Proprioceptive Neuromuscular Facilitation (PNF) is a therapeutic approach with principles and procedures used to promote motor rehabilitation. One of the principles of PNF is motor irradiation, in which the application of resistance to a stronger body segment promotes muscle contraction in another weaker limb. This is useful in motor recovery when voluntary contraction is not possible, such as in cases of paralysis of a segment. Abdominal strength is fundamental to the effectiveness of various training and rehabilitation techniques. But, in clinical practice, it is commonly observed that people with greater abdominal strength produce weaker contractions in target muscle during the application of irradiation maneuvers.

To investigate the relationship between abdominal strength and the amplitude of activation of the biceps brachii muscle provoked by the execution of four FNP motor irradiation maneuvers.

A cross-sectional observational study was conducted with 33 healthy participants (19 women and 14 men, with 27 ± 5 years of age, 1.70 ± 0.09 m in height, 68.6 ± 14.47 kg in body mass, and BMI of 23.64 ± 3.90 kg/m²). Inclusion criteria were age between 18 and 45 years and agreement to sign the Free and Informed Consent Form. Individuals with uncontrolled cardiovascular conditions, musculoskeletal alterations, post-COVID sequelae, and other contraindications to the use of EMG were excluded. After signing the consent form, a screening session was held to identify the demographic characteristics of the sample and identify the non-dominant side. Next, the maximum voluntary contraction of each participant was measured. Then, surface electromyography (EMG) (Delsys trigno) of the biceps brachii on the non-dominant side was obtained, while the patterns of (i) flexion, adduction and external rotation with knee flexion in lateral decubitus; (ii) extension, adduction and internal rotation of the upper limb in dorsal decubitus; (iii) rotation of the lower trunk in prone; and (iv) pre-bridge position on the elbows (upper limb along the body without supporting it on the stretcher) in an adapted position (standing with the knees semi-flexed) were applied on the dominant side against manual resistance. Each pattern was repeated 3x and the contraction maintained for 5s on each attempt, with a 10s rest interval. Abdominal strength tests were performed according to the American College of Sports Medicine protocol.

The amplitude of the EMG of the biceps brachii was significantly different between the irradiation maneuvers applied, but there was no group effect (divided according to performance in the abdominal test), nor any significant interaction between group and maneuver.

The data indicate that the level of performance in the abdominal test may not significantly influence the specific recruitment of the biceps brachii during the application of irradiation maneuvers. Future studies should consider a larger sample size, with different age groups and body composition, and with different kind of diseases.

This study challenges the assumption that stronger abdominal muscles decrease recruitment of distal muscles through irradiation techniques. If future studies confirm these findings, abdominal strength will not need to be considered when healing weakness of a segment with motor irradiation.