Changes in reciprocal and transjoint inhibition induced by muscle fatigue in man

Utilising redundancy in musculoskeletal systems for adaptive stiffness and muscle failure compensation: a model-free inverse statics approach

Vertebrates possess a biomechanical structure with redundant muscles, enabling adaptability in uncertain and complex environments. Harnessing this inspiration, musculoskeletal systems offer advantages like variable stiffness and resilience to actuator failure and fatigue. Despite their potential, the complex structure presents modelling challenges that are difficult to explicitly formulate and control. This difficulty arises from the need for comprehensive knowledge of the musculoskeletal system, including details such as muscle arrangement, and fully accessible muscle and joint states. Whilst existing model-free methods do not need explicit formulations, they also underutilise the benefits of muscle redundancy. Consequently, they necessitate retraining in the event of muscle failure and require manual tuning of parameters to control joint stiffness limiting their applications under unknown payloads. Presented here is a model-free local inverse statics controller for musculoskeletal systems, employing a feedforward neural network trained on motor babbling data. Experiments with a musculoskeletal leg model showcase the controller's adaptability to complex structures, including mono and bi-articulate muscles. The controller can compensate for changes such as weight variations, muscle failures, and environmental interactions, retaining reasonable accuracy without the need for any additional retraining.

Changes in the input-output relation of the Hoffmann reflex recruitment curve after submaximal fatiguing contraction of the antagonist muscles

PURPOSE

The central nervous system adapts strategies to compensate the decreased motor capacities of a fatigued muscle. However, data on neurophysiological adaptations of muscles other than those under fatigue are scarce. The present study was designed to evaluate the effects of submaximal fatiguing contraction (leading to a task failure) induced in ankle dorsiflexors muscles on the excitability of the Hoffmann reflex (H-reflex) of an ankle plantarflexor (soleus muscle).

MATERIALS AND METHODS

Twenty-three physically active males (75.5 ± 8.3 kg; 1.77 ± 0.08 m; 27.0 ± 8.0 years) were asked to maintain the contraction level of the right ankle dorsiflexors at 60% of the maximal isometric voluntary contraction (MIVC). Task failure was defined when the force level dropped below 40% MIVC for 5 consecutive seconds. The input-output relation of the ascending limb of the recruitment curve of the soleus H-reflex was examined at 0 min, 5 min, 10 min, 15 min and 20 min after the task failure.

RESULTS

The amplitude parameter representing the first recruited motoneurons (threshold H-reflex - H@th) was significantly higher at 5 min, 10 min, 15 min and 20 min after task failure as compared to control (Hth) (p < 0.05). On the other hand, the parameter that represents the activation of the relatively higher threshold motoneurons (H@100) was reduced (as compared to control - H100), but only at 20 min after the task failure (p < 0.05).

CONCLUSIONS

These results suggest differential reflex modulation of the soleus H-reflex after fatigue of the ankle dorsiflexors, that probably reflects neuronal adaptations underlying motor control around the ankle joint.

Impairment in postural control is greater when ankle plantarflexors and dorsiflexors are fatigued simultaneously than when fatigued separately

The aim of the present study was to compare the effects of ankle muscle fatigue on postural control when plantarflexors (PFs) and dorsiflexors (DFs) are fatigued simultaneously compared with separately. This study also investigated the recovery of postural control after fatigue. Sixteen adults (eight women and eight men) performed postural trials before and after an isokinetic fatigue task involving either (i) only PFs (30°s(-1)), (ii) only DFs (120°s(-1)), or (iii) both PFs and DFs simultaneously. The fatigue task involved maximal contractions repeated until the torque produced decreased below 50% of the maximal torque. Postural trials lasted 30s and were performed on one leg with eyes open (EO) or eyes closed (EC). Sway area, medio-lateral (ML) and antero-posterior (AP) positions and velocities were calculated from the center of pressure displacements. With EO, no effect of fatigue was found on postural variables. With EC, sway area and AP velocity increased only when both PFs and DFs were fatigued simultaneously. An effect of fatigue present only when both muscle groups are fatigued simultaneously could be due to impairment in the compensatory activity between agonist and antagonist muscles and/or a greater decrease in proprioception due to a greater number of fatigued muscles. In addition, when PFs and DFs were fatigued simultaneously, sway area and AP velocity returned to pre-fatigue values within 2min, whereas a posterior shift in AP position persisted for 10min. This last result may suggest a longer-lasting change in postural strategy needed for optimal postural control.

Impact of precision grip tasks on cervical spinal network excitability in humans

Motor skill acquisition in the lower limb may induce modifications of spinal network excitability. We hypothesized that short-term motor adaptation in precision grip tasks would also induce modifications of cervical spinal network excitability. In a first series of experiments, we studied the impact of two different precision grip force control tasks (a visuomotor force-tracking task and a control force task without visual feedback) on cervical spinal network excitability in healthy subjects. We separately tested the efficacy of two key components of the spinal circuitry: (i) presynaptic inhibition on flexor carpi radialis (FCR) Ia terminals, and (ii) disynaptic inhibition directed from extensor carpi radialis (ECR) to FCR. We found that disynaptic inhibition decreased temporarily after both force control tasks, independently of the presence of visual feedback. In contrast, the amount of presynaptic inhibition on FCR Ia terminals decreased only after the visuomotor force tracking task. This temporary decrease was correlated with improved tracking accuracy during the task (i.e. short-term motor adaptation). A second series of experiments confirmed these results and showed that the visuomotor force-tracking task resulted also in an increase of the Hmax/Mmax ratio and the slope of the ascending part of the H-reflex recruitment curve. In order to address the role of presynaptic inhibition in the motor adaptation process, we conducted a third series of experiments during which presynaptic inhibition was recorded before and after two consecutive sessions of visuomotor force tracking. The results showed that (i) improved tracking accuracy occurred during both sessions, and (ii) presynaptic inhibition decreased only after the first session of visuomotor force tracking. Taken together, these results suggest thus that the nature of the motor task performed has a specific impact on the excitability of these cervical spinal circuits. These findings also suggest that early motor adaptation is associated with a modulation of presynaptic Ia inhibition in the upper limb.

