Biomedical Sciences, Biomed Biopharm Res., 2022; 19(1):58-71
Short duration – low intensity isometric plantar flexion increases distal perfusion: observations from a healthy cohort
Margarida Florindo 1,2,3 , João Gregório 1 & Luis Monteiro Rodrigues 1,*
1Universidade Lusófona CBIOS - Research Center for Biosciences and Health Technologies, Av Campo Grande, Lisboa, Portugal (EU); 2U Alcalá PhD Program Health Sciences-Alcalá (Madrid) Spain ; 3ESSCVP - the Portuguese Red Cross Health School. Dep. Physiotherapy, Lisboa, Portugal (EU)
Controlled physical activity might help as a preventive and therapeutic tool in vascular pathology. In this study we aimed to understand how lower limb exercise impacts perfusion in a healthy cohort. The study involved a convenience sample of eighteen previously selected healthy individuals of both sexes (n=9 each), mean age 32.8 ± 12.7 years. Procedures respected all principles of good clinical practice. Blood perfusion changes were simultaneously assessed in the dorsal region of both feet by laser Doppler flowmetry (LDF) and polarised spectroscopy (PSp). Measurements were taken at baseline, after stabilization (phase I), following 1 minute of bipedal isometric plantar flexion (phase II), and during recovery (phase III). Descriptive and comparative statistics were performed. Plantar flexion evoked significant perfusion changes in both feet, but in opposite directions – increasing with LDF and decreasing with PSp. These changes indicate that this approach promotes an adaptive mobilisation of blood from superficial to the deeper plexus. No significant changes in arterial blood pressure or cardiac frequency were detected. This manoeuvre, needing no specialised supervision, is capable of promoting significant perfusion changes in the lower limb, showing potential to be further explored in future studies with a prospective design in a preventive/recovery context.
Keywords: plantar flexion, foot perfusion, laser Doppler flowmetry, polarised spectroscopy, PAHR - prompt adaptive hemodynamical response, physical activity, home-health
Received: 26/03/2022; Accepted: 10/06/2022
Physical activity and controlled exercise are commonly regarded as indispensable components of well-being, as well as useful in the prevention (or slowing the progression) of disease and favoring recovery from both injury and disease (1,2), and both are seen as principal to provide the necessary cardio-respiratory and microcirculatory operations that ensure normal tissue “performance” (2-5). A precise balance involving structural integrity to respond to neuroendocrine output affecting cardiorespiratory performance, systemic vascular resistance, and local microcirculatory (endothelial and myogenic) activity is permanently required (3). When unmet, the link between vascular damage and mobility loss is clear, as impairment regularly emerges with disease progression (6-9).
Routine daily exercise has been within the first line of therapeutic options in managing these patients. However, data on the impact of its implementation within an integrated healthcare strategy are still insufficient. Recent trials seem to confirm the beneficial potential of regular physical exercise, including walking, in the prevention of and recovery from cardiovascular conditions (10-12), yet some controversy persists. Regular physical activity seems to improve or even retard severe frailty in adults, although aerobic and resistance exercise may not be recommended (5,13). Exercise has also been recommended to improve lower limb hemodynamics in diabetic patients (9,14-16), even when neuropathy is already present (15). Results are not as clear in the presence of peripheral vascular disease (PVD), as the benefits of exercise seem to depend on the severity of the existing lesions and the presence of intermittent claudication (17,18). The patient’s compliance, motivation, and communication within the “rehabilitation cluster” (patient, therapist, family, other professionals) to support positive progress toward both clinical and meaningful goals are also important determinants (19). According to various studies, the main reason for failing prescribed (therapeutic) physical activity is the “lack of time” (20-22). Some pathological processes are known to create and perpetuate a cycle that discourages the patient from performing activities requiring mobility (4,6,8,18,23). Thus, the patient’s engagement is crucial, especially if accepting that even “small” exercises can be useful in vascular rehabilitation (19,23,24). Specific guidance involving a complex combination of aerobic exercise, strength education, flexibility coaching, and nutrition has been proposed to address many of these concerns (4,12,14,19,24,25).
