C. C. A. M. Gielen

Relation between location of a motor unit in the human biceps brachii and its critical firing levels for different tasks

Critical firing levels (CFLs) of single motor units (MUs) in the long head of the human biceps brachii muscle were determined in combinations of two isometric tasks: flexion of the elbow, supination of the lower arm, and exorotation of the humerus, as well as the corresponding antagonistic tasks. The MU activity was recorded by 25-micron bipolar wire electrodes. Four main patterns of MU recruitment, related to the recording location in the muscle, were found: (i) MUs active only when flexing the elbow were located mostly laterally. (ii) MUs active only when supinating were all located medially. (iii) MUs whose CFL depended on a linear combination of flexion and supination forces were all located medially. Some of these MUs could not be recruited during pronation. (iv) Nonlinearly behaving MUs, located centrally. The relative weights of flexion and supination input were constant for all units, whose CFL depended on a linear sum of flexion and supination forces, as well as for the nonlinearly behaving units. Supination and exorotation showed equivalent CFL changes when they were combined with the flexion task. Extension did not change the CFL for supination- or exorotation tasks. No clear difference was found between the ratios of the peak twitch forces in flexion and supination direction for laterally and medially located small muscle areas or single MUs. A simple model of the motoneuron pool organization is proposed to explain our findings.

Changes in recruitment order of motor units in the human biceps muscle

Changes in recruitment threshold of individual motor units of the human biceps (caput longum), a multifunctional muscle, were investigated during different tasks, i.e., isometric flexion of the elbow, isometric supination of the forearm, and isometric exorotation of the humerus of the 110° flexed semiprone horizontal arm. The activity of 17 motor units was recorded by means of fine wire electrodes. Some units were found that could be recruited only by one force, e.g., flexion. In such cases recruitment did not depend on other forces. Most units, however, were recruited when a linear combination of exerted forces exceeded a certain threshold. The contribution of a force to this combination could be different for different motor units. Units with a high threshold for flexion tended to show a lower threshold while simultaneously exerting force in another direction. Units with a low threshold for flexion were more difficult to recruit under this condition. The findings support the view that movements are programmed “directionally”.

Differences in the activation of m. biceps brachii in the control of slow isotonic movements and isometric contractions.

SummaryWe have compared muscle activation in the control of slow isotonic movements and isometric contractions. Specific attention has been given to the contribution of the two force-grading mechanisms, the recruitment of motor units and the modulation of firing frequency in motor units that have already been recruited. The recruitment order of the m. biceps motor units under study was the same during isometric contractions and slow isotonic movements. However, the recruitment thresholds of the m. biceps units were considerably lower for both isotonic flexion and extension movements, even at velocities as low as 2 deg/s, than for isometric contractions. Furthermore, firing frequency at recruitment was found to depend on the motor task: at recruitment the motoneurone starts firing with a higher firing frequency during isotonic flexion movements and a lower firing frequency during isotonic extension movements than during isometric contractions. Two main conclusions can be drawn from these results. First of all, the concept of one single activation parameter (total synaptic drive?) cannot account for the motor-unit behaviour observed during our experiments: the relative contribution of the two forcegrading mechanisms is different for different tasks. Secondly, the distribution of activity among flexor motoneurone pools is different for isometric contractions and isotonic movements.

Postural adjustments induced by simulated motion of differently structured environments

SummaryWe have investigated how visual information of a scene, moving along the line of sight of a subject, affects postural readjustments made by a subject when instructed to maintain an upright posture. Two different types of stimulus patterns were presented each inducing a different optic flow field. In one case an optic flow field was induced by simulating motion of a subject relative to a wall and in the second case by stimulating motion of a subject through a tunnel. In both cases clear effects on postural balance were observed. It suggests that postural responses are invariant for the structure of the moving environment. The amplitude of the postural responses did not depend on the velocity of the simulated motion, and therefore did not depend on the absolute magnitude of the optic flow components. The amount of texture in the moving scene proved to be an important factor. In addition, it was found that the control of postural balance is not exclusively dominated by information provided by the peripheral part of the subjects visual field. Moreover, the results indicate that the divergence component in the optic-flow field alone is not sufficient to control posture in forward/backward direction.

