Changes in firing rate of human motor units during linearly changing voluntary contractions

Abstract

SUMMARY1. Human subjects generated approximately linearly increasing or decreasing voluntary, isometric contractions using the first dorsal interosseus muscle of the hand.2. Single motor units began firing at 8-4+ 13 impulses/sec (mean + S.D. of an observation) and increased their firing rate 1*4 + 0-6 impulses/ sec for each change of 100 g in voluntary force. These values were independent of the threshold force for recruiting motor units.3. At intermediate rates of increasing and decreasing voluntary force (one complete cycle every 10 sec) the firing rate of single motor units varied linearly with force over the entire range of forces studied. However, during slow increases in voluntary force, the firing rate tended to reach a plateau, while during rapid increases an initial train of impulses at a roughly constant rate was observed.4. The relative importance of recruitment and increased firing rate, as mechanisms for increasing the force of voluntary contraction, was determined. Only at low levels of force is recuitment the major mechanism. Increased firing rate becomes the more important mechanism at intermediate force levels and contributes the large majority of force if the entire physiological range is considered.

Quantitative and Qualitative Evaluation of Knee Electromyograms by a Bluetooth-communication Gait Analyzer: Integration and Power Spectral Analysis of Surface Electromyograms

“…Also, Henneman et al 24) proposed the size principle that type I fibers with a smaller diameter are initially recruited at the beginning of muscle contraction followed by the recruitment of type II fibers with a larger diameter with increased contraction strength. In this study, we obtained a result in which there was increased gait velocity despite increased amounts of muscle action in the vastus medialis muscle, which is inconsistent with these earlier reports 23,24) . This suggests that an increase in gait speed forced the vastus medialis muscle to act as a braking muscle, while the biceps femoris muscle acted as an accelerating muscle.…”

“…Milner-Brown et al 23) noted that motor unit recruitment is dominant during weak muscle contraction, while an increased discharge frequency is increasingly apparent during stronger muscle contraction. Also, Henneman et al 24) proposed the size principle that type I fibers with a smaller diameter are initially recruited at the beginning of muscle contraction followed by the recruitment of type II fibers with a larger diameter with increased contraction strength.…”

Nonlinear surface EMG analysis to detect changes of motor unit conduction velocity and synchronization

“…Recruitment of MUs was assumed to take place at up to 80% of the maximal force, simulating a muscle that recruits MUs almost until the maximum contraction level, as is the case of the biceps brachii (26). The firing rates were inversely related to the recruitment threshold (34), with a minimum of 8 pulses/s at the recruitment and a maximum of 35 pulses/s. The standard deviation of the interpulse interval was fixed for all MUs to 15% of the mean interpulse interval (Gaussian distribution).…”

Mechanisms controlling accurate changes in elbow torque in humans

“…Consequently, corrections for overshooting the target were produced by decreasing agonist muscle activity. The differences in shape of overshoot and undershoot corrections might then be explained by parallel differences in the rise and relaxation times of a muscle twitch (Milner-Brown et al, 1973).…”