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1. The aim of this study was to examine the extent of muscle-unit force modulation due to motoneuron firing-rate variation in type-identified motor units of the cat medial gastrocnemius (MG) muscle, and to investigate the contribution of muscle-unit force modulation to whole-muscle force regulation. The motoneuron discharge patterns recorded from 8 pairs of motor units during 12 smoothly graded muscle contractions evoked by stimulation of the mesencephalic locomotor region (MLR) were used to reactivate those units in isolation to estimate what their force profiles would have been like during the evoked whole-muscle contractions. 2. For most motor units, muscle-unit force modulation was similar to motoneuron firing-rate modulation, in that muscle-unit force increased over a limited range (120-600 g) of increasing whole-muscle tension and was then maintained at a near maximal (> 70%) output level as muscle force continued to rise. Most muscle units also decreased their force outputs over a slightly larger range of declining whole-muscle force before relaxing. This second finding was best explained by the counterclockwise hysteresis recorded in the motor units' frequency-tension (f-t) relationships. 3. In those instances when whole-muscle force fluctuated just above the recruitment threshold of a motor unit, a substantial percentage (10-25%) of the change in whole-muscle force could be accounted for by force modulation in that motor unit alone. This finding suggested that few motor units in the pool were simultaneously simultaneously undergoing force modulation. To evaluate this possibility, the extent of parallel muscle-unit force modulation within the 8 pairs of simultaneously active motor units was evaluated. As with parallel motoneuron firing-rate modulation, the extent of parallel muscle-unit force modulation was limited to unit pairs of the same physiological type and recruitment threshold. In several instances, pairs of motor units displayed parallel motoneuron firing-rate modulation but did not show parallel muscle-unit force modulation because of the nature of the motor units' f-t relationships. 4. The limited extent of parallel muscle-unit force modulation seen in these experiments implies that the major strategy for force modulation in the cat MG muscle, involving contractions estimated to reach 30-40% of maximum, may be motor-unit recruitment rather than motor-unit firing-rate variation with resulting force modulation. Given, however, that the majority of motor units are already recruited at these output levels (< 40%), it is proposed that motor-unit firing-rate variation with resulting force modulation may take over as the major muscle force modulating strategy at higher output levels.

1. The recruitment order of 64 pairs of motor units, comprising 21 type-identified units, was studied during centrally evoked muscle contractions of the cat medial gastrocnemius (MG) muscle in an unanesthetized, high decerebrate preparation. Motor units were functionally isolated within the MG nerve by intra-axonal (or intramyelin) penetration with conventional glass microelectrodes. 2. Graded stimulation of the mesencephalic locomotor region (MLR) was used to evoke smoothly graded contractions, which under favorable conditions was estimated to reach 40% of maximum tetanic tension of the MG muscle. With this method of activation, 100% of slow twitch (type S) units, 95% of fast twitch, fatigue-resistant (type FR) units, 86% of fast twitch, fatigue-intermediate (type FI) units, and 49% of fast twitch, fatigable (type FF) units studied were recruited. 3. Motoneuron size as estimated by axonal conduction velocity (CV) was correlated with muscle-unit size as estimated by maximum tetanic tension (Po). Although the correlation between these properties was significant among type S and FR units, no significant correlation was found for these properties among type FI and FF units. 4. Motor-unit recruitment was ordered by physiological type (S > F, 100% of pairs; S > FR > FI > FF, 93% of pairs). Although none of the motor-unit properties studied predicted recruitment order perfectly, motor-unit recruitment was found to proceed by increasing Po (89% of pairs), decreasing contraction time (79% of pairs), decreasing fatigue index (80% of pairs), and increasing CV (76% of pairs). These percentages were significantly different from random (i.e., 50%). Statistically, all four motor-unit properties were equivalent in predicting recruitment order. These results are similar to those reported by other investigators for motor-unit recruitment order evoked from other supraspinal centers, as well as from peripheral sites. 5. When, however, motor-unit recruitment within pairs of motor units containing two fast-twitch (type F) units was examined, Po was a significantly better predictor of recruitment order than CV (85% vs. 52% of pairs). One explanation for this observation is that the correlation between Po and CV is high among type S, type FR units, and possibly among the lower-tension type FF units, but not among the remaining higher-tension type FF units. 6. The reproducibility of recruitment order in multiple contractions was investigated in 16 motor-unit pairs. Recruitment order was found to be variable in only three motor-unit pairs, all of which contained units of similar physiological type and recruitment threshold. 7. Analysis of recruitment order by pair-wise testing confirms the general conclusion reached in human studies that the muscle force level at recruitment for a motor unit is highly correlated with its strength. As an additional confirmation, the whole-muscle force level at recruitment for 41 units was measured in a series of contractions in which the rate of rise of muscle tension was limited to rates < 1,000 g/s. For these contractions, a significant correlation was found between muscle tension at recruitment and motor-unit Po.

