Interferential current electro-massage (ICE) is defined as a technique which combines simultaneously manual therapy (massage) and ICT. We used a current bipolar mode, using a carrier frequency of 4000 Hz at constant voltage and an amplitude-modulated frequency of 100 Hz (Sonopuls 692®; Enraf-Nonius BV, Rotterdam, The Netherlands), was administered. The intensity was set to provide a strong and comfortable tingling, without evoking muscle twitches even though a slight vibration (fasciculation) was allowed. The sequence combined (A) superficial stroke over the neck-shoulder for 30-45 seconds; (B) deep sliding movements, alone or (C) combined with shoulder drop, for 4-5 minutes; (D) bilateral kneading of the upper trapezius (4-5 minutes); (E) slight stretching of cervical muscles (upper trapezius, sternocleidomastoid, and levator scapulae); and repetition of step (A). The electro-massage protocol lasted 15 minutes.

A mechanical pressure Fisher algometer (Force Dial model FDK 40) with a 1 cm² area contact head was used to measure the pressure pain threshold. The reliability of pressure algometry has been found to be high [intraclass correlation coefficient = 0.91 (95% confidence interval, 0.82-0.97)]. With the participant in supine, the pressure pain threshold of the the myofascial trigger point nº2 of the upper trapezius muscle according to Travell and Simons and the central trigger point of the sternocleidomastoid muscle was bilaterally evaluated. Also, in sitting position the pressure pain threshold of the myofascial trigger point of the scapula elevator muscle was bilaterally evaluated. The minimal clinically important difference (MCDI) is 1.2 Kg/cm2.

It indicates the sensitivity of short-term variability in HRV non-linear spectrum. It is considered an indicator of parasympathetic activity.

It is a diameter from Poincaré plot which indicates the degree of longitudinal dispersion. It is thought to reflect long-term changes in RR intervals and it is considered an inverse indicator of parasympathetic activity.

Minimum heart rate variability.

Maximum heart rate variability.

It corresponds to the interval between two beats (R peaks on the ECG).

Percentage of consecutive RR intervals that differ by more than 50 ms from each other. A high value of pNN50 provides valuable information about high spontaneous HR.

Situated between 0.04 and 0.15 Hz. In long-term recordings it provides us with more information about the activity of the SNS.

They are located between 0.15 and 0.4 Hz. HF is clearly related to PNS activity and has a relaxation-related effect on HR2.

From low frequency and high frequency ratio of the HRV spectral analysis result we can estimate the vagal (related to relaxation and HF) and sympathetic (related to stress and LF) influence. Thus we can estimate sympathetic-vagal balance.

The electromyography (EMG) signal of the upper trapezius muscle was recorded during 3 step contractions of shoulder elevation force (15%-30% maximal voluntary contraction). The highest value of the three contractions was taken. The signal of the sternocleidomastoid muscle was recorded during 3 billateraly step contractions of neck flexión and antepulsion neck force. Both contractions were performed in a combined and simultaneous movement, recreating the movement produced by the reaction forces in the takeoff and landing of the fighter jet. Both movements were made at 15%-30% maximal voluntary contraction, as used by Calamita et al on the same musculature in subjects with nonspecific neck pain.

The Spanish version of the TSK-11 was used to measure fear of movement. Higher scores indicate greater fear-avoidance behaviors. The TSK-11 has demonstrated acceptable internal consistency and validity.

The Pain Catastrophizing Scale (PCS) is a self-administered scale of 13 items and one of the most used to assess catastrophism of pain. The subjects take their past painful experiences as a reference and indicate the degree to which they experienced each of the 13 thoughts or feelings on a 5-point Líkert scale ranging from 0 (never) to 4 (always). The theoretical range of the instrument is between 13 and 62, indicating low scores, little catastrophism, and high values, high catastrophism.

S/PS is expressed as the quotient of SS and SD1, and it is considered to reflect autonomic balance - that is, the relationship between sympathetic and parasympathetic activity

It is an index to facilitate physiological interpretation of Poincaré plot. It is expressed as the inverse of the SD2 diameter multiplied by 1000 and is considered directly proportional to the sympathetic activity in the sinus node.

The degree of cervical disability involvement was measured through the Neck Disability Index (NDI), translated into Spanish, presenting optimum reliability and internal validity. It consists of 10 sections, 4 of them are related to subjective symptoms and the other 6 are related to basic activities of daily life.

Each of the sections presents 6 possible responses, scoring these from 0 to 5 according to progression of functional disability.

Scores less than 5 points indicate non-disability, between 5-14 points indicates mild disability, values between 30-48 points moderate disability, between 50-64 points severe disability and those that exceed 70 points represent complete disability.

It indicates the degree of activation of the Parasympathetic Nervous System on the cardiovascular system. It is obtained from the square root of the mean value of the sum of the squared differences of all successive RR intervals. This parameter reports the short-term variations of the RR intervals. It is directly associated with short-term variability.