Extra force from asynchronous stimulation of cat soleus muscle results from minimizing the stretch of the common elastic elements

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

Age-related alterations of skeletal muscle are numerous and present inconsistently, and the effect of their interaction on contractile performance can be nonintuitive. Hill-type muscle models predict muscle force according to well-characterised contractile phenomena. Coupled with simple, yet reasonably realistic activation dynamics, such models consist of parameters that are meaningfully linked to fundamental aspects of muscle excitation and contraction. We aimed to illustrate the utility of a muscle model for elucidating relevant mechanisms and predicting changes in output by simulating the individual and combined effects on isometric force of several known ageing-related adaptations. Simulating literature-informed reductions in free Ca2+ concentration and Ca2+ sensitivity generated predictions at odds qualitatively with the characteristic slowing of contraction speed. Conversely, incorporating slower Ca2+ removal or a fractional increase in type I fibre area emulated expected changes; the former was required to simulate slowing of the twitch measured experimentally. Slower Ca2+ removal more than compensated for force loss arising from a large reduction in Ca2+ sensitivity or moderate reduction in Ca2+ release, producing realistic age-related shifts in the force-frequency relationship. Consistent with empirical data, reductions in free Ca2+ concentration and Ca2+ sensitivity reduced maximum tetanic force only slightly, even when acting in concert, suggesting a modest contribution to lower specific force. Lower tendon stiffness and slower intrinsic shortening speed slowed and prolonged force development in a compliance-dependent manner without affecting force decay. This work demonstrates the advantages of muscle modelling for exploring sources of variation and identifying mechanisms underpinning the altered contractile properties of aged muscle.

Spinal motoneurons of the human newborn are highly synchronized during leg movements

Motoneurons of neonatal rodents show synchronous activity that modulates the development of the neuromuscular system. However, the characteristics of the activity of human neonatal motoneurons are largely unknown. Using a noninvasive neural interface, we identified the discharge timings of individual spinal motoneurons in human newborns. We found highly synchronized activities of motoneurons of the tibialis anterior muscle, which were associated with fast leg movements. Although neonates' motor units exhibited discharge rates similar to those of adults, their synchronization was significantly greater than in adults. Moreover, neonatal motor units showed coherent oscillations in the delta band, which is directly translated into force generation. These results suggest that motoneuron synchronization in human neonates might be an important mechanism for controlling fast limb movements, such as those of primitive reflexes. In addition to help revealing mechanisms of development, the proposed neural interface might monitor children at risk of developing motor disorders.

Mitigation of excessive fatigue associated with functional electrical stimulation

OBJECTIVE

Restoration of motor function in paralyzed limbs using functional electrical stimulation (FES) is undermined by rapid fatigue associated with artificial stimulation. Typically, single electrodes are used to activate muscles with FES. However, due to the highly distributed branching of muscle nerves, a single electrode may not be able to activate the entire array of motor axons supplying a muscle. Therefore, stimulating muscle with multiple electrodes might enable access to a larger volume of muscle and thereby reduce fatigue.

APPROACH

Accordingly, we compared the endurance times that ankle dorsiflexion could be sustained at 20% maximum voluntary force using feedback controlled stimulation (25 Hz) of human tibialis anterior (TA) using one or four percutaneous intramuscular electrodes. In addition, we measured endurance times in response to direct stimulation of the nerve supplying TA and during voluntary contraction. In all sessions involving electrical stimulation, an anesthetic nerve block proximal to the site of stimulation was used to isolate the effects of stimulation and alleviate discomfort.

MAIN RESULTS

Endurance time associated with stimuli delivered by a single intramuscular electrode (84  ±  19 s) was significantly smaller than that elicited by four intramuscular electrodes (232  ±  123 s). Moreover, endurance time in response to nerve stimulation (787  ±  201 s) was not significantly different that that produced during voluntary contraction (896  ±  272 s).

SIGNIFICANCE

Therefore, excessive fatigue associated with FES is probably due to the inability of conventional FES systems to enlist the full complement of motor axons innervating muscle and can be mitigated using multiple electrodes or nerve-based electrodes.

Effect of stimulation frequency on force, net power output, and fatigue in mouse soleus muscle in vitro

The effects of electrical stimulation frequency on force, work loop power output, and fatigue of mouse soleus muscle were investigated in vitro at 35 °C. Increasing stimulation frequency did not significantly affect maximal isometric tetanic stress (overall mean ± SD, 205 ± 16.6 kN·m–2 between 70 and 160 Hz) but did significantly increase the rate of force generation. The maximal net power output during work loops significantly increased with stimulation frequency: 18.2 ± 3.7, 22.5 ± 3.3, 26.8 ± 3.7, and 28.6 ± 3.4 W·kg–1 at 70, 100, 130, and 160 Hz, respectively. The stimulation frequency that was used affected the pattern of fatigue observed during work loop studies. At stimulation frequencies of 100 and 130 Hz, there were periods of mean net negative work during the fatigue tests due to a slowing of relaxation rate. In contrast, mean net work remained positive throughout the fatigue test when stimulation frequencies of 70 and 160 Hz were used. The highest cumulative work during the fatigue test was performed at 70 and 160 Hz, followed by 130 Hz, then 100 Hz. Therefore, stimulation frequency affects power output and the pattern of fatigue in mouse soleus muscle.