Studies were made on the mechanism of high frequency fatigue with in vivo Phosphorus-31 Magnetic Resonance Spectroscopy (³¹P-MRS) and compound muscle action potential. The sciatic nerve of Wistar-Kyoto rats (WKY) was stimulated electrically at 67Hz and 4V to induce high frequency fatigue of the gastrocnemius and soleus muscles. Energy metabolism in the skeletal muscles was evaluated by ³¹P-MRS, and the compound muscle action potential (M wave) was measured to assess neuromuscular transmission. The energy level of the muscles was evaluated by the phosphocreatine (PCr)/{inorganic phosphate (Pi)+phosphomonoester (PME)} ratio, PCr/(Pi+PME), and the intracellular pH was calculated by chemical shift of Pi.
The initial tension of the muscles was 383.8±35.8 dynes. During the remainder of the periods of stimulation, the tension decreased gradually. The PCr/(Pi+PME) and the intracellular pH in the first 2min of stimulation rapidly decreased to 0.6±0.1 and 6.62±0.04, respectively. Later, both the PCr/(Pi+PME) and the intracellular pH returned with time. The percentage M wave amplitude of the control value of gastrocnemius muscle rapidly dropped to 33.7±10.2% in the first 2min of stimulation, which indicated neuromuscular transmission failure during stimulation, and later maintained lower levels of M wave amplitude during stimulation.
These results suggest that high frequency fatigue is ascribable not to a decreased energy level and intracellular pH of the muscles, but to neuromuscular transmission failure resulting from a raised threshold intensity of excitation-contraction coupling during stimulation.

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Motor unit firing behavior in human tibialis anterior (TA) muscle and extensor carpi radialis longus (ECRL) muscle was studied during controlled isometric contraction.
In TA muscle, during voluntary isometric contraction myoelectric activity was collected from healthy subjects using two quadrifilar electrodes inserted 3-4 inches apart. Motor units collected from each needle were decomposed separately to each motor unit action potential train, using the EMG signal decomposition technique developed by De Luca and colleagues. Crosscorrelation function was calculated from motor unit firing rate fluctuations. High crosscorrelation values were found between motor units collected within individual needle, but also between motor units collected from each needle. And about the relationship between the force at recruitment and the force at derecruitment, positive linear correlation was observed. So it is concluded that the nervous system does not control the firing rates of motor units individually, instead it acts on the pool of the homonymous motorneurons in a uniform fashion when the muscle acts in one direction wherever the motor unit locates in the muscle. Highly ordered recruitment and derecruitment scheme remains stable wherever the motor unit locates in the muscle. It suggests that the size principal act well when the muscle acts in one direction wherever the motor unit locates in the muscle.
In ECRL muscle, during voluntary isometric contraction myoelectric activity was also collected from healthy subjects using the same technique. And it is found that some motor units respond more to extension force and some more to abduction force. It is concluded that in multifunctional muscle the motor units are not activated homogeneously, because the input to the motorneurons of the ECRL is to be two dimensional and consist of an extend channel and abduct channel or to be multidimensional and consist of a spread of channels from extension to abduction and the channels have differences in the pattern of synaptic connections to the motorneurons. The behavior of motorneurons is consistent with a multichannel control of a muscle's motorneurons. It appears that the brain motor commands are patterned in terms of movements rather than in terms of muscles since the final path is not common, but rather involves different connectivities in the motorneuron pools for movements in different directions.

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It is said that plastic ankle foot orthoses (AFOs) may easily fail due to repeated deformation during walking. In order to improve the durability of plastic AFOs, investigation of the stress distribution in the AFOs is necessary. The purpose of the present study was to determine the relationship between the surface stress distribution in a shoe horn brace (SHB) and the sites of failure. The direction and the magnitude of the principal stresses acting on the SHB under plantarflexion and dorsiflexion loads (29.4N) were measured using the photoelastic technique. The results were as follows:
1) The principal stress trajectories showed that compression and tension stresses alternated along the trim line at the narrowest part of the SHB under plantarflexion and dorsiflexion loading. 2) The sites of stress concentration were the ankle joint area of the SHB during dorsiflexion and the narrowest part of the SHB during plantarflexion. 3) The maximum compression stress at the ankle joint area of the SHB was 90.3×10³N/m² during dorsiflexion, while the maximum tension stress at the narrowest part of the SHB was 76.8×10³N/m² during plantarflexion. The maximum stress caused by dorsiflexion was larger than that caused by plantarflexion. 4) These results indicated that there were two reasons why the SHB frequently failed at the narrowest part and at the ankle joint area. One was that the direction of the principal stresses alternated at the narrowest part of the SHB during plantarflexion and dorsiflexion, and the other was that the magnitude of stress was greatest at the ankle joint area during dorsiflexion.

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