De Rol van de Intrinsieke Voetspieren bij het Lopen en de Balans – en hun Trainbaarheid – bij Oudere Volwassenen in de Context van Valpreventie.

Falling among older adults is a growing concern and requires optimal prevention. Incidence rates are rising as the population continues to age and because of the increasing prevalence of multimorbidity and frailty. Among the diverse fall risk factors, impaired gait and balance are, second to muscle weakness, the strongest predictors of falling. Effective fall prevention programs are those that contain physical exercise. More specifically, exercise programs are recommended to include at least balance and functional exercises to target impaired gait and mobility. The majority of existing fall prevention exercise programs are not focused on foot muscles. However, several studies have shown the importance of targeting foot muscles to treat gait and balance impairments in older adults. In addition, growing evidence highlights the key role of a specific group of foot muscles, the plantar intrinsic foot muscles (PIMs), in this context. If these muscles can be trained in older adults, and this results in improved gait and balance, this will benefit existing fall prevention programs. Therefore, the overall aim of this doctoral project is to improve fall prevention by gaining insight into the trainability of the PIMs and their role in gait and balance in older adults. We hypothesize that training the PIMs in older adults will increase their force capacity and, concomitantly, improve gait and balance. This is examined in a final intervention study investigating the effect of a PIM strengthening program on PIM force capacity, gait, and balance. This intervention study is preceded by several preparatory studies to substantiate choices in the design of the intervention study. Therefore, the methodological approach of this project consists of a chronological sequence of studies, each with its own methodology and purpose. Chapter 2 presents the study investigating the intra-assessor reliability and measurement error of ultrasound-assessed morphology - an estimate of force capacity - of selected foot muscles in older adults using a tablet-based device. As a secondary aim, the measurement error was compared between older and younger adults and between the tablet-based device and a mainframe system. Ultrasound images were collected from 18 older and 10 younger adults on two occasions using the two ultrasound systems. In older adults, the standard error of measurement ranged from 2.8% to 11.9%, with most intra-class correlation coefficients being excellent. Although younger adults showed smaller error variance for six morphology measures, errors in older adults were small as well. The tablet-based device demonstrated superior repeatability compared to the mainframe machine. These findings suggest that a tablet-based ultrasound can be used to reliably assess foot muscle morphology in older adults and is suitable for detecting group-level hypertrophy following training. It presents a practical alternative to expensive mainframe systems, which often have limited access. In addition, the use of a tablet-based device enables the researcher or clinician to perform ultrasound measurements at any location, even at the home of older adults. Chapter 3 reports the systematic literature review that examined the evidence on whether interventions anticipated to strengthen the PIMs improve dynamic balance control and foot function during gait in adults. Eleven studies were included, with the findings of five high- or moderate-quality studies synthesized. Most research involved younger populations and applied isolated foot exercises. Results showed low-certainty evidence that PIM exercises improve vertical ground reaction force - a measure for foot function during gait - and very low-certainty evidence for improved dynamic balance. No effect was found on medial longitudinal arch kinematics. Overall, findings suggest potential benefits of PIM strengthening for outcomes that are associated with falling in older adults. However, high-quality intervention studies targeting older adults are needed, using outcome measures related to fall risk and PIM function, such as propulsive forces and balance during gait, in addition to PIM strength or capacity measures. Chapter 4 examines whether functional foot exercises can activate the PIMs to a comparable degree as isolated foot exercises. Although isolated exercises are traditionally used in clinical practice, they may be difficult for older adults to perform. Functional exercises, by contrast, are more accessible, but their effectiveness in specifically targeting the PIMs remains unclear. In this experimental study, surface electromyography was used in 29 younger adults to compare the activation of three PIMs during four functional exercises (normal and unstable toe stance, toe walking, and hopping) with the activation during the isolated foot exercise provoking the largest level of activation for the respective PIM. Results showed