Histomorphological evidence of muscle tissue damage and recording area using coiled and straight intramuscular wire electrodes

While intramuscular wire electrodes (IWE) for the measurement of neuromuscular function offer high spatial resolution for examining single motor unit activity, the resulting damage to muscle tissue and mechanical instability should be considered. We examined the influence of IWE type and component parts on muscle damage using light microscopy in rats and confirmed that intramuscular pressure influences the mechanical stability of IWE. Three types of electrode, coiled electrodes with or without suture material inside and a straight electrode, were inserted into the soleus, gastrocnemius and tibialis anterior muscles. Transverse serial sections (5 μm) of these muscles in the vicinity of the electrodes were stained with haematoxylin and eosin. Less structural damage was observed in the vicinity of the recording points (leading-off surface; 50 μm diameter) for all electrode types compared to the electrode body. No differences in the extent of tissue damage were observed around the recording points for all electrodes. However, compared to straight electrodes, the extent of damaged tissue around the bodies of coiled electrodes was significantly (P < 0.0001) greater. The average distance between the recording points and the electrode body was <1 mm for all electrodes. Intramuscular pressure at rest and maximal twitch contraction were 1.1 ± 0.5 and 49.4 ± 4.0 mmHg, respectively. Coiled IWEs became well integrated with muscle fibres, stabilizing electrode localization and facilitating electromyographic recordings without causing significant muscle damage.