4 mA tDCS, Estrogen, and Leg Muscle Fatigability

Not Applicable

Completed

Conditions: Healthy

Interventions: Device: Sham transcranial direct current stimulation 4 mA
Device: Transcranial direct current stimulation 4 mA

Registration Number: NCT04471805

Lead Sponsor: University of Iowa

Brief Summary: The majority of transcranial direct current stimulation (tDCS) studies have failed to consider sex as a modulating factor. This neglect may partly account for the high inter-subject variability bemoaned by many tDCS investigators (e.g., approximately 50% of participants do not respond to tDCS) and has certainly delayed progress in the field. Therefore, research into how sex influences stimulation-related outcomes is vital to fully understand the underlying mechanisms of tDCS, which has shown great inconsistency.

Because of the menstrual cycle, the hormonal levels of women fluctuate considerably more than in men. Importantly, these hormonal variations might impact the efficacy of neuromodulatory tools, like tDCS. It is suggested that estrogen, which is high in the second follicular phase, reinforces excitatory mechanisms in the motor cortex. However, because anodal tDCS enhances cortical excitation there is also a possibility of excessive excitability. For instance, anodal tDCS may lead to overexcitation and non-optimal performance when it is applied in the second follicular phase of the menstrual cycle. Currently, there is a lack of knowledge on how the phases of the menstrual cycle affect tDCS performance outcomes in healthy young women because no studies have examined if and how the phases of the menstrual cycle alter tDCS efficacy.

This study is critical for determining the optimal time to administer anodal tDCS, and the ideal intensity for that administration, to achieve the most beneficial results. Furthermore, this investigation will emphasize the need for future tDCS studies to test women during the same menstrual cycle phase.

Detailed Description: The majority of transcranial direct current stimulation (tDCS) studies have failed to consider sex as a modulating factor. This neglect may partly account for the high inter-subject variability bemoaned by many tDCS investigators (e.g., approximately 50% of participants do not respond to tDCS) and has certainly delayed progress in the field. Therefore, research into how sex influences stimulation-related outcomes is vital to fully understand the underlying mechanisms of tDCS, which has shown great inconsistency.

Because of the menstrual cycle, the hormonal levels of women fluctuate considerably more than in men. There are two main phases of the menstrual cycle: 1) the follicular phase, characterized by low levels of estradiol and progesterone (first follicular phase, days 1-7) followed by increased levels of estradiol and low levels of progesterone (second follicular phase, days 7-14); and 2) the luteal phase (days 14-28), characterized by moderate estradiol and high progesterone levels. Importantly, these hormonal variations might impact the efficacy of neuromodulatory tools, like tDCS.

It is suggested that estrogen, which is high in the second follicular phase, reinforces excitatory mechanisms in the motor cortex. Thus, it appears that higher levels of estradiol increase cortical excitability. However, because anodal tDCS enhances cortical excitation there is also a possibility of excessive excitability. For instance, anodal tDCS may lead to overexcitation and nonoptimal performance when it is applied in the second follicular phase of the menstrual cycle. Currently, there is a lack of knowledge on how the phases of the menstrual cycle affect tDCS performance outcomes in healthy young women because no studies have examined if and how the phases of the menstrual cycle alter tDCS efficacy.

This research will be significant because the changing hormone levels during the different phases of menstruation in women is an especially important factor for minimizing response variability from tDCS. Thus, this study is critical for determining the optimal time to administer anodal tDCS, and the ideal intensity for that administration, to achieve the most beneficial results. Furthermore, this investigation will emphasize the need for future tDCS studies to test women during the same menstrual cycle phase.

Recruitment & Eligibility

Status: COMPLETED

Sex: Female

Target Recruitment: 10

Inclusion Criteria

Has a regular menstrual cycle
Young adult (18-35 years)
Right-side dominant
At least 30 min of moderate-intensity, physical activity on at least 3 days of the week for at least the last 3 months
Without chronic neurological, psychiatric, or medical conditions
Not taking any psychoactive medications.

