The aging process is associated with the decline of physical performance, which increases the risk of falls in the older population. Falls have been considered as a public health problem, because they increase the risk of negative health outcomes, like fractures, loss of independence, depression, social restrictions, hospitalization, institutionalization, and death.1 The impairment of physical function performance has been associated with reduced muscle lower limb strength.1-4 To create a physical therapy program targeting enhancement of physical performance through muscle improvement, the assessment of specific muscle groups may be needed.

However, besides muscle strength, other muscle variables, i.e., rate of torque development (RTD) and torque steadiness (TS),5,6 should be taken into consideration to better understand the muscle contribution to physical function in older adults. The RTD is defined as the capacity to rapidly generate torque at the beginning of muscle contraction (200 to 300 ms).5 Neural factors seem to have a greater influence on RTD in the initial phase of muscle contraction (< 40 ms) and maximum muscle strength seems to have more influence in the later stages of muscle contraction (> 90 ms).7 On the other hand, TS is defined as the ability to maintain a submaximal contraction at a given torque level,8 which means that a greater torque contraction variability may be associated with motor control deficit, increasing the risk for physical performance impairment in older adults.8

Preventing impairment of physical function and loss of independence is an essential goal among health care professionals to help older adults achieve a successful aging process and good quality of life. To the best of our knowledge, no study has so far proposed a conceptual framework for trunk and lower limb muscle roles regarding tests widely used in the Geriatric and Gerontological clinical practice and recommended by the World guidelines for falls prevention and management for older adults,9 the American Geriatrics Society,10 and European Working Group in Sarcopenia in Older People.11 Previous studies have suggested assessing the ability of older adults to perform challenging tasks, such as step tests and tandem gait.4,12 Although previous studies have shown that there are sex differences related to functionality and risk of falls,13-15 recruiting and engaging older men in research remains challenging.16,17 Therefore, this study aimed to verify the association between the trunk and lower limb muscles and physical performance in community-dwelling older women.

Our study presents a conceptual framework regarding the association of physical performance and muscle variables in community-dwelling older women. The result overview shows that different physical performances are associated with different muscle variables, and although PT is an important muscle parameter, TS rather than RTD is a parameter involved in physical performance, which means that besides the maximum muscle strength, the ability to maintain a submaximal contraction at a given torque level (TS) must also be considered for future longitudinal research that aims at verifying the role of muscle groups and muscle variables to predict changes in physical performance over time.

Gait speed has been used as an important health status indicator. When it is slow, the older adult has a higher risk of functional decline, social restriction, depression, falls, institutionalization, and mortality.29,30 Therefore, the improvement of gait speed usually is a target in a physical rehabilitation program. Our results are in agreement with previous studies that have shown that walking does not require a maximum muscle strength generation,31,32 because we identified that habitual gait speed was only associated with PT of the hip flexors, and higher RTD of the hip adductors and knee flexors, which represents the ability to generate torque at the beginning of muscle contraction. Also, gait speed was associated with torque steadiness at 10 % of PT of the trunk flexors. Our results point out to the direction that gait does not require stronger muscle, which is probably the reason that older adults, even with some degree of dysfunction, are able to walk short distances in unchallenged situations, such as flat surface, even though adopting a reduced gait speed.33

On the other hand, in clinical practice, there are many other physical tests that demand more from trunk and lower limb muscles. Mobility performance has been widely evaluated using the TUG test, which requires different motor tasks, as standing up from a chair, walking at a usual pace, turning, and returning to sit down on the chair.34 For this task, stronger trunk flexors and extensors, hip flexors and extensors, and ankle plantar flexors were associated with a better performance, probably because these muscle groups are involved in the movement of standing up from and sitting down on the chair. Similarly to gait speed, in TUG performance, the RTD 200 ms of the hip adductors and RTD 30 ms of the knee flexors are involved, probably associated with the gait domain. Also, the rapid hip extensors and adductors strength generation may be required to achieve the fast upright posture after standing from the chair, and the rapid action of the hip adductors is required to maintain hip alignment. The ability of the adductor muscles to rapidly generate torque may be related to the fact that in complex movements, where transfer and rotation components are required, it is likely that the hip adductor muscles are bilaterally and simultaneously active to control both femoral-on-pelvic and pelvic-on-femoral hip movements.35