The impact of common low-intensity movement in the lower limb circulation has been a theme of study for our research group as we have sought relationships between movement and blood perfusion to identify and characterise the adaptive mechanisms involved (26-30). Recently we identified a centrally mediated response – the Prompt Adaptive Hemodynamic Response - in place of what we had believed to result from local interactions as in reactive hyperemia or in the venoarteriolar reflex (31). Moreover, we noted that simple challenges applied to a single limb, such as the reactive hyperemia associated with massage or a unipodal squat, would consistently impact blood perfusion in the other (contralateral) limb through this PAHR (28-31). Thus, we extended our research to investigate the impact of low-intensity short duration localised activity such as plantar flexion on foot microcirculation in the absence of disease. The objective of this exploratory study is to identify the distal perfusion variations related to the foot isometric plantar flexion in the upright position as close as possible to the normal physiological state.
Material and Methods
A convenience sample of sixteen healthy volunteers of both sexes (n=8 per sex) with a mean age of 31.9 ± 12.9 years old was chosen from our university community. Selected participants were required to be normotensive, non-smokers, and free of any medication or food supplementation. Blood pressure, cardiac frequency, the Ankle-Brachial Index a recognised indicator of vascular health (32) and the Body Mass Index calculated by the Quetelet‘s formula (BMI = weight/height2, expressed in kg/m2) (33,34) were also calculated. All participants reported some degree of physical activity, and some reported regular exercise, although none were athletes. All young women reported regular menstrual cycles. The general characteristics of the participant panel is summarized in Table 1.
The absence of recent pathology of the foot that could influence the ankle joint during plantar flexion was an exclusion criterion and confirmed upon selection. Other restrictions included refraining from caffeine and alcohol consumption 24 hours prior to measurements, as well as any topical (including cosmetic) application in the assessment areas.
Footedness, regarded as a measure of the preference (dominance) and performance of one limb, was determined by the validated Portuguese version of the Lateral Preference Inventory (LPI) (35) and compared with the laser Doppler flowmetry perfusion values, which were used as a biomarker.
All participants were previously informed of the objectives and phases of the study and provided their informed written consent. Procedures fully respected the principles of good clinical practice defined for human research (36) and were evaluated by the institutional Ethics Committee (EC.ECTS/P03.20 de 2020) prior to study initiation.
Measurements were conducted in the research lab with controlled temperature (21 ± 2 ºC), light and humidity (40 - 60%) conditions and performed by the same experienced operator-researcher. After adapting to the room conditions (approximately 15 minutes) in the standing position, participants completed a protocol divided into three phases: five minutes baseline recordings in a stable standing position with parallel feet (phase I); one minute of comfortable isometric plantar flexion of both feet (phase II); five minutes recovery from movement, returning to the baseline position (phase III). Blood perfusion was continuously measured in both feet simultaneously using non-invasive technology, specifically, laser Doppler flowmetry (LDF) (Perimed PF5010 System, Stockholm, Sweden) and polarised spectroscopy (PSp) via Tissue Viability Imaging® (TiVi) (TiVi701cam, WheelsBridge, Sweden). The LDF detects cutaneous blood perfusion (BP) variations through the Doppler effect (3,37). For this study, LDF probes were placed in the anterointernal region of each foot, one centimeter posterior to the first metatarsal-phalangeal joint (Figure 1).
The polarized spectroscopy system includes a digital camera equipped with polarized filters placed perpendicular to the skin, without contact, to record and analyse changes in the Concentration of Red Blood Cells (CRBC) in a relatively large chosen region of interest (ROI) (38). The chosen ROI was the dorsal region of both feet. Figure 2 shows a typical blood perfusion record obtained under these conditions. Pulse rate (PR) and arterial blood pressure were also monitored with a digital sphygmomanometer (Pic 22012000200 Esfigm Classic Check, Artsana S.p.A, Italy).
