Lennart Grimby

The fatigue and voluntary discharge properties of single motor units in man.

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.

Firing rate of the lower motoneuron and contractile properties of its muscle fibers after upper motoneuron lesion in man.

We studied motor unit (MU) firing rate and contractile properties and myosin isoform composition of single muscle fibers after upper motoneuron lesion. Single MUs and surface electromyogram (EMG) were recorded during voluntary contractions and locomotion in the paretic (P) and nonparetic (NP) tibialis anterior (TA) of 15 hemiparetics. P TA low‐threshold MUs fired within the lower end of their normal range. High‐threshold MUs fired below their normal range or were not recruited. Surface EMG was abnormally low and high in the P TA and NP TA, respectively. On muscle cross sections stained with histochemical methods, type I fibers represented 99.4%, 74.3%, and 66.6% of NP, P, and control TA, respectively. P TA fibers expressing type I myosin heavy chain (MyHC) were smaller, weaker, and slower. In conclusion, low MU firing rate and activity in the P TA was associated with slower type I MyHC fibers, while increased activity in NP TA resulted in homogeneous expression of type I MyHC.

Contraction time and voluntary discharge properties of individual short toe extensor motor units in man.

1. The contraction time and the voluntary discharge properties of forty‐five short toe extensor motor units were studied in man. 2. The contraction time of the individual motor unit was studied by using selective electrical nerve stimulation or by averaging the increase in force related to its electromyographic potential in tonic voluntary contraction. 3. Both methods showed a range of contraction times from 40 to 90 ms. 4. The discharge properties of the individual motor unit were studied with e.m.g. techniques, permitting the identification of its potentials during maximum voluntary effort. 5. A motor unit which could be driven continously and had a minimum rate of about 10/s and a maximum rate of about 30/s had a contraction time between 60 and 90 ms. 6. A motor unit which could not be driven continously and had a minimum rate of about 20/s and a maximum rate above 40/s had a contraction time between 40 and 55 ms. 7. A motor unit with intermediate voluntary discharge properties had an intermediate contraction time. 8. It is concluded that each motor unit fires at its fusion frequency in voluntary contraction and that the voluntary discharge frequency range of a motor unit can be used as an indication of its contraction time.

Axonal conduction velocity and voluntary discharge properties of individual short toe extensor motor units in man.

1. The axonal conduction velocity and the voluntary discharge properties of 120 short toe extensor motor units were studied in man. 2. Electromyographic techniques were used which permitted the identification of individual motor unit potentials after proximal and distal electrical nerve stimulation and during maximum voluntary effort. 3. The necessary selectivity of the e.m.g. recordings was achieved in two circumstances. In some subjects, previous motor nerve lesions distal to the point of stimulation had led to collateral sprouting with larger motor unit potentials. In other subjects an accessory deep peroneal nerve was present, so that lidocaine block of the main motor nerve left a small number of innervated motor units. 4. The axonal conduction velocities of the individual motor units ranged from 30 to 54 m/sec with most motor units between 35 and 45 m/sec. 5. Motor units which voluntarily could be driven continuously at frequencies below 10/sec had axonal conduction velocities between 30 and 45 m/sec. 6. Motor units which on voluntary drive responded only in high frequency bursts had axonal conduction velocities between 40 and 54 m/sec. 7. Motor units with intermediate voluntary discharge properties had intermediate axonal conduction velocities. 8. Thus a relationship was established between voluntary discharge properties and axonal conduction velocity.

Disuse of anterior tibial muscle during locomotion and increased proportion of type II fibres in hemiplegia

The tibialis anterior (TA) is a muscle activated mainly during walking. Its use during the step cycle was studied in 10 patients (55.8 +/- 8.8 years) with chronic hemiplegia (duration 3-18 years) and related to the muscle fibre composition, size and expression of isoforms of myosin heavy chains (MHCs). In the average step cycle the integrated surface EMG of the paralysed TA did in the majority of the hemiplegic patients not exceed 10% of that recorded during maximal contraction of the normal leg. The type I fibre percentage in the paralysed TA subject was 57.4% as compared with 79.4% in normal muscles (P less than 0.05). The range of axonal conduction velocities in the peroneal nerve did not differ in paralysed and non-paralysed leg, suggesting that there was no selective loss of one class of motoneurons. The type II fibres consisted of IIA (66%) and IIB (31%), in contrast to the normal TA muscle where less than 1% of the muscle fibres are of type IIB. The incidence of fibres in the biopsies with both slow and fast MHCs had a mean value of 3.5% (range 0.7-9%). The type I and type II muscle fibres had normal sizes with cross-sectional area 4511 +/- 962 microns 2 and 6181 +/- 1062 microns 2. No selective type II atrophy was seen. Occasional hypertrophic type I and II fibres were seen in 4 patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractile properties in single muscle fibres from chronically overused motor units in relation to motoneuron firing properties in prior polio patients

