Author:

Abstract:

Aging is accompanied by a gradual loss of functional ability and independence, which has a negative effect on individual quality of life and presents a significant and increasing burden on our society. The age-related loss of muscle mass and strength, also known as sarcopenia, is a major contributor to this loss of functional ability. Older women are particularly susceptible to sarcopenia due to the post-menopausal acceleration in loss of muscle mass. This is reflected by a relatively high rate of falling incidents and concurrent injuries like hip fractures in older women compared to men. Physical exercise is considered to be the most effective way to combat sarcopenia. Muscle plasticity, and therefore the ability to improve strength, appears to be well preserved with age. By improving muscle strength we can improve the functional limits of older adults and increase the reserve capacity for safe performance of everyday tasks. However little is known about the best training type and intensity to improve muscle mass and strength in older adults. Furthermore, training-based adaptations in older adults are often highly task-specific, meaning that improvements strength do not necessarily translate to improvements in functional ability. Evidence from previous intervention studies indicates that training programs for older adults should combine resistance exercise and task-specific exercise to achieve optimal results. However, time-constraints and limited long-term adherence of older adults to resistance exercise present a challenge for designing effective prevention training programs. Bench-stepping is a weight-bearing exercise with high functional similarity to activities of daily life such as stair negotiation. Because it is a weight-bearing exercise that requires vertical displacement of the body's center of mass, it also shows potential for transfer effects on muscle mass and strength. However, previous studies investigating the effects of bench-stepping and stair based exercise have found little to no improvements in muscle mass and strength, likely due to sub-optimal training intensities. Kinetic analyses show that increasing step height beyond heights commonly encountered in daily life results in significant increases in peak power output. This indicates that, by using incremental step heights, bench-stepping could be a viable and task-specific exercise to improve muscle mass and strength. An important factor to take into account is that different muscles play specific roles during motor tasks such as stair negotiation and that muscle weakness in any of these muscles can compromise correct and safe task performance. Unfortunately, previous studies found that correct and safe task performance of everyday tasks in older adults is often compromised, as indicated by modulation of their motor strategies. Although bench-stepping with incremental step heights may improve overall power output, it is essential to assess which stepping modalities (e.g. step height, step direction) are required to sufficiently recruit all muscles involved. However, due to the mechanical indeterminacy of the musculoskeletal system and the inability to measure individual muscle loading in vivo it remains hard to directly assess if bench-stepping with incremental step heights can elicit sufficient muscle loading to increase mass and strength in individual major lower limb muscles of older adults. The core of this doctoral thesis consists of four studies, divided into three sections. In Section 2.1. we used EMG to compare peak muscle activation between bench-stepping and resistance exercises performed at 60% of one-repetition maximum (1-RM), which is the recommended intensity for hypertrophy and strength gains defined by the American College of Sports Medicine. Using this comparison, we assessed the training potential provided by different step heights and directions. 1-RM measurements are commonly applied to calculate the relative intensity of exercises. However, older adults have more difficulty performing maximum contractions. Moreover, previous research has found that EMG signals obtained from maximum contractions can be unreliable. This has important implications for the selection of appropriate signal normalization, which is an essential methodological consideration when comparing muscle activation between subjects and estimating individual muscle loads. In study 1, comparisons between maximum isometric and (sub-)maximum dynamic contractions revealed that differences in maximum excitation between different contractions types are age-dependent and that normalization to sub-maximum dynamic contractions appears to be the best approach for older adults. In study 2, nineteen older women (69.3 ± 3.4 years) performed stepping trials with step heights of 10, 20 and 30 cm in forward and lateral directions and upper leg resistance exercises. Our results revealed that, for most upper leg muscles, a step height of 20-30 cm was required to achieve similar EMG amplitudes to resistance exercise at the recommended threshold of 60% 1-RM. For the gluteus medius, which is an important muscle for medio-lateral stability, a step height of at least 30 cm was required to reach threshold EMG amplitudes. Although these findings could be used to design effective prevention training programs for older women, they are still only based on an approximation of relative training intensities. Therefore, a randomized controlled trial was needed to assess the clinical impact. In Section 2.2. we used the findings from study 2 to design a bench-stepping program, dubbed the Strength Training for Elderly through Elevated stePping (STEEP) program. By adapting a task-specific exercise modality such as bench-stepping to incorporate both incremental step heights and forward and lateral stepping directions, we aimed to simultaneously improve muscle mass (volume), strength, power, functional ability and balance performance in older women. Forty-five community-dwelling older women (68.8 ± 3.9 years) were assigned to the intervention (STEEP) group or a non-exercise (CONTROL) group. The STEEP group received 12 weeks of bench-step exercise and training intensity was primarily determined by incrementing step heights. Results from this intervention study showed that the STEEP program increased muscle volume, isometric peak torque, power, unloaded rate of velocity development and improved performance on all functional tests with exception of countermovement jump height and static balance. Additionally, low drop-out and positive scores on feelings related to the exercises indicated a high likelihood for long-term training adherence in older women. These findings show that bench-stepping with step heights that exceed those encountered in daily life can counteract the age-related declines in muscle mass, strength and functional ability in community-dwelling older women. In response to the age-related declines in muscle strength, older adults tend to modulate their motor strategies. This allows them to perform tasks such as stair ascent within their functional limits and compensate for reduced balance control. In Section 2.3. we investigated if muscle synergy analysis could detect age-related differences in motor strategies of older adults during step ascent. Our results showed that synergy analysis during more challenging tasks, such as bench-stepping with increased step height, was able to detect subtle age- and step height-related differences in synergy organization and activation patterns, but did not show differences in synergy number. Therefore, assessment of synergy recruitment during more challenging tasks, such as bench-stepping with increased step height, might provide a way to detect early-onset deterioration of functional performance. However, additional longitudinal studies are needed to assess if synergy analysis during step ascent can identify older adults with increased risk of developing disability to target for prevention training. Overall, this thesis shows that we need to rethink the resistance training/functional training dichotomy and that task-specific exercise can be adapted to simultaneously improve muscle characteristics and functional ability. By employing EMG analyses, we can overcome challenges with regard to estimation of training intensity imposed by the redundancy of the musculoskeletal system.