Introduction

There is extensive scientific evidence that the ocular physiology is sensitive to circadian variations [1], as well as to a wide variety of daily activities such as sleeping position [2], caffeine consumption or dietary supplementation [3, 4], physical exercise [5, 6], playing wind instruments [7], or mental stress [8]. The impact of the aforementioned activities on ocular physiology has been explored due to their influence on ocular health, and especially in glaucoma [9]. Glaucoma is the leading cause of irreversible vision loss worldwide [10] and, thus, the scientific community has focused on identifying possible strategies to minimize the side effects associated with this disease.

Activities that comprise the exchange of gases (e.g., isometric efforts) are of particular interest because they seem to promote abrupt changes in intraocular pressure (IOP) and different anterior eye segment parameters [11,12,13,14], being commonly discouraged for subjects at high risk for glaucoma onset or progression [15]. Some studies have directly tested the influence of the exchange of gases on IOP and anterior segment morphology asking the subjects to execute the Valsalva maneuver [12, 16], whereas other studies have assessed the effects of different physical efforts on IOP [6]. Nevertheless, as indicated by Wang et al. [11], there are a wide range of daily activities that may promote the execution of the Valsalva maneuver (e.g., lifting heavy weights, forceful coughing, and sneezing), leading to significant alterations of IOP and anterior segment morphology (narrowing of the iridocorneal angle). However, the possible detrimental effects of these daily activities on IOP and anterior eye biometrics have not been investigated in detail.

To address the limitations found in the scientific literature, we designed the present study to determine the effect of holding a weight corresponding to the 10% and 20% of participants’ body weight, which is considered as a very frequent daily activity (e.g., carrying shopping bags), during 5 min on IOP and anterior segment biometrics. We hypothesized that holding weight would promote an acute IOP rise and a reduction in anterior chamber angle (ACA), as it has been demonstrated during the execution of the Valsalva maneuver [11, 12, 16]. We also hypothesized that these changes will be heightened when using heavier loads [17].

Methods

Participants

Eighteen healthy university students (10 women and 8 men: age = 23.4 ± 4.7 years, body mass = 67.7 ± 13.4 kg, height = 171.0 ± 9.8 cm) took part in this study. They were screened to accomplish the following inclusion criteria: (i) be free of any ocular disease, as assessed by slit lamp and direct ophthalmoscopy examination, (ii) had no history of refractive surgery or orthokeratology, and (iii) not currently taking any medication. Participants were also instructed to refrain from alcohol and caffeine-based drinks for 12 h and to sleep regularly the night prior to the experiment. Additionally, contact lens wearers were asked to avoid the use of their contact lenses for 8 h prior to attending the laboratory. This study adhered to the guidelines of the Declaration of Helsinki and was approved by the University of Granada Institutional Review Board (IRB approval: 438/CEIH/2017).

Instruments and measurements

IOP was measured with a portable rebound tonometer (Icare Tonometer, Tiolat Oy, INC., Helsinki, Finland), which has demonstrated to provide accurate IOP reading when compared with Goldman applanation tonometry [18], as well as to have an excellent intraobserver reproducibility [19]. Participants were instructed to fixate on a distant target while six rapidly consecutive measurements were performed against the central cornea. The instrument displays the mean reading and indicates whether differences between measurements are acceptable. We only considered values with low standard deviations (ideal measures).

The Pentacam system (OCULUS Optikgeräte Inc., Wetzlar, Germany) allowed us to measure the eye anterior segment morphology. This non-contact instrument uses a rotating Scheimpflug camera, which rotates 360° around the optical axis, and permits to acquire multiple photographs of the anterior segment of the eye in 2 s, considering the corneal vertex as the reference point [20]. After it, the Pentacam software uses the elevation data and constructs a 3-dimensional image of the anterior segment, which permits to calculate numerous anterior segment parameters. For this study, the anterior chamber angle (ACA; the smaller of the 2 angles taken in the horizontal meridian), anterior chamber depth (ACD; the distance from the corneal endothelium to the anterior surface of the crystalline), and central corneal thickness (CCT; centered at the corneal apex) were considered.

Experimental design and procedure

A repeated measures design (3 loads × 7 points of measure) was used to test the acute effects of holding weights (control (0%), 10%, and 20% of participants’ body weight) at different points of measure (baseline, while holding the weight for 0.5, 2, 3.5, and 5 min, as well as after 0.5 and 2 min of recovery) on IOP and anterior eye biometrics measures.

