1. The discharge properties of single motor units during prolonged maximal voluntary effort have been studied using electromyographic recordings, mainly in the short big toe extensor muscle but also in the anterior tibial muscle. 2. The required selectivity of the e.m.g. recordings was achieved in the short big toe extensor muscle after previous mechanical lesions to the terminal nerve twigs and muscle fibres and consequent collateral sprouting, and in the anterior tibial muscle with the use of a high impedance wire electrode. 3. During the first few hundred milliseconds of sustained maximal effort the motor units fired at rates ranging from about 30‐60 Hz, and the tension was the same as that obtained on electrical tetanization of the nerve to the toe extensor muscles above 50 Hz. 4. During prolonged maximal effort the firing rates and the proportion of motor units firing successively decreased. Motor units initially firing at 30 Hz continued to fire tonically but at 15‐20 Hz. Motor units initially firing at 60 Hz ceased to fire tonically but could still be made to discharge phasically. The period of time during which all motor units responded tonically could be increased from some seconds up to 20 sec by long‐term training. 5. Motor units with a limited endurance fired at a lower tension in the early than in the late stages of maintained contraction. 6. It is suggested that motoneurones innervating slow twitch muscle fibres respond continuously to prolonged voluntary drive at rates sufficient for full fusion but that the threshold of motoneurones innervating fast twitch muscle fibres increases so that they finally mainly fire phasically thus protecting the peripheral excitation and contractile mechanisms from excessive exhaustion.

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The use of the musculus tibialis anterior during walking and the type and size of its muscle fibers were determined in 15 young normal subjects (29 +/- 4 years) and in 13 old healthy subjects (70 +/- 3 years). In the average step cycle 16 +/- 6% of the maximal surface EMG was recorded during the swing phase and 44 +/- 15% during 100 msec at the heel strike. Single motor unit recordings showed that low-threshold units fired at 10-15 Hz during the swing and at 20-25 Hz during the heel strike peak. Usually, high-threshold units did not participate in the ordinary step cycle. A significantly higher mean use was found in the old subjects as compared with the young ones presumably due to lower muscle power in relation to body weight. In the young subjects 76 +/- 7% of the muscle fibers were type 1 and 22 +/- 7% were type 2A. In the old subjects 84 +/- 9% were type 1 and 15 +/- 8% were type 2A. In the young subjects the mean cross-sectional area of type 1 fibers was 3950 +/- 950 microns2 and of type 2 fibers was 8070 +/- 1850 microns2. In the old subjects the corresponding numerals were 4050 +/- 890 microns2 and 5700 +/- 1970 microns2, respectively. The significance of physiological variations in use for muscle fiber type composition and size is discussed.

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