This study was designed to determine the changes in stature (loss and recovery) in response to 6h of different working journeys performed by patrolling and administrative MPOs. It also aimed to identify if the stature variations (loss and recovery) are related to the ability of the trunk flexor and extensor muscles to generate force and anthropometric and working characteristics. Stature loss was noted for all working conditions but was greatest in those MPOs involved in patrolling activities, irrespective of the vehicle type (car or motorcycle).
Stature variation measures have been used as an index of spinal loading in a range of tasks . It has been shown that extended sitting causes stature reductions , which act as a high-risk factor for LBP . The stature loss in the administrative MPOs (2.80 mm) was smaller than that observed in other studies (from 5.6 to 6.3 mm) that have analyzed stature changes while sit-stand and sitting during a regular office workday [24, 25]. In contrast, others have demonstrated comparable stature losses during a short period of sitting of 15 minutes (from 2.95 to 3.47mm; ) and 1h (from 1.5 to 2.6 mm; ). Possibly, differences in office chair types and the amount of sit-stand activity, as well as other body movements during the working day, may have clouded comparisons between studies. It has been demonstrated that some movements of the body may help to recover stature . The chair type  and the use of backrests , the lack of instructions regarding sit-stand ratio  and the uncontrolled movements performed during the workday (e.g., intermittent hyperextensions; ) may have also played a role.
The lower stature change observed in the administrative MPOs in comparison to the patrolling MPOs can be partially explained by the fact that they are less overloaded (50–77% less load) and not exposed to vibration effects. Indeed, the reduced load they sustain during their workdays is the lowest in comparison to that imposed by the weight of the mandatory protective equipment used in patrolling activities. It is known that the greater the load, the greater is the stature loss , which may have a superimposed effect of vibration. Magnusson and colleagues  reported that 5 minutes of sinusoidal vibration exposure causes greater stature loss when compared to regular sitting (i.e., without vibration). It has been evidenced that workers exposed to vibration are more likely to experience muscle fatigue and reduce the mechanical properties of soft tissues, which may make them more prone to injuries . It is especially concerning because the low frequencies caused by the vehicle’s tires hitting the road is in the resonant range of the low back (5-7Hz) and may cause further discomfort when the vehicle speed increases . Therefore, the compounded effect of vibration and overloading may explain the larger stature changes in MPOs after a working journey involving vehicle patrolling in comparison to those involved in administrative tasks.
Although the MPOs that patrolled using a motorcycle were subject to approximately 40% greater loads due to protective equipment (i.e., 16% of BM; see Fig. 1), the stature loss was comparable to that observed while patrolled by car. Generally, MPOs on motorcycle patrol are recommended to dismount for 15 minutes each hour of the workday. Changing postures during the working journey may have reduced the magnitude of the stature loss and resulted in comparable losses to that imposed while patrolling by car. The spinal overloading from while patrolling by car may have suffered the influence of the backrest inclination. Magnusson and colleagues  reported that seats with a backrest tilted at 120o cause less stature loss in comparison to seats without a backrest. On the other hand, sitting without a backrest causes a pelvic retroversion, which changes the physiological curve of the spine and modifies the compressive load distribution .
Wearing heavy protective apparatus may have also fatigued the muscles as the working day unfolds. It is known that muscle's ability to generate and sustain torque is essential to enhance and provide stability . Besides, the muscles surrounding the spine are known to support and stabilize the spinal column may have their protective functions reduced. Abdominal pressure increases are associated with the activation of the abdominal flexor muscles and have been thought to be beneficial by producing spinal unloading during extension efforts – especially by the transverse abdominal . It is also possible that the continuous activation of the muscles to increase the stiffness of the spine and improve stability may have caused an unceasing loading effect. These compressive forces may have increased spinal loading and, consequently, more substantial stature losses.
The ability of the trunk extensor and flexor muscles to generate torque was not relevant to reduce the spinal loading, irrespective of the patrolling vehicle (car or motorcycle). The borderline (non-significant) influence of the trunk extensors to generate torque in the stature loss may indicate that MPOs with greater strength may experience larger compressive loads on the intervertebral discs and so experience a considerable stature loss. The ability to produce large amounts of force was not a protective component to reduce spinal loading as measured by stature loss. Additional studies involving larger samples and more uniform working loads are required to test these arguments.
