showed that the fatiguing procedure had no effect on muscle excitation or timing in any of the recorded postural muscles, regardless of APA, OPA or CPArelated time-window. Therefore, the postural drive did not change with fatigue. Furthermore, the peakto-peak motion at postural joints did not change. Postural maintenance was therefore not additionally challenged. These results are in line with the hypothesis that the effect of fatigue on postural adjustments is dependent on the adequacy between fatigued motor units (MUs) and MUs recruited during the postural adjustments. Increasing IC intensity during the fatiguing procedure might therefore not necessarily exacerbate the effect of fatigue on postural control highlighted during lower level IC.

Does symmetrical upper limb task involve symmetrical postural adjustments

Voluntary arm movements are preceded, accompanied and followed by a complex pattern of muscle activation/deactivation in lower limbs and trunk muscles called anticipatory (APA), on-line (OPA) and corrective (CPA) postural adjustments, respectively. It is generally admitted that their functional goal is to counter the perturbation to posture and equilibrium elicited by the focal movement (Bouisset and Zattara 1987). One way that is most often used in the literature to reduce the complexity in the analysis of these postural adjustments (PA) is to investigate the electrical activity of muscles in one single body side and to restrict the analysis to the sole APA time window. These ‘short-cuts’ have been typically used during bilateral arm movements performed symmetrically with respect to the sagittal plane, e.g. bilateral isometric ramp push (Le Bozec and Bouisset 2004), load releasing/catching from extended arms (Aruin et al. 1998), bilateral shoulder and elbow flexion/extension (Fujiwara et al. 2003), etc. If electromyographical (EMG) patterns are analysed unilaterally, then the assumption is implicitly made that changing the postural constraints will induce symmetrical EMG response in homologous muscles of dominant and nondominant leg. In the literature (Aruin et al. 1998; Fujiwara et al. 2003; Le Bozec and Bouisset 2004), this assumption is, however, exclusively based on visual inspections of EMG traces obtained in both sides and has never been subjected to experimental verification. This study tested the hypothesis that symmetrical upper limb task might involve asymmetrical PA with respect to lower limb dominance. Results of the present research might have consequences on studies reducing the postural response recordings to one leg and generalising the findings to the other one. Results may also have relevant implications in clinical investigations where postural asymmetry is generally considered as reflecting postural impairment.

Modulation of slow and fast elbow extensor EMG tonic activity by stretch reflexes in man

SummaryReflex EMG responses to sudden passive flexion of the elbow were recorded from anconeus and triceps brachii in 5 human volunteers.While the subjects were required not to resist the flexion movement, they were required to maintain an extension torque of 3.5 or 7.0 Nm prior to its onset.Under these isotonic conditions, the latency and amplitude of the reflex activities from anconeus and triceps brachii did not differ significantly, in contrast to the findings of Le Bozec (1986) in actively relaxed subjects. The myotatic/postmyotatic EMG amplitude ratio did not provide a further quantitative way to distinguish between these muscles.The absence of a difference between the reflex activities of a slow (anconeus) and a fast (triceps brachii) muscle is interpreted as resulting from a strong drive of spindle activity on the whole extensor motoneuron pool, which outweights the differences in recruitment due to the differing relative amounts of type I and type II fibres in the two muscles. Differences like those described between finger and calf muscles by other authors are though to be due to the relative degree of corticalization of these muscles.All short and long latency responses of the muscles increased in magnitude and decreased in latency with increasing background EMG activity as well as with increasing initial length. The position and tonic activity dependency of these responses is explained in terms of alpha-gamma coactivation.

Adherence modulation in single stepping

Humans have to comply with mechanical constraints, such as the ground reaction forces (GRFs) that are developed at the interface with the physical environment, when they move on the earth track. Moreover, it is well known that the GRFs depend on ground adherence and rigidity. The problem of adherence has been considered during walking by authors (Carlsöö 1962; Strandberg 1983; Tisserand 1985). They were conducted with the aim of measuring floor/shoe slip resistance to prevent slipping and fallrelated injuries. The aim of this preliminary study was to characterise the adherence modulation associated with single stepping, in order to document its role in motor control later on. To this end, a specific paradigm was considered, that consisted in the execution of one single step between two marks, with the foot landing flat on the ground at the foot contact (FC).

Consecutive postural adjustments in a single step

The role of postural adjustments in relation to voluntary movements has been described since Babinski’s observations in 1899 (Babinski 1899). Although anticipatory postural adjustments (APAs) have been studied extensively in recent years (Dietrich et al. 1994; Bouisset and Le Bozec 2002), consecutive postural adjustments (CPAs), which occur after the end of the voluntary movement, have received less attention from authors Le Bozec et al. (2008) who have focused primarily on upper-limb tasks. The aim of this study was to emphasise the role of CPAs when compared with their corresponding APAs, and to identify certain aspects of postural control during the locomotion process. To this end, a single step (SS) paradigm was considered as it provides the advantage of allowing comparison of the upper and lower limb related experiments because upper limb related movements do not perturb body balance as intensively as lower limb related ones.

The role of consecutive postural adjustments (CPAs) during postural control in isometric ramps pushes

Postural adjustments involved in the stabilising reactions, related to voluntary movement, have been described at least since Babinski’s (1899) observations. Since then, many experimental studies have been run. They focused mainly on anticipatory postural adjustments (APAs), following the pioneering work of Hopf and Hufschmidt (1963) and Belenkii et al. (1967). On the other hand, very little attention has been paid to the consecutive postural adjustments (CPAs), which occur after the end of voluntary movement. The aim of this paper was to stress the role of CPAs, as compared to APAs, and to specify certain aspects of postural control. To this end, maximal isometric ramp pushes were studied. They were performed by subjects in two sitting postures differing in the ischio-femoral contact with the seat. This task was chosen because it offers many advantages, which have already been developed (Le Bozec and Bouisset 2004). In this work, two of them can be emphasised: (i) since push originates in the focal chain, and as the upper limbs are fully extended, the kinetics should be located in the postural chain; and (ii) since the subjects are seated, a change in ischio-femoral contact with the seat is not difficult to obtain, which produces two different peak push force values, each of them resulting from a maximal effort, without perturbing gross body balance.