Descriptive and comparative statistics were performed with SPSS v.22.0 (IBM Corp. Amrock, NY, USA) and Jamovi software Version 2.2 (jamovi project, Sydney, AU). A 95% level of confidence was adopted throughout the analysis. The Shapiro-Wilk test was used to detrmine the normality of data distribution. The Student’s t-test or the Mann-Whitney non-parametric test was used to assess differences for independent samples. Following normality and homogeneity testing, pairwise comparisons between feet were performed with Repeated Measures ANOVA with the post-hoc Tukey test to evaluate differences among variables. A post-hoc power analysis using the Jamovi software was also performed.
Blood perfusion values for both feet are shown in Table 2, using mean and standard deviation (SD) for all variables representing the totality of each period register. Results of the comparison analysis (p-value) between feet and across phases are also shown (Table 2).
The plantar flexion challenge (phase II) caused obvious and statistically significant blood perfusion changes in both feet, as could be expected. Blood perfusion measured by LDF significantly increased in both feet in phase II, and these differences disappeared in the recovery period. The CRBC index indicated a significant decrease of perfusion in both feet with a reduction of the previously detected perfusion asymmetries (Table 2).
Although not significant, there were different blood perfusion values obtained between paired feet with both (LDF and PSp) instruments. LDF recorded higher in the right foot in 67% of men and 44% of women. These results disagreed with the results of the LPI applied to determine footedness, which depicted a 94% dominance of the right foot for all participants. These perfusion asymmetries between right and left feet were always present in all phases when measured by LDF, more or less pronounced although not statistically significant (Figure 3).
Percentage differences between phases for both limbs and technologies are presented in Table 3. As shown, the LDF system detects higher perfusion amplitude differences compared with PSp. No differences between limbs were found, indicating that the plantar flexion induces the same response in both limbs. Exploratory tests to assess these delta differences between different age groups were also performed. The delta between phase II and III measured by the PSp for both limbs was significantly lower in the more mature adults (p=0.019 right foot; p=0.027 left foot), suggesting that age might influence the response. However, we should emphasize that due to the low number of participants these differences must be interpreted with caution and should be better explored in future studies with different design.
Our objective in the presented experimental setting was to characterise the impact of isometric plantar flexion of both feet, in an upright position, on blood perfusion.
Baseline analysis immediately revealed asymmetric blood perfusion between right and left feet, although not statistically significant (Table 2). Asymmetries have been described as differences between right and left, dominant and nondominant, preferred and nonpreferred or stronger and weaker limbs (39-41). Also referred to as footedness (or handedness for the upper limb), it reveals a complex aspect of human performance and cognitive-motor processes (39). In the absence of vascular disease, the physiological meaning of these asymmetries is far from being fully understood, but recent evidence suggests it to be particularly relevant in sports medicine. Footedness has been classified as a lesion risk factor for the preferred foot in the lower extremity (40,42) and is likely to be considered in the design of training or recovery programs. Despite the relevancy of this information, most of the procedures are used by convenience, and this lack of normalisation complicates the comparison of results and the determination of any true meaning (39,43). Additionally, some studies have associated blood flow with muscle mass (40,41), suggesting that circulatory stress might produce more asymmetries and promote muscle-perfusion lesions (40,43). Hemodynamics, muscle activation, and force generation have been proposed as explanations for this scale of strength or force competence between the paired limbs (40,43-46).
Under this view, the self-report limb preference to execute specific tasks seems to represent a poor indicator for this assessment. As previously mentioned, the Portuguese version of the LPI revealed a 94% dominance of the right foot for all participants. However, using LDF perfusion as a (reliable) biological marker (39,43,47) indicated higher perfusion in the right foot in 67% of men and 44% of women. Recent data has shown that even common activities such as gait require comparable amounts of blood flow for distal muscle activation in dominant and nondominant limbs (43,46). Therefore our results justify this option to establish the limb preference according to the higher blood perfusion values as measured by LDF.