The 5TSST has been used to evaluate muscle strength and power of lower limb muscles,36 and a poor test performance may predict future falls and disability in the older population.37 For this test, stronger trunk extensors and flexors, hip flexors, knee flexors and extensors, ankle plantar flexors and dorsiflexors were associated with a better test performance. Our results are aligned with those by Porto et al.,38 who showed that 5TSST is able to discriminate older women with reduced global muscle strength. The submaximal control (TS) of the hip extensors and abductors contraction seems to influence the test, as the ability to rapidly generate strength (RTD) of the trunk flexors and ankle dorsiflexors were associated with the test performance, probably because it is required to sit down again quickly.

Step tests have been proposed as a challenging test that can evaluate the physical performance of older adults during a motor activity that requires single lower limb support.4,12,22 For the forward step test, stronger PT of the trunk flexors and extensors, hip flexors and adductors, knee flexors and extensors, and ankle plantar flexors and dorsiflexors were associated with a higher step that a woman could climb. Our findings are in line with the concept that step climbing requires a higher demand for muscle strength.4,39 Our results also demonstrate that the ability to control the muscle contraction of the trunk flexors, hip extensors, abductors, adductors and ankle plantar flexors, as the ability to generate rapid strength (RTD) of the hip adductors and knee flexors are important components for achieving a higher step.

For the lateral step test, stronger trunk flexors and extensors, hip flexors and abductors, knee extensors, and ankle plantar flexors and dorsiflexors were associated with a higher step that women could climb. After a fatigue protocol of hip abductor muscles, Martins and collaborators40 observed a worse performance of lateral step, which is in line with our findings. The ability to control the muscle contraction at 10 % of PT for the hip extensors, abductors, and adductors and 50 % of PT for ankle dorsiflexors was associated with a higher step, probably because it allows better control of the movement, allowing better trunk-pelvic alignment and lifting the foot during step climbing.

Tandem gait has been used to challenge mediolateral stability because it is a dynamic task performed on a narrow support base. Aligned with our results, many previous studies12,41 have suggested that this task demands lumbopelvic stability. Porto et al.,12 have proposed the combination of lateral step and tandem gait to identify older women with reduced abductor muscle strength and in agreement with our results, for a better performance of lateral step and tandem gait, greater hip abductor muscle strength is required. Additionally, the greater the strength of the trunk extensors and hip extensors, the faster the women completed the test, probably due to the need of keeping the trunk-pelvic alignment during the task.

Van Cutsem et al.42 showed that neural adaptations caused by explosive strength training are the main factors for an increase in RTD. However, heavy-resistance strength training can also lead to improvements in the capacity of torque generation during muscle contraction, in addition to increasing PT.43 So far, it is not possible to define an optimal mode of training to improve TS. However, studies show that Tai Chi44 and resistance training with heavy load45 can improve TS in older adults. Therefore, because muscle performance adaptations are highly specific depending on the training program, understanding different muscle parameters in relation to functional performance allows the elaboration of a more specific physical therapy program. Then, our results may guide the study design of future longitudinal research to confirm causality and assist physical therapists in their decision-making.

The community-dwelling older women population was chosen to establish a referential base for the understanding of muscle variable role on physical performance. One of the limitations of the study was the fact that we only included older women, because we did not have a representative male sample size, and the inclusion of men in our results could be a bias because we have previously shown that the performance is different between older men and women, probably because men are stronger and taller than women.13 Another limitation is that this was a cross-sectional study; therefore, to confirm causality, a longitudinal cohort study is needed. Furthermore, studies are needed to confirm whether the training of specific muscles, suggested in the conceptual framework, will improve physical performance.

The results show that different physical performances are associated with different muscle variables. Gait speed was the test the least associated with muscle strength performance. The 5TSST and the forward and lateral step tests seem to be the physical tests most associated with muscle strength. Also, performance on the lateral step and tandem gait were associated with stronger hip abductor muscles. The forward step, lateral step, and 5TSST were the tasks most associated with the ability to control a submaximal torque. The ability to rapidly generate torque seems to have an important role mainly for the TUG and gait speed, although it also showed association with the 5TTST. Trunk muscles, mainly PT, must also be considered for the performance of physical function.