1. The aim of this study was to examine the nature of motoneuron firing-rate modulation in type-identified motor units during smoothly graded contractions of the cat medial gastrocnemius (MG) muscle evoked by stimulation of the mesencephalic locomotor region (MLR). Motoneuron discharge patterns, firing rates, and the extent of firing-rate modulation in individual units were studied, as was the extent of concomitant changes in firing rates within pairs of simultaneously active units. 2. In 21 pairs of simultaneously active motor units, studied during 41 evoked contractions, the motoneurons' discharge rates and patterns were measured by processing the cells' recorded action potentials through windowing devices and storing their timing in computer memory. Once recruited, most motoneurons increased their firing rates over a limited range of increasing muscle tension and then maintained a fairly constant firing rate as muscle force continued to rise. Most motoneurons also decreased their firing rates over a slightly larger, but still limited, range of declining muscle force before they were derecruited. Although this was the most common discharge pattern recorded, several other interesting patterns were also seen. 3. The mean firing rate for slow twitch (type S) motor units (27.8 imp/s, 5,092 activations) was found to be significantly different from the mean firing rate for fast twitch (type F) motor units (48.4 imp/s, 11,272 activations; Student's t-test, P < 0.001). There was no significant difference between the mean firing rates of fast twitch, fatigue-resistant (type FR) and fast twitch, fatigable (type FF) motor units. When the relationship between motoneuron firing rate and whole-muscle force was analyzed, it was noted that, in general, smaller, lower threshold motor units began firing at lower rates and reached lower peak firing rates than did larger, higher threshold motor units. These results confirm both earlier experimental observations and predictions made by other investigators on the basis of computer simulations of the cat MG motor pool, but are in contrast to motor-unit discharge behavior recorded in some human motor-unit studies. 4. The extent of concomitant changes in firing rate within pairs of simultaneously active motor units was examined to estimate the extent of simultaneous motoneuron firing-rate modulation across the motoneuron pool. A smoothed (5 point sliding average) version of the two motoneurons' instantaneous firing rates was plotted against each other, and the slope and statistical significance of the relationship was determined. In 16 motor-unit pairs, the slope of the motoneurons' firing-rate relationship was significantly distinct from 0. Parallel firing-rate modulation (< 10-fold difference in firing rate change reflected by a slope of > 0.1) was noted only in pairs containing motor units of like physiological type and then only if they were of similar recruitment threshold. 5. Other investigators have demonstrated that changes in a motoneuron's "steady-state" firing rate predictably reflect changes in the amount of effective synaptic current that cell is receiving. The finding in the present study of limited parallel firing-rate modulation between simultaneously active motoneurons would suggest that changes in the synaptic drive to the various motoneurons of the pool is unevenly distributed. This finding, in addition to the findings of orderly motor-unit recruitment and the relationship between motor-unit recruitment threshold and motoneuron firing rate, cannot be adequately accommodated for by the existing models of the synaptic organization in motoneuron pools. Therefore a new model of the synaptic organization within the motoneuron pool has been proposed.