Conditions determining early modification of motor programmes in response to changes in target location

SummaryArm movements were studied in response to double-step stimuli in two-dimensional space. The inter-stimulus interval (ISI) between the two successive target presentations varied randomly between 25, 50, 75 and 100 ms. When ISI was 25 or 50 ms movement trajectories were sometimes modified by the second target already at the very onset of the movement. This modification was apparent from a change in the initial movement direction. A change in the initial movement direction occurred when the reaction time of the movement minus ISI was greater than 200 ms. The initial movement direction of these modified movements was in between the first and second target. To explain the results we conclude that 1) the internal representation of target position shifts gradually to its final position and 2) the motor programme uses as the goal of the movement this current internal representation that may not yet have reached that final position.

Differences in central control of m. biceps brachii in movement tasks and force tasks

SummaryMotor-unit activity in m. biceps brachii during isometric flexion contractions has been compared with motor-unit activity during a slow voluntary movements against constant or increasing preloads and b flexion contractions while movements were imposed by a torque motor. Recruitment levels and firing frequency behaviour of the motor units were found to be very similar when torques were generated during isometric contractions and during the imposed movements. However, these characteristics of the biceps motor units were quite different during the slow voluntary movements. It is suggested that the central activation of the α and/or γ motoneurone pools of m. biceps brachii is different for force tasks and slow movement tasks, even if the same torques are exerted and/or movements are made.

Inhomogeneous activation of motoneurone pools as revealed by co-contraction of antagonistic human arm muscles.

SummaryThere is now considerable evidence that motoneurone pools in several human arm muscles are activated inhomogeneously during isometric and reflex-induced contractions in different directions (van Zuylen et al. 1988; Gielen et al. 1988; ter Haar Romeny et al. 1984). In this paper we investigate the activation of antagonist muscles (m. brachialis, m. brachioradialis, m. biceps and m. triceps) during cocontraction of the upper arm muscles. The results show that there is a marked difference between the distribution of the activities of synergistic flexor muscles, or even within these muscles, in co-contraction tasks and in flexion tasks. This discrepancy may be attributed to the existence of inhibitory mechanisms between motoneurone pools of antagonist muscles. These mechanisms can also account for different types of recruitment behaviour of motor unit populations in a single muscle.

The attainment of target position during step-tracking movements despite a shift of initial position.

SummarySubjects performed a step-tracking forearm movement at maximum velocity without visual guidance. A considerable shift of initial forearm position, which remained unnoticed by the subject, was induced by vibration of the biceps tendon. Notwithstanding the shift of initial position subjects attained the correct final position, irrespective of whether the vibration was switched off before the movement or continued during the movement. Recordings of biceps and triceps electromyograms show that muscle activities were programmed to produce a movement from the actual initial position to the target position. The findings indicate that correct information on forearm position is available to the central nervous system at a subconscious level even if position perception is disturbed through vibration.

Intrinsic and afferent components in apparent muscle stiffness in man.

The subject was asked to bring his lower arm to a prescribed position against an applied torque. He was instructed not to intervene when the torque was unexpectedly changed and this resulted in a movement to a new position. During these experiments motor unit activity of m. biceps or triceps brachii was recorded. It was found that changes in the torque are compensated by reflex-induced recruitment of motor units and variations in their firing frequency. Recordings of motor unit activity revealed that in the range investigated the firing frequency of the motoneurones varied approximately linearly with the applied torque. From the results, it is concluded that afferent input plays a more important role in position control than intrinsic muscle properties. It is argued that the afferent feedback is controlled by central activation of the gamma-motoneurones.

The contribution of afferent information on position and velocity to the control of slow and fast human forearm movements

SummaryWe applied vibration at various rates to the biceps tendon of a passive, restrained arm in normal human subjects and measured its effect on the perception of forearm position and the perception of forearm velocity. The disturbances of limb position perception and limb velocity perception depended on the vibration rate in distinctly different ways. We thereupon applied vibration at various rates to the biceps tendon during the performances of non-visually-guided slow and fast forearm movements. The vibration-rate-dependence of the disturbance of slow movements matched the vibration-ratedependence of the disturbance of limb position perception. The vibration-rate-dependence of the disturbance of fast limb movements matched the vibration-rate-dependence of the disturbance of limb velocity perception. It is concluded that afferent position information is dominant in the control of slow movements, whereas mainly afferent velocity information is used in the control of fast movements.