Participants attended to the laboratory in only one occasion, and the three experimental conditions were performed in a randomized order with a 10-min break between two consecutive conditions. At the beginning of the session, participants were screened for the accomplishment of the inclusion criteria (see above) and weighted to determine the load used in the experiment. For the load manipulation, two 10-l volume capacity jugs with comfort-grip handles placed at the upper part of the jug (similar to a kettlebell) were used, and they were equally filled with water in order to adjust the load to the 10% and 20% of participants’ body weight. For example, for a participant weighing 80 kg, each jug was filled with 8 l of tap water for the 20% of body weight condition (16 kg). An experienced optometrist took the anterior segment measurements with the Oculus Pentacam and, subsequently, the IOP measurement was taken by another experimenter with the Icare rebound tonometer. For this purpose, participants were seated in front of the Pentacam apparatus, and after the baseline measurement, they were asked to hold the corresponding load during 5 min, being the anterior segment and IOP measurements performed after 0.5, 2, 3.5, and 5 min of holding the weight. After it, they were asked to leave the jugs on the floor and two measurements were taken after 0.5 and 2 min. Both jugs were opaque and filled by a researcher that was not present during the data collection. The examiners responsible for data collection were masked of the load used in each experimental condition. All measurements were obtained under constant environmental (∼ 22 °C and ∼ 60% humidity) and illumination conditions (~ 30 l×).

Statistical analyses

Before any statistical analysis, the normal distribution of the data (Shapiro-Wilk test) and the homogeneity of variances (Levene’s test) were confirmed (p > 0.05).

To assess the impact of holding weights on IOP and anterior eye biometrics (ACA, ACD, and CCT), we performed a repeated measures ANOVA for each dependent variable with the load (control, 10%, and 20%) and the point of measure (baseline, 0.5 min, 2 min, 3.5 min, 5 min, and after 0.5 min and 2 min of recovery) as the within-participant factors.

For all analyses, an α of 0.05 was adopted to determine the significance of the main effects, and the Holm-Bonferroni correction was adopted for multiple comparisons. The magnitude of the differences was assessed by Cohen’s d effect size (d) and eta squared (ƞp2) for T and F tests, respectively.

Results

Table 1 shows the descriptive values for all the variables assessed at the different points of measure.

Table 1 Average ± standard deviation values for the intraocular pressure and anterior eye biometric parameters at the different points of measure in each of the three experimental conditions
Full size table

Regarding IOP, there was a main effect of load (F2, 34 = 4.64, p = 0.016, ƞp2 = 0.215), point of measure (F6, 102 = 8.15, p < 0.001, ƞp2 = 0.324), and the interaction (F12, 204 = 5.05, p < 0.001, ƞp2 = 0.229). Post hoc comparisons evidenced greater IOP rises in the 20% condition compared with the control condition (corrected p value = 0.035, d = 0.67) (Fig. 1).

Fig. 1
figure 1

Effects of holding weights on intraocular pressure at the different points of measure. The asterisk, number sign, and dollar sign denote statistically significant differences for the comparisons 20% vs. control, 20% vs. 10%, and 10% vs. control, respectively (corrected p value < 0.05). The error bars represent the standard error. Bas = baseline; s = seconds; rec = recovery

Full size image

The analysis of ACA yielded a statistically significant effect for the load (F2, 34 = 4.55, p = 0.018, ƞp2 = 0.211), whereas the point of measure (F6, 102 = 1.17, p = 0.326) and the interaction (F12, 204 = 1.17, p = 0.307) did not reach statistical significance. Post hoc comparisons showed that the 20% load condition induced a narrower ACA in comparison with the control condition (corrected p value = 0.029, d = 0.69) (Fig. 2).

Fig. 2
figure 2

Effects of holding weights on anterior chamber angle at the different points of measure. The asterisk and dollar sign denote statistically significant differences for the comparisons 20% vs. control, and 10% vs. control, respectively (corrected p value < 0.05). Error bars represent the standard error. Bas = baseline; s = seconds; rec = recovery

Full size image

The ACD was far from showing any significance for either the load, the point of measure, or their interaction (F < 1 in the three cases). Similarly, no effects of load (F2, 34 = 0.81, p = 0.455), point of measure (F6, 102 = 1.12, p = 0.355), or the interaction (F12, 204 = 1.12, p = 0.348) were found for the CCT (Table 1).

Discussion

The present study is aimed at examining the effects of holding weight on IOP and anterior eye biometrics. Our data show, for the first time, that holding weight increases IOP and narrows ACA, being these changes only significant when using the load corresponding to the 20% of body weight. The IOP and ACA values returned to baseline levels after 30 s of recovery. However, we did not find any effect on CCT and ACD. The current findings highlight that daily life activities such as holding weights may have an acute impact on the ocular physiology, and these results should be considered by eye care professionals involved in the management of different ocular diseases, especially glaucoma.