On the other hand, several studies have indicated that fast and prompt responses are more relevant to stabilize the spinal column than large amounts of torque . Thus, peak torque may not be the most relevant component to stabilize the spine, and other parameters (e.g., muscle activation ratio and the rate of torque development) may be more significant. Exercise interventions designed to improve muscle strength and neuromuscular coordination are considered as the most effective treatments to prevent and to reduce LBP . Indeed, some investigations aimed to reduce low back pain have demonstrated that exercises performed with low muscle activation (i.e., unlikely to promote substantial strength and torque gains) are effective in increasing spinal stabilization and to reduce chronic low back pain . Therefore, the ability to produce large amounts of torque failed to explain stature loss in MPOs.
On the other hand, BMI was the only significant variable to explain stature loss. It has been suggested that individuals with greater BMIs (i.e., with higher body mass) sustain a “chronic” loading condition and are more prone to experience adverse effects . Even though the weight of the protective apparatus was not a significant factor, it may have amplified the chronic loading condition and increased the stature loss. The weight of the protective equipment of both patrolling groups was approximately three times greater than the additional load of the group involved in administrative tasks (i.e.,13.3% vs. 4.9% BW).
Therefore, the acute loading imposed by the protective apparatus may have caused an additive loading in both patrolling groups, which explains the smaller stature loss experienced by the group involved in administrative tasks. It is interesting to note that the univariate analysis failed to identify the task performed by the MPOs as a relevant factor. It can be speculated that stature loss was similarly influenced during the patrolling tasks, as both groups (MOT and CAR) showed comparable reductions. Thus, from a mechanical point of view, patrolling by CAR or MOT produced similar spinal loadings. Thus, it seems that efforts to reduce the spinal loading by decreasing body mass and the protective apparatus are a plausible solution to avoid spinal overloading, as the peak torque of the trunk flexor and extensor muscles were not identified as explanatory factors. Studies designed to analyze the way several apparatuses are transported (handcuffs, ammunition, gun, etc.) are required. Indeed, it has been demonstrated that the overload caused by the bulletproof vest can cause muscular fatigue, especially on the side the gun is transported. Thus, future studies to redesign the way MPOs carry additional apparatuses are required, as well as alternative materials to reduce the weight of the bulletproof vest without diminishing its protective effect.
At the end of the working day, the 20 minutes period of unloading was sufficient to allow a partial restoration of the stature loss in all groups. The ADM group recovered 77% of the initial stature, with MOT 40% and CAR 46% recovery. Although the relative recovery was lower for the motor vehicle groups, the absolute change in stature was similar, indicating an equivalent rate of recovery regardless of stature loss. The univariate analysis failed to identify task differences as a relevant factor to recovery between the patrolling groups. A reduced rate of stature recovery has been shown by Rodacki and colleagues  and Healey and colleagues  to be associated with low back pain. The absence of any difference in recovery rates between groups is in line with expectations as existing low back pain was an exclusion criterion for the study. The findings do demonstrate that MPOs who have been on patrol in a car or motorcycle need to allow a greater period after the work to recover stature loss.
This study has some limitations, and the reader should bear them in mind while interpreting the results. The first refers to the limited number of participants that may have reduced the power of the analysis and impeded establishing a more detailed group comparison. However, the number of participants is comparable to similar studies. As the results were gathered on participants while fulfilling their normal daily activities, it was not possible to control the nature of the activities and movements performed during the working routines (e.g., sit-stand ration, hydration). The posture assumed while sitting in the car or the motorcycles were not controlled, although they represent that real working conditions. It was not possible to separate the effects of the weight of the equipment and vibration, which requires other studies. Finally, differences in patrolling routines (mainly due to the weight of the protective equipment) may have influenced the results, as participants were subjected to different spinal loadings. Other studies applying a more uniform spinal loading is required to confirm if muscle strength presents a protective effect.