The relation between force and integrated electrical activity in fatigued muscle

Abstract

It has been shown by Lippold (1952) and Bigland, Hutter & Lippold (1953) that the force of an isometric contraction of a mammalian muscle is proportional to the voltage-time integral of the electrical activity recorded from it. For practical use to be made of these observations, it is necessary to determine the limits within which this relationship holds.If, for instance, a voluntary contraction is to be maintained at a constant tension, it might be expected that more motor units would be recruited as the tension developed by fatigued units declined. Such an occurrence would alter, with the degree of fatigue, the proportionality between electrical activity and tension. Other factors in fatigue may well influence the relation, such as neuromuscular block (Brown & Burns, 1949) or the effect of activity on the size of action potentials (Brown & von Euler, 1938).The experiments to be reported here were designed to investigate the relation between the electrical activity and the tension of isometrically contracting human muscle under conditions of fatigue. METHODSThe tension in the calf muscles of human subjects was measured as described before (Lippold, Naylor & Treadwell, 1952). Action potentials were recorded by means of silver suction electrodes placed over the medial border of soleus muscle, where this is subcutaneous, and were integrated electronically, the integral being displayed by a Dekatron scaling unit. Calibration confirmed that the count in 5 sec was proportional to the input voltage, within the range utilized in the experiment. A parallel channel recorded the electromyogram and its integral (as pips) on moving paper to enable a subsequent check to be made. RESULTSSubjects were first required to make 5 sec periods of isometric contraction at a series of constant tensions. These were usually in steps of approximately 3 % over a range of 9-45 % of the maximum voluntary strength. During each 5 sec period the electrical activity was recorded. After this procedure a con-

Rehabilitative Ultrasound Imaging of the Lower Trapezius Muscle: Technical Description and Reliability

“…EMG has been used by many investigators to analyze scapular muscle activity in symptomatic groups and controls. However, EMG has several limitations for clinical use, including difficulty of use and interpretation, cross-talk of signals between muscles, dissociation from force when muscle is fatigued, 9 and access to equipment. The advantages of RUSI over EMG are that the imaging technique is more widely accessible, quick to use, and can easily differentiate between adjacent muscles.…”

Shear elastic modulus can be used to estimate an index of individual muscle force during a submaximal isometric fatiguing contraction

“…tigue induces changes in the shape of the motor unit action potentials due to changes in the shape of the intracellular action potential and/or in the propagation velocity of these potentials. Because these changes do not relate fully to the change in the motor unit twitch forces (9,11), EMG amplitude (i.e., sum of motor unit action potentials) and force level (i.e., sum of motor unit twitches) change differently with fatigue.…”

“…5). This impairment in the precision of the force estimation may be explained by the alteration of the EMG activity level-force relationship when fatigue occurs (11). Indeed, fa- For each subject, RMS deviation of Tlim (RMSdevTlim) values (in %MVC) were averaged over 10 equal time-windows for both EMG and SSI sessions.…”

“…The relationship between muscle activity level assessed by surface electromyography (EMG) and force is thus altered in a way that depends on the task (12). Consequently, as initially suggested by experimental work (6,11,27) and recently confirmed by a modeling approach (10), EMG cannot be used to estimate individual muscle force during a sustained submaximal fatiguing exercise. Since information about muscle force is crucial to study muscle coordination strategies during tasks where neuromuscular fatigue occurs, alternative approaches are necessary.…”

Effects of Rate of Stimulation and of Fatigue on the Response to Neuromuscular Blocking Agents

“…This intensification precedes any diminution in twitch tension, for it occurs at a time when fatigue -of the neuromuscular junction only can be involved. In fatigue, the decrease of twitch tension is not due to a depression of neuromuscular transmission but to a decline in contraction strength as the biochemistry of the contractile process becomes defective (Merton, 1954;Edwards and Lippold, 1956). This mechanism clearly differs from that of blocking drugs which reduce the twitch tension during partial block by preventing some of the muscle fibres from responding to stimulation.…”