Force Maintenance With Submaximal Fatiguing Contractions

Whereas many definitions of fatigue include externally measurable decrements in force or performance, fatigue can be present with no change in the external output of the muscle. The maintenance of submaximal forces can be considered a compromise between neuromuscular force enhancement and competing inhibitory influences. An example of a muscle facilitatory process includes postactivation potentiation that results in an increased sensitivity to Ca++. The neuromuscular system copes with metabolic disruption and subsequent loss of force by recruiting additional motor units and increasing the firing frequency. If the contraction persists, firing frequency may decrease so as to optimize the stimulus rate with the prolonged duration of the muscle fibre action potential (muscle wisdom). The insertion of additional neural impulses into the train of stimuli can result in force potentiation (catch-like properties). Furthermore, there is evidence of neural potentiation and a dissociation of muscle activity with submaximal fatigue. Conversely, inhibition may be derived supraspinally or at the spinal level. While there may be some evidence of intrinsic motoneuronal fatigue, inhibitory afferent influences from chemical, tensile, pressure, and other factors play an important role in the competing influences on force output. Key words: postactivation potentiation, recruitment, rate coding, inhibition, catch-like properties

Spinal interneurones; how can studies in animals contribute to the understanding of spinal interneuronal systems in man?

The first part of this review deals with arguments that the essential properties and organization of spinal interneuronal systems in the cat and in man are similar. The second part is concerned with the possibility that some interneuronal systems may be responsible for motor disturbances caused by spinal cord injuries and that these interneurones may be defined. This possibility is discussed with respect to the hyperexcitability of alpha-motoneurones and the exaggeration of stretch reflexes in spastic patients. To this end, what is known about cat spinal interneurones and about the neuronal basis and pharmacological treatment of spasticity, is put together. Interneurones in di- and trisynaptic reflex pathways from group II muscle afferents are singled out, since they are depressed by the alpha(2) noradrenaline receptor agonists clonidine and tizanidine, which is a critical feature of interneurones expected to contribute to exaggerated stretch reflexes which are reduced by alpha(2) noradrenaline receptor agonists. Recent observations that reflex excitation of extensor motoneurones from group II afferents is enhanced in spastic patients and that the pathologically strong reflex actions of group II afferents are reduced by clonidine and tizanidine support this proposal. On the other hand, a lack of effect of clonidine and tizanidine upon other types of excitatory or inhibitory interneurones argues against any major contribution of such interneurones to the abnormally strong responses of alpha-motoneurones to muscle stretch.

Alternate muscle activity observed between knee extensor synergists during low-level sustained contractions

To determine quantitatively the features of alternate muscle activity between knee extensor synergists during low-level prolonged contraction, a surface electromyogram (EMG) was recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) in 11 subjects during isometric knee extension exercise at 2.5% of maximal voluntary contraction (MVC) for 60 min (experiment 1). Furthermore, to examine the relation between alternate muscle activity and contraction levels, six of the subjects also performed sustained knee extension at 5.0, 7.5, and 10.0% of MVC (experiment 2). Alternate muscle activity among the three muscles was assessed by quantitative analysis on the basis of the rate of integrated EMG sequences. In experiment 1, the number of alternations was significantly higher between RF and either VL or VM than between VL and VM. Moreover, the frequency of alternate muscle activity increased with time. In experiment 2, alternating muscle activity was found during contractions at 2.5 and 5.0% of MVC, although not at 7.5 and 10.0% of MVC, and the number of alternations was higher at 2.5 than at 5.0% of MVC. Thus the findings of the present study demonstrated that alternate muscle activity in the quadriceps muscle 1) appears only between biarticular RF muscle and monoarticular vasti muscles (VL and VM), and its frequency of alternations progressively increases with time, and 2) emerges under sustained contraction with force production levels < or =5.0% of MVC.

The effect of a fatiguing exercise by the index finger on single- and multi-finger force production tasks

We studied the effects of fatigue, induced by a 60-s maximal isometric force production with the index finger, on multi-finger coordination and force production by the other fingers of the hand. Finger forces were measured during single- and multi-finger maximal voluntary force production (MVC) at two sites, the middle of the distal or the middle of the proximal phalanges. Two fatiguing exercises involving force production by the index finger were used, one at the distal phalanx and the other at the proximal phalanx. The MVC of the index finger dropped by about 33% when it was produced at the site involved in the fatiguing exercise. In addition, large transfer effects of fatigue were observed across sites of force application and across fingers. Force deficit increased under fatigue, especially due to a drop in the recruitment of the index finger. Under fatigue, the index finger was less enslaved during force production by other fingers. During multi-finger tasks, the percentage of total force produced by the index finger was significantly reduced after the fatiguing exercise. The principle of minimization of secondary moments was violated under fatigue. We suggest that the most impaired (fatigued) finger shows less interaction with other fingers or, in other words, is being progressively removed from the multi-finger synergy. Some of the observed changes in finger coordination suggest effects of fatigue at a central (neural) level.