Optometry & Visual Performance 262 Volume 13 | Issue 4 | December 2025 only one study has investigated the causal effect of neck activity on accommodation.15 Han and Lennerstrand, in 1998, applied highfrequency mechanical vibrations at 70 Hz to the sternocleidomastoid muscle or splenius muscle of the shoulder to stimulate muscle proprioceptors, which in turn could indicate a change in head position.15 Thus, changes in head posture were only simulated in their study. A control condition had no mechanical vibration. Five subjects aged 25–35 years took part in the study. Accommodative demands were 1 D and 5 D, and the accommodative vergence (rather than accommodation) was measured. According to this study, neck proprioception can influence certain aspects of accommodation. When the accommodative target moved in a step motion, the response latency was unaffected by neck vibration. However, the time constant was shorter in the non-dominant eye with neck vibration. The response amplitude may have differed a little due to neck vibration based on inspection of the tabled data (their Table 3). Unfortunately, the authors did not perform a test of statistical significance on these specific data. Nevertheless, no statistically significant effect of neck vibration was found when the target moved sinusoidally in depth. Thus, there may be some minor effects of neck vibration stimulation on accommodation based on this study. While mechanical vibrations to the neck have been used in laboratory and clinical settings, the current PRI hypothesis of a neck-stretch effect on refraction can be best addressed by having individuals adopt a genuine neck posture in the absence of mechanical vibration. We decided to build on Han and Lennerstrand’s study15 by using genuine neck postures and measuring accommodation directly and objectively. Another rationale for this study came from suggestions that neck posture may influence subjective refraction. This idea comes from physical therapists who are part of the Postural Restoration Institute (PRI). They treat musculoskeletal problems while considering the natural imbalances in the human body. They work in a multi-disciplinary way with optometrists, podiatrists, and dentists. There are approximately 260 physical therapists worldwide who follow the PRI approach. Some PRI practitioners hypothesize that posture during subjective refraction influences the refractive outcome and, reciprocally, that small differences in spherical equivalent refraction in the eyes can lead to changes in neck and body posture.16,17 However, PRI does not hypothesize a specific direction (for example, hyperopic or myopic) or magnitude of the accommodation change. In summary, we investigated the causal effect of neck posture on accommodation to address Richter’s observation8,14 that the coupling between eye accommodation and neck muscles has received little attention. We also addressed informal observations by PRI practitioners that neck posture may influence subjective refraction. We specifically studied a natural and a forward head posture, with the head held erect in both cases. Methods Subjects Twenty-four young adult individuals were recruited for this study. They were recruited from students and employees of Marshall B. Ketchum University and other members of the public in the surrounding area. We included individuals aged 18–35 years, thus excluding any who were pre-presbyopic. Individuals were excluded if they had any history or showed signs of amblyopia, strabismus, or accommodative anomalies. Subjects had age-normal subjective amplitudes of accommodation18 in the right and left eyes and monocular visual acuities better than or equal to 0.1 log MAR (20/25 Snellen equivalent) in both eyes. Individuals were excluded if they had a history of significant ocular disease; a history of head, neck, or back injuries; or a history of systemic conditions with a greater risk of contracting COVID, according to the Centers for Disease Control and Prevention criteria. Individuals using mydriatic or cycloplegic drugs had to undergo a washout period if they wished to be enrolled in the study. This period was 48 hours for tropicamide and phenylephrine and 72 hours for cyclopentolate. Subjects must have had a comprehensive eye exam within the previous 24 months to qualify for the study. Subjects sat for a onehour session in a ‘phoropter-like’ setting. This naturally excluded those with neurological or muscular deficits that might have led to discomfort. The study was approved by the Institutional Review Board and HIPAA Compliance Committee of Marshall B. Ketchum University, and it followed the tenets of the Declaration of Helsinki. The informed consent of subjects to participate in the study was obtained after explaining the nature and possible consequences of the study and before any study procedures were performed.
RkJQdWJsaXNoZXIy MjEyMDQ0Nw==