Intramuscular Contributions to Low-Frequency Force Potentiation Induced by a High-Frequency Conditioning Stimulation

Figures

• 

  Figure 1. Increased low-frequency force potentiation after a high-frequency conditioning tetanus in human plantar flexors. (A) Original records of the force signal illustrating the low-frequency force potentiation at an ankle angle of 90°. (B) Mean data (± SD) (n = 11) of the relative low-frequency force potentiation induced by the conditioning 100-Hz tetanus at short, intermediate, and long muscle length (i.e., ankle angle of 105, 90, and 75°, respectively) showed no length-dependency in force potentiation. (C) Original traces of soleus electromyographic activity from the same subject as in (A), recorded at the time indicated by the vertical arrows in (A). Note that no obvious difference was observed between the M wave following each stimulation artifact before (full line) and after (dashed line) the 100-Hz stimulation. (D) Mean data (±SD) (n = 8) of relative changes in soleus M-wave amplitude after the conditioning 100-Hz stimulation at the three muscle lengths showed no length-dependent difference in membrane excitability. *Significantly increased relative to the initial low-frequency tetanus, p < 0.05.

• 

  Figure 2. The increase in low-frequency force and [Ca2+]i after a high-frequency conditioning tetanus is not muscle length-dependent in mouse flexor digitorum brevis intact single fibers. Original recordings of the force (A) and [Ca2+]i (B) during stimulation in a mouse flexor digitorum brevis single fiber, and mean (±SD) values (n = 8) of the relative increase in force (C) and [Ca2+]i (D) for the low-frequency tetanus following the high-frequency conditioning tetanus. The horizontal dashed lines in the [Ca2+]i recording reflect the mean [Ca2+]i. *Significantly increased relative to the initial low-frequency tetanus at each length, p < 0.05.

• 

  Figure 3. Increased tetanic [Ca2+]i can increase low-frequency force. Original recordings of the force (A) and [Ca2+]i (B) from a mouse flexor digitorum brevis single fiber during stimulation which introduced a 200-ms pause between the high-frequency conditioning tetanus and the following low-frequency tetanus. Mean (±SD) values of the relative change in low-frequency force and tetanic [Ca2+]i (n = 10) show that the increased force corresponds to increased tetanic [Ca2+]i (C). Original recordings of the force (D) and [Ca2+]i (E) under a stimulation paradigm that minimally increased tetanic [Ca2+]i by evoking a brief 100-ms 100-Hz tetanus. (F) Mean (±SD) values of the relative change in low-frequency force and tetanic [Ca2+]i (n = 3) show that force is not significantly increased when the increase in tetanic [Ca2+]i is minimal. The horizontal dashed lines in the [Ca2+]i recording reflect the mean [Ca2+]i. *Significantly increased relative to the initial low-frequency tetanus, p < 0.05.

• 

  Figure 4. Increased low-frequency force without increased tetanic [Ca2+]i in mouse single fibers. Original recordings of force and [Ca2+]i in mouse flexor digitorum brevis intact single fibers cooled to 18°C during the stimulation sequence (1 s at 20 Hz, 100 ms at 100 Hz, and 1 s at 20 Hz) without (A) or with (B) a 200-ms pause following the 100-Hz tetanus. Mean (±SD) values showing relative increases of force and tetanic [Ca2+]i (n = 4) with and without a 200-ms pause (C). The horizontal dashed lines in the [Ca2+]i recording reflect the mean [Ca2+]i. Arrows indicate the 500-ms time point from the start of the 20-Hz tetanus where force was measured. *Significantly different force between stimulations without and with a pause after the 100-Hz tetanus, p < 0.05.

• 

  Figure 5. Low-frequency force potentiation is abolished by adding a 200-ms pause after the high-frequency conditioning stimulation in human plantar flexors. Original recording of the force (A) and mean (±SD) values (n = 6) (B) show that the low-frequency force is not different before and after the high-frequency stimulation.