1. This study was designed to determine whether human thenar motor units can be classified into types by the same physiological criteria used for other mammalian limb motor units and to consider whether such classification is functionally relevant. 2. Contractile responses of 25 human thenar single motor units were examined when their motor axons were stimulated intraneurally at rates from 1 to 100 Hz and intermittently at 40 Hz in a conventional 2-min fatigue test. Twitch and tetanic forces were measured together with various indexes of contractile rate. 3. Twitch contraction times and subtetanic to maximum tetanic force ratios were both distributed continuously. "Sag" in tension was not evident in unfused force profiles. Thus these units could not be divided into fast and slow types by the use of traditional contractile rate criteria. 4. Most units were fatigue resistant, with force fatigue indexes (FI) ranging from 0.33 to 1.14. None could be classified as fatiguable (FI less than 0.25). Seven units (28%) fell into the fatigue-intermediate (FI = 0.25-0.75) category, whereas 18 units (72%) had FI greater than 0.75, i.e., they were fatigue-resistant units. However, these units could not be classified by conventional FI and contractile rate criteria, because fatigue-resistant and fatigue-intermediate units had similar contractile rates. 5. Additional FI were calculated to describe changes in contractile rate. During the fatigue test, units behaved in one of three ways, showing 1) little change in either force or rate; 2) contractile slowing during the contraction and relaxation phases, with little or no force loss; or 3) both force and rate reduction.(ABSTRACT TRUNCATED AT 250 WORDS)
[1]
D. Kernell,et al.
Properties of motor units in the first deep lumbrical muscle of the cat's foot
,
1975,
Brain Research.
[2]
D N Levine,et al.
Motor units in cat soleus muscle: physiological, histochemical and morphological characteristics
,
1974,
The Journal of physiology.
[3]
M. Johnson,et al.
Data on the distribution of fibre types in thirty-six human muscles. An autopsy study.
,
1973,
Journal of the neurological sciences.
[4]
B. Calancie,et al.
Limitations of the spike‐triggered averaging technique
,
1986,
Muscle & nerve.
[5]
R S Johansson,et al.
A comparison of human thenar motor-unit properties studied by intraneural motor-axon stimulation and spike-triggered averaging.
,
1990,
Journal of neurophysiology.
[6]
R E Burke,et al.
Motor unit types of cat triceps surae muscle
,
1967,
The Journal of physiology.
[7]
R. Johansson,et al.
Twitch properties of human thenar motor units measured in response to intraneural motor-axon stimulation.
,
1990,
Journal of neurophysiology.
[8]
R S Johansson,et al.
Measurement of contractile and electrical properties of single human thenar motor units in response to intraneural motor-axon stimulation.
,
1990,
Journal of neurophysiology.
[9]
D. Levine,et al.
Physiological types and histochemical profiles in motor units of the cat gastrocnemius
,
1973,
The Journal of physiology.