Effects of a single bout of isometric neuromuscular electrical stimulation on rat gastrocnemius muscle: a combined functional, biochemical and MRI investigation

Neuromuscular electrical stimulation training induces myonuclear accretion and hypertrophy in mice without overt signs of muscle damage and regeneration

BACKGROUND

Skeletal muscle is a plastic tissue that adapts to increased mechanical loading/contractile activity through fusion of muscle stem cells (MuSCs) with myofibers, a physiological process referred to as myonuclear accretion. However, it is still unclear whether myonuclear accretion is driven by increased mechanical loading per se, or occurs, at least in part, in response to muscle injury/regeneration. Here, we developed a non-damaging protocol to evaluate contractile activity-induced myonuclear accretion/hypertrophy in physiological conditions.

METHODS

Contractile activity was generated by applying repeated electrical stimuli over the mouse plantar flexor muscles. This method is commonly referred to as NeuroMuscular Electrical Simulation (NMES) in Human. Each NMES training session consisted of 80 isometric contractions delivered at ∼15% of maximal tetanic force to avoid muscle damage. C57BL/6J male mice were submitted to either a short (i.e., 6 sessions) or long (i.e., 12 sessions) individualized NMES training program while unstimulated mice were used as controls. Histological investigations were performed to assess the impact of NMES on MuSC number and status, myonuclei content and muscle tissue integrity, typology and size.

RESULTS

NMES led to a robust proliferation of MuSCs and myonuclear accretion in the absence of overt signs of muscle damage/regeneration. NMES-induced myonuclear accretion was specific to type IIB myofibers and was an early event preceding muscle hypertrophy inasmuch as a mild increase in myofiber cross-sectional area was only observed in response to the long-term NMES training protocol.

CONCLUSION

We conclude that NMES-induced myonuclear accretion and muscle hypertrophy are driven by a mild increase in mechanical loading in the absence of overt signs of muscle injury.

Acute microcirculatory effects of medium frequency versus high frequency neuromuscular electrical stimulation in critically ill patients - a pilot study

BACKGROUND

Intensive care unit-acquired weakness (ICUAW) is a common complication, associated with significant morbidity. Neuromuscular electrical stimulation (NMES) has shown promise for prevention. NMES acutely affects skeletal muscle microcirculation; such effects could mediate the favorable outcomes. However, optimal current characteristics have not been defined. This study aimed to compare the effects on muscle microcirculation of a single NMES session using medium and high frequency currents.

METHODS

ICU patients with systemic inflammatory response syndrome (SIRS) or sepsis of three to five days duration and patients with ICUAW were studied. A single 30-minute NMES session was applied to the lower limbs bilaterally using current of increasing intensity. Patients were randomly assigned to either the HF (75 Hz, pulse 400 μs, cycle 5 seconds on - 21 seconds off) or the MF (45 Hz, pulse 400 μs, cycle 5 seconds on - 12 seconds off) protocol. Peripheral microcirculation was monitored at the thenar eminence using near-infrared spectroscopy (NIRS) to obtain tissue O2 saturation (StO2); a vascular occlusion test was applied before and after the session. Local microcirculation of the vastus lateralis was also monitored using NIRS.

RESULTS

Thirty-one patients were randomized. In the HF protocol (17 patients), peripheral microcirculatory parameters were: thenar O2 consumption rate (%/minute) from 8.6 ± 2.2 to 9.9 ± 5.1 (P = 0.08), endothelial reactivity (%/second) from 2.7 ± 1.4 to 3.2 ± 1.9 (P = 0.04), vascular reserve (seconds) from 160 ± 55 to 145 ± 49 (P = 0.03). In the MF protocol: thenar O2 consumption rate (%/minute) from 8.8 ± 3.8 to 9.9 ± 3.6 (P = 0.07), endothelial reactivity (%/second) from 2.5 ± 1.4 to 3.1 ± 1.7 (P = 0.03), vascular reserve (seconds) from 163 ± 37 to 144 ± 33 (P = 0.001). Both protocols showed a similar effect. In the vastus lateralis, average muscle O2 consumption rate was 61 ± 9%/minute during the HF protocol versus 69 ± 23%/minute during the MF protocol (P = 0.5). The minimum amplitude in StO2 was 5 ± 4 units with the HF protocol versus 7 ± 4 units with the MF protocol (P = 0.3). Post-exercise, StO2 increased by 6 ± 7 units with the HF protocol versus 5 ± 4 units with the MF protocol (P = 0.6). These changes correlated well with contraction strength.

CONCLUSIONS

A single NMES session affected local and systemic skeletal muscle microcirculation. Medium and high frequency currents were equally effective.