Exclusion Criteria

Pregnant
Known holes or fissures in the skull
Metallic objects or implanted devices in the skull (e.g., metal plate)
Women on hormonal contraceptives/supplements.

Study & Design

Study Type: INTERVENTIONAL

Study Design: SINGLE_GROUP

Arm && Interventions

Group	Intervention	Description
Eumenorrheic Women	Transcranial direct current stimulation 4 mA	Participants will have the anode (active electrode) placed over the brain area that controls their dominant leg and the cathode (return electrode) above the ipsilateral eyebrow. tDCS is administered in the early follicular phase, late follicular phase, and mid-luteal phase of their menstrual cycle. tDCS: Stimulation is ramped up to 4 mA over the first 30 seconds and stays at 4 mA for the remainder of the simulation time. Sham: Stimulation is turned on (4 mA) for 30 seconds at the beginning and the end of the trial but stays at 0 mA in the intervening time.
Eumenorrheic Women	Sham transcranial direct current stimulation 4 mA	Participants will have the anode (active electrode) placed over the brain area that controls their dominant leg and the cathode (return electrode) above the ipsilateral eyebrow. tDCS is administered in the early follicular phase, late follicular phase, and mid-luteal phase of their menstrual cycle. tDCS: Stimulation is ramped up to 4 mA over the first 30 seconds and stays at 4 mA for the remainder of the simulation time. Sham: Stimulation is turned on (4 mA) for 30 seconds at the beginning and the end of the trial but stays at 0 mA in the intervening time.

Primary Outcome Measures

Name	Time	Method
Muscle Activity During the Strength and Fatigue Tests	Completed at each visit, spaced approximately 14 days apart for 2 consecutive months	Collect electromyographic (EMG; muscle activity) information during the fatigue tests. Muscle activity is measured as electrical signals/voltages. The muscle activity of the knee extensors (rectus femoris, vastus medialis, and vastus lateralis) was averaged to represent the cumulative activity of this muscle group. The first two repetitions of the fatigue test were considered adaptation repetitions and were removed. Therefore, the remaining 38 repetitions were used for the average EMG analyses. The subsequent 38 repetitions were also organized into 8 windows. The first seven windows consisted of five consecutive and non-overlapping repetitions (e.g., window 2 = reps 8-12; window 3 = reps 13-17, etc.) while the last (eighth) window was comprised of the final three repetitions.
Fatigue Index From the Isokinetic Fatigue Test	Completed at each visit, spaced approximately 14 days apart for 2 consecutive months	Perform 40 consecutive flexion and extension repetitions of the knee on the dominant leg. After a 10 minute rest, do the same task on the non-dominant leg. The fatigue index was calculated using the greatest torque from the relevant repetitions of the fatigue test as follows: (\[mean of reps 3 through 7-mean of last five reps\]/mean of reps 3 through 7) X 100 and is expressed as a percentage of decline in torque production.

Secondary Outcome Measures

Name	Time	Method
Estrogen Level	Completed at each visit, spaced approximately 14 days apart for 2 consecutive months	Staff nurses collected 4.5 mL of blood from the median cubital vein of the left arm (total volume collected per subject = 9 mL) for the estrogen assay. Samples were immediately analyzed for serum estrogen levels after the blood draws by University of Iowa Hospitals and Clinics Pathology technicians using an Electrochemiluminescence Assay (Roche Diagnostics, Basel, Switzerland). The estrogen assay had a lower limit of detection of 5 pg/mL and a coefficient of variation of 8%. Because menstrual cycles have great inter- and intrasubject variability, the peak estrogen levels of the subjects were not consistently found in the late-follicular phase, which is a common failing of menstrual cycle phase calendar estimation. Thus, estrogen levels were grouped as high or low according to each individual subject's estrogen serum levels, irrespective of the anticipated/targeted phase.