Ballistic contractions in man: characteristic recruitment pattern of single motor units of the tibialis anterior muscle.

Abstract

SUMMARY1. Single motor units were recorded with highly selective electrodes from intact tibialis anterior muscle in the adult man. A detailed parametric analysis was made of the discharge patterns during voluntary isometric contractions of different peak forces carried out at various rates of force development.2. During the smooth tracking of a ramp force, the different motor units recorded from a given muscle site were recruited in a consistent order, each unit becoming active when the muscle developed a certain level of force. The threshold of some of the units in such slow ramp contractions exceeded 8 kg. By contrast, in brisk ballistic contractions reaching a peak force of 12 kg in less than 0 15 sec, the same motor units discharged in a transient burst which largely preceded the muscle force production.3. In slow tracking ramp contractions, the instantaneous frequency of single motor units was initially rather low (5-15/sec) and it increased as the ramp force augmented. By contrast, in (strong) ballistic contractions, the same units discharged at an unusually high instantaneous frequency (60-120/sec) early in the burst and the firing frequency decreased thereafter. Such hitherto unknown pattern appears characteristic of ballistic contractions and it was not found in even fast tracking ramp contractions achieving 12 kg in only 0 4 sec. 4. The potentials of the different motor units activated are rather crowded at intervals of a few msec in the early burst of a strong ballistic contraction and observations on the rank activation of the different motor units do not provide reliable data for the analysis of the recruitment order of units in ballistic contractions.5. A new method is described for estimating ballistic force threshold of single motor units. When a large series of brisk ballistic contractions with peak forces ranging from 005 to 12 kg was carried out any given motor J. E. DESMEDT AND E. GODA UX unit only became active when the ballistic peak force exceeded a certain reproducible value. A detailed analysis of the recruitment order based on these ballistic force thresholds showed it to be virtually identical to the recruitment order of the same units in slow tracking ramp contractions (correlation = 0.95).6. Ballistic contractions are graded in force both by the recruitment of additional motor units in stronger contractions, and by an increase in their rate of firing. These gradation mechanisms are discussed.

Motor Unit Recruitment Strategies Are Altered during Deep-Tissue Pain

“…This theory predicts recruitment of units from smallest to largest (Desmedt and Godaux, 1977;Veale, 1978). Departures in recruitment order have been identified due to nonphysiological stimuli (Garnett and Stephens, 1981;Semmler and Türker, 1994), changes in rate of force change, and in some voluntary tasks (Thomas et al, 1978;ter Haar Romeny et al, 1982;Riek and Bawa, 1992;Butler et al, 1999).…”

What is cough and what should be measured?

“…It has been demonstrated that the rate of rise of the integrated EMG activity reflects the rate of recruitment of motor units, as well as of the increase in their firing frequency, while the peak of the integrated EMG activity is an expression of the total number of units recruited and of their maximum firing frequency [30]. Thus, EMG recordings of expiratory muscle output during cough can provide insights into the neural mechanisms activated during voluntary and reflex cough and during the ER.…”

Changes in the recruitment threshold of motor units produced by cutaneous stimulation in man.

“…It is widely agreed that the recruitment order of motor units recorded during voluntary muscle contraction in man is fixed in that it cannot be altered at will, nor is it different during different sorts of fast ballistic or slow ramp contractions (Denny-Brown & Pennybacker, 1938;Tanji & Kato, 1973;Bfidingen & Freund, 1976;Desmedt & Godaux, 1977a, 1978. In spite of the known specialization of motor unit mechanical properties within individual muscles, the controlling motoneurone pool and its input would appear to be organized in such a way as to prevent rather than allow the potentially useful selective activation ofunits according to mechanical type.…”