The relation between motoneuron firing rate in vivo and maximum velocity of unloaded shortening (Vmax) and myosin isoform composition in single chemically skinned muscle fibres was investigated in chronically overused motor units. Ten patients with loss of a large proportion of the motoneuron pool due to a prior polio lesion and compensatory overuse of residual neurones were studied and compared with normal individuals. The tibialis anterior muscle (TA) was chosen and prior polio patients who used all residual TA motor units at high rates during the normal step cycle were selected. In prior polio patients, all motor units fired at approximately 40 Hz when maximum voluntary force was reached. A common firing rate of 30 Hz yielded 70-90% maximum force. In normal subjects, on the other hand, maximum TA force was reached when low threshold units fired at 25-30 Hz and high threshold units at 50 Hz. Myosin heavy chain (MHC) and light chain (MLC) isoforms were resolved by 6% and 12% sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), respectively, and quantified densitometrically. In the whole biopsy cross-sections, types I, IIA and IIB MHC proportions were 97, 3 and 0% in a typical prior polio patient and 65, 25 and 10% in an age- and sex-matched control subject. Vmax differed significantly (p < 0.001) between type I fibres from the patient (0.54 +/- 0.12 ML/s) and the control subject (0.29 +/- 0.08 ML/s). The composition and relative contents of essential and regulatory MLC isoforms differed in single type I MHC fibres from the control subject and prior polio patient. 65% of the fibres co-expressed the fast and slow isoform of the regulatory light chain (MLC2) in the patient, while this combination was only observed in one of the control type I fibres. All prior polio fibres with a Vmax higher than 0.45 ML/s, except one, co-expressed MLC2s and MLC2f and the only control fibre co-expressing the slow and fast MLC2 isoform had the highest Vmax (0.50 ML/s) among control fibres. On the other hand, a high relative content of MLC3 was not associated with a high Vmax in type I MHC fibres. It is suggested that the composition of fast and slow isoforms of MLC2 has a significant modulatory influence on Vmax within type I MHC fibres. This combination of MLCs and high Vmax in type I MHC fibres is probably induced by chronic motor unit overuse and an altered motoneuron firing pattern.

Motoneuron firing and isomyosin type of muscle fibres in prior polio.

In patients with prior polio there was an excessive use of remaining motor units and an absence of type II muscle fibres in the tibialis anterior (TA). In the present study, eight subjects with prior polio with more than 90% type I fibres in the TA were examined. The aim was to elucidate whether the lack of type II muscle fibres was due to a selective loss of motoneurons with high threshold and high axonal conduction velocity or due to a muscle fibre transition from type II to type I. There was no decrease of the proportion of motoneurons with high threshold and high axonal conduction velocity. Monoclonal antibodies against fast and slow myosin heavy chains (MHC) were used as histochemical markers and many muscle fibres of type I according to ATPase stainability showed a binding of both anti-fast and anti-slow MHC. It is suggested that the type I muscle fibre dominance in prior polio subjects with excessive use of TA during walking is due to a muscle fibre transition from type II to type I and not to a loss of one class of motor units.

Fatigue of chronically overused motor units in prior polio patients.

This study was undertaken to investigate the mechanisms underlying fatigue of chronically overused motor units (MUs). The force of the tibialis anterior muscle (TA) and the firing properties of single MUs were studied during prolonged maximum voluntary effort in 10 prior polio patients selected such that daily living required all residual TA power. Almost all TA fibers were hypertrophic type I. Activities of intermyofibrillar succinate dehydrogenase (SDH) and calcium‐stimulated myofibrillar adenosine triphosphatase (ATPase) were measured in single TA fibers from a representative patient. Neither insufficient motoneuron activation nor peripheral blocking of the electrical impulse played a major role in the loss of force during prolonged contraction or for slow recovery after contraction. The ratio of SDH to calcium‐stimulated ATPase, representing the relation between energy resynthesis and energy utilization, was significantly (P < 0.001) lower in prior polio patients (0.230 ± 0.096) compared to control (0.515 ± 0.097) type I fibers.

Differences in recruitment order of motor units in phasic and tonic flexion reflex in `spinal man'

The recruitment order of motoneurones in muscle contractions has been held to be largely constant and determined by the size of the cell. However, as shown in a previous investigation using electromyographic techniques, the order in which different motor units are activated during voluntary muscle contractions changes in normal human subjects on shifts from phasic to tonic contraction. In order to investigate these two types of activity also in cases in which the cerebral influence on the motoneurone pool is blocked, an analysis was made of the recruitment order in phasic and tonic flexion reflexes in 10 patients with total interruption of the spinal cord. The following four principles were found to apply and presumed to be generally valid for the isolated human spinal cord: (1) in the phasic exteroceptive reflex, the order of recruitment varies despite application of a standardized stimulus; (2) in the tonic reflex, the first unit to be recruited is usually the same even with widely different types of stimuli; (3) a shift from phasic to tonic reflex activation may result in considerable changes in recruitment order; (4) after facilitation by a subliminal long-lasting stimulus, the first unit to be recruited in the phasic reflex is also the first to be recruited in the tonic reflex. It is suggested that a tonic influence on the motoneurone pool is required for the presupposed constancy of the recruitment order.

Disturbances in the voluntary recruitment order of anterior tibial motor units in spastic paraparesis upon fatigue

The recruitment order of motor units in the tibialis anterior muscle upon fatigue of tonic voluntary contraction was studied in 20 patients with severe spastic paraparesis. An electromyographic technique for secure identification of single motor units was used. Before fatigue the recruitment order is stable and low-frequency units are recruited before high-frequency units; this recruitment pattern agrees with that in normal voluntary activity. When fatigue appears, however, the recruitment order becomes indefinite and high-frequency units can be recruited before low-frequency units; this recruitment pattern agrees with that in normal phasic voluntary activity. Finally, all voluntary activation power disappears, even in the case of units which have never been active. Contraction ability and original recruitment order are restored upon rest but also upon tonic reflex support of the voluntary drive. Whether the fatigue reaction may be due to insufficient gamma motoneurone innervation and whether it is related to spasticity are discussed. The practical physiotherapeutic implications are reviewed.