Plantar flexion significantly increased blood perfusion as measured by LDF in both feet. Simultaneously, we observed an opposite effect with the PSp instrument, indicating a significant reduction of perfusion measured by CRBC (Table 3, Figure 3). To understand these two observations, we must keep in mind the particular vascular structure of human skin and the technologies used for blood perfusion measurement. The vasculature of the skin is organised in two plexus at different depths parallel to its surface (3). The superficial plexus involves numerous capillary loops that extend to the epidermis connecting small arterioles and venules near the papillary dermis. The lower plexus, near the dermal-hypodermal interface, includes arteries and veins from the underlying muscle and adipose tissue that perforate the fascia to form ascending arterioles and descending venules, connected to the superficial plexus (3). This peculiar structure allows blood to move between these two planes through that anastomosis network (31,48). In addition, the isometric contraction of the calf muscle displaces plantar pressure to the forefoot, facilitating the blood movement to the deeper structure (49,50).
The quantitative measurement of skin blood perfusion is commonly assessed by optical-based technologies, with LDF still regarded as the gold standard. These technologies use different lights and laser frequencies to operate, however, and thus measure at different depths and reveal different features (50). Our own experience supports the information provided by the manufacturer - that these LDF frequencies allow measurements up to 1 mm (50), while the PSp system measures more superficially, likely reaching less than 0.5 mm (38). Considering these arguments, the blood perfusion increase detected by LDF and the blood perfusion decrease detected by PSp are coherent, signifying that plantar flexion is likely displacing blood along the superficial plexus to deeper levels. A previous study has shown that a short-term isometric contraction recruiting a large number of muscular fibres demands an increase in the local blood supply (51). Permanent communication between the lower limb muscle pump and the superficial plantar venous plexus also seems to contribute to these adaptive mechanisms (52). However, this substantial blood perfusion increment registered with the plantar flexion does not result from a local response but rather from the PAHR previously described by our group (28,31,53). Similar adaptive responses were observed in unipodal exercises with the contralateral limb at rest, with blood perfusion modification noted in both active and resting feet (29). Repeated plantar flexion seems to trigger this mechanism, resulting in rapid constriction of the superficial vessels and blood mobilization to the deeper vascular structures. The sustained pressure in the anterior plantar region associated with the posterior (leg) muscular pumping ensures local mechanics and hemodynamics (52). The rapid recovery to baseline values noticed in phase III (when movement ceased and the volunteers returned to a stable upright position) agrees with the previously observed and described mechanisms (26-28).
In our opinion, unsupervised home-based exercise including walking is still an underused and poorly explored therapeutic tool. Recent studies in vascular patients suggested more consistent outcomes with supervised exercise when compared with unsupervised approaches (54). However, results are ambiguous and analysis is still limited by the reduced number of studies and participants (54).
Our results suggest that this easy-to-execute activity, requiring no specialised supervision, might be useful to promote muscular health and, in that direction, to be explored as a component of a person-centred home-based physical activity program. Relevant limitations to be pointed out include that: (i) the observational nature of our study, with a reduced number of participants, limits the extrapolation for the general population, and the identification of the influence of sex, age, and other potential determinants as well; (ii) results were obtained from healthy participants; thus time-related impacts in (specific) groups of cardiovascular patients were not established; (iii) experiments were conducted in the laboratory, different from the home-based scenario; (iv) a proper validation of procedures is needed; and (v) the proposed strategy is only applicable to individuals with adequate mobility. As we move forward and expand this research, these limitations will be fully addressed.
To all participants, and to all CBIOS researchers involved in the reported studies.
Conceptualization, LMR; Data curation, JG and MF; Investigation, MF; Methodology, LMR and MF; Supervision, LMR; Validation, LMR; Writing – original draft, LMR MF and JG; Writing – review & editing, LMR.
The senior editor co-authoring this manuscript had no participation in the review nor in the decision process. All authors declare there were no financial and/or personal relationships that may present a potential conflict of interest.
This study is supported by FCT - Foundation for Science and Technology, I.P., by the grants UIDB/04567/2020 and UIDP/ 04567/2020." The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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