Measurement of force and both surface and deep M wave properties in isolated rat soleus muscles

In isolated soleus muscles of 4-wk-old rats, M wave parameters were recorded with surface and deep recording electrodes and examined in relation to both twitch and tetanic force. Addition of ouabain (10(-5) M for 16 min) to isolated muscles caused an similar to 40% decrease in twitch amplitude and area (P < 0.01) that was associated with a 98% decrease in surface M wave amplitude, a 78% decrease in deep M wave amplitude (both P < 0.001), a 98% decrease in surface M wave area (P < 0.01), 48% of which occurred within 60 s of addition of ouabain (P < 0.05), and a 55% decrease in deep M wave area (P < 0.05). The decrease in twitch parameters on addition of ouabain was most closely correlated with deep M wave area (r = 0.92). Direct tetanic stimulation at a frequency of 30 Hz resulted in an initial potentiation of M waves, which was not seen at a frequency of 90 Hz. Instead, 90 Hz stimulation resulted in a prompt decrease in tetanic force that was correlated with a decrease in both deep M wave amplitude (r = 0.94; P < 0.01) and deep M wave area (r = 0.96; P < 0.01). It is concluded that simultaneous surface and deep recordings involving area and amplitude are fundamental to analysis of the effects of pharmacological agents on muscle performance and the use of M waves as predictors of muscle excitability.