The IOP rise associated with holding weight observed in the present study is in line with the results reported by Baser et al. [21], who found an IOP increment (~ 1 mmHg) just after carrying a 5-kg shopping bag. In the current study, we found an acute IOP rise of up to ~ 2.3 mmHg while holding a weight corresponding to the 20% of participants’ body weight, being these changes in agreement with previous studies that explored the IOP response to activities that compromise the exchange of gases, such as performing the Valsalva maneuver [16], playing wind instruments [7], or weightlifting [22, 23]. Regarding anterior eye biometrics, our results are in line with Sihota et al. [24] and Wang et al. [11] who evidenced an abrupt narrowing of the eye angles when participants performed the Valsalva maneuver. It should be noted that in our study, participants did not voluntarily perform the Valsalva maneuver. Therefore, it seems that holding weight induces a narrower ACA even when the participants are instructed to breathe normally (i.e., without performing the Valsalva maneuver). It seems plausible to consider that the mechanisms involved in the ACA narrowing linked to holding weight may be similar to those occurring when the exchange of gases is compromised [25].

The changes induced by holding weight on IOP and ACA were positively associated with the loading condition (higher changes for the 20% condition). This set of results is in agreement with the accumulated evidence on the IOP responses to resistance training, showing a strong positive association between the magnitude of the load and the level of accumulated effort with IOP levels [17, 22]. Nevertheless, to our knowledge, no previous study has examined the influence of physical effort on anterior chamber morphology. In the current study, we revealed that the narrowing of the ACA is modulated as by the magnitude of the load used, being these changes only evident for the 20% load condition. Here, IOP evidenced a progressive IOP rise as a function of time-on-task; however, the iridocorneal angle presented a constant reduction during 5-min of effort (holding 20% of body weight). Taken together, our results suggest that holding heavy weights may have detrimental effects on the ocular health and, thus, these situations should be discouraged for individuals who need to avoid IOP peaks and narrowing of the anterior segment angles (e.g., glaucoma patients).

The significant impact of holding weight on both IOP and ACA may demonstrate that the IOP rises associated with physical effort could be caused by ACA reduction, which may add resistance to the outflow of aqueous humor [10]. Although these changes may seem modest, it is important to highlight that small variations in IOP (1 mmHg) are linked to a higher risk (10%) for glaucoma [26] and, thus, our findings could be of special relevance for people who routinely perform activities that require to carry or lift weights. Indeed, eye care specialists should be aware of the influence of carrying weights on ocular health, and especially for the management of primary angle closure glaucoma patients or susceptible individuals. Note that diurnal IOP fluctuations or responsiveness to different stress tests have demonstrated to be higher for these patients compared with healthy individuals [27, 28].

The main novelty of this investigation is the simultaneous assessment of the effect of holding a weight on IOP and ACA. However, there are some aspects that may limit the validity of our findings to the general population, and especially to glaucoma patients. First, the experimental sample included in this study consisted of healthy young adults and, therefore, the applicability of these results to glaucoma patients needs further investigation. Note that the functioning of the autoregulatory mechanisms of ocular hemodynamics in glaucoma patients is altered [29] and they show a heightened response to different stress tests when compared with healthy individuals [28, 30]. Second, we did not include participants with a narrow eye angle. Although Wang et al. [11] did not observe significant differences in ACA between individuals with a narrow angle compared with a control group during the execution of the Valsalva maneuver, the possible difference between subjects with narrow and normal anterior segment angles should be further explored. Third, we observed that the ACA was reduced during the most physically demanding condition (20% of participants’ body weight); however due to technical limitations, we did not assess the possible impact of holding weights on the trabecular meshwork, which is known to play an important role in aqueous humor regulation [31]. Recent developments in the assessment of the trabecular meshwork (Fourier domain optical coherence tomography) may help to explore the impact of holding weights on the trabecular meshwork outflow pathways [32]. Fourth, recent studies have demonstrated that the ocular changes induced by physical efforts are modulated by participants’ fitness level [33] and, thus, we consider of interest to test whether subjects that commonly carry or lift heavy loads may show a less prominent effect of holding weights on ACA. Fifth, the holding weight task lasted only 5 min, and the effects of more prolonged periods need further investigation. Lastly, no studies have assessed the long-term effect of heavy lifting on ocular health, which should be desirable to discern whether certain lifestyle habits (e.g., lifting or carrying weights regularly) should be discouraged for subjects at high risk for glaucoma onset or progression.

Conclusions

Holding a weight corresponding to 20% of body weight during 5 min narrows the acute anterior segment angle and increases IOP values in young healthy adults. These changes returned to baseline levels after 30 s of recovery. This study suggests that holding heavy loads may be associated with modest changes in IOP (rise) and ACA (narrowing); however, the clinical relevance of these effects should be further investigated. The external validity of these results for glaucoma patients or subjects at high risk of developing glaucoma needs to be addressed in future studies.