that functional exercises elicited similar or even greater muscle activation than the muscle-specific isolated exercise in all 12 comparisons. Specifically, functional exercises showed higher activation in 25% of the comparisons, with no significant differences in the remaining 75%. These findings suggest that functional exercises may serve as a viable alternative or as a complementary exercise mode to isolated foot exercises in rehabilitation or prevention programs targeting older populations. Chapter 5 outlines the study protocol for a randomized controlled trial (RCT) designed to evaluate the effects of a 12-week PIM strengthening program on gait, dynamic balance and foot muscle function in older adults. Recognizing that effective fall prevention programs typically include exercise but still require enhancement, the study investigates whether targeted PIM training can improve gait and dynamic balance and, thereby contribute to reducing fall risk. The protocol describes that community-dwelling older adults (≥65 years) who participate in a group-based functional exercise program and have an indication of possibly increased fall risk are eligible. Participants are randomly assigned to either an intervention or control group. The intervention group follows a progressive 12-week training program combining isolated and functional foot exercises, to be performed five times per week at home of which one supervised by a trainer. The control group continues usual activities but keeps a diary. The primary outcome is the change in maximum gait speed, reflecting propulsive capacities and dynamic balance; secondary outcomes include the change in PIM capacity and strength, foot and ankle biomechanics, and fall-related risk factors (e.g. concerns about falling). Data are collected at baseline and post-intervention in a laboratory setting, with between-group differences in change from baseline analyzed using ANCOVA. This protocol provides the foundation for determining whether PIM strengthening may enhance the effectiveness of existing fall prevention strategies for older adults. Chapter 6 presents the results of the RCT introduced in Chapter 5. Thirty-three participants (15 men, 18 women) were randomly allocated to an intervention group receiving the PIM strengthening program (n = 16; median age: 78.4 years (IQR: 73.3 - 83.6)) or a control group (n = 17; median age 76.6 years (IQR: 71.3 - 82.1)). The median adherence to the prescribed number of training sessions was 99 % (IQR:97 % - 100%). While no significant between-group difference was found for the change in maximum gait speed, the intervention group demonstrated a larger reduction in concerns about falling. The reduction in concerns about falling was most notable for activities that involve walking. In addition, the intervention group showed increased capacity and strength for a selection of PIMs, affirming the trainability of the PIMs in this population. However, these muscle adaptations were not associated with other findings. The findings support further research on the benefits of integrating PIM strengthening exercises in physical exercise programs, including fall prevention programs, for mobility and fall risk. In addition, future studies are needed to unravel the mechanism behind the reduction in concerns about falling. Chapter 7 presents a qualitative study exploring how adherence to a 12-week PIM strengthening program is experienced and influenced in older adults. Understanding their perspectives may improve adherence of the target population, which is generally low to moderate for foot exercise programs hampering the effectiveness Twelve participants (aged 69-91), all enrolled in the intervention arm of the randomized controlled trial reported in Chapters 5 and 6, were interviewed to gain in-depth insight into factors related to capabilities, opportunities, and motivation that shaped their adherence. The median total score on the six items of the Exercise Adherence Rating Scale (EARS) was 23.5 out of 24 (IQR: 20-24) and showed excellent adherence to the prescribed unsupervised home exercise session. Using reflexive thematic analysis with an inductive, experiential approach, four key themes were identified: "Getting started," "Hanging in," "Settled into the rhythm," and "Perceiving health benefits is the ultimate." These central themes were additional to the positive evaluation of intervention features: clear instructions from the trainer, supportive materials, and flexibility in timing and content of the training program. The identified phases reflect a temporal progression in participants' perception and illustrate how the appearance of factors influencing adherence is dynamic and phase-dependent. Surprisingly, health benefits (e.g., improved gait, enhanced self-confidence in walking, increased stability) were spontaneously revealed by a large proportion of the participants. The findings underscore that a sense of benefit, combined with well-designed support and a personalized approach, support continued adherence. These ins