Number of Chair Stands Should Not Be Considered a Muscle Function Measure, But a Physical Performance Measure. What Can We Do Then?

We read with great interest the recent Letter to the Editor published in this journal about the use of the 30-s chair stand test as a measure of muscle function in older people (1). As it is stated in the letter, the assessment of muscle function in the clinical setting is of high relevance, since muscle dysfunction can be the predecessor of frailty and disability among older people. In this sense, the assessment of handgrip strength may be considered the most popular measure of muscle function that is being used in the clinical setting, and it is the preferred choice by the European Working Group on Sarcopenia in Older People (2). The main limitation derived from handgrip strength testing is that it poorly reflects lower limb muscle function or changes in lower limb muscle function resulting from interventions targeting the lower limbs. We could fairly assert that lower limb muscle function has a higher relevance than handgrip strength for some of the main activities required for an independent living: walking, chair rising and stair climbing. Therefore, we agree with Prof. McGrath about the necessity of a lower limb muscle function measure suitable for the clinical setting in terms of feasibility and clinical relevance. However, we disagree with Prof. McGrath’s proposal on the use of chair stands as a measure of lower limb muscle function (1). As noted by Prof. McGrath, the Short Physical Performance Battery, which includes the chair stand test, is used to examine physical performance, and not muscle function. Indeed, the 30-s chair stand test is widely considered a physical performance assessment. Although chair stand performance can be correlated to lower limb power and endurance, it is not a measure of muscle function, as well as gait speed is correlated to lower limb muscle function, but it is a measure of physical performance. Both tests indicate the rate at which an individual is able to perform a certain functional task (meters per second in the case of gait speed, and chair stands in a certain time period for the chair stand test). Importantly, the use of the chair stand test as a measure of muscle function can lead to erroneously diagnose muscle dysfunction in some – not infrequent – cases. For example, a lower count in the chair stand test might be the result of the individual presenting obesity, while he/she might present a normal lower limb muscle function (simply the excess of body mass impeded them to perform better in this functional task). So in this case the conclusion should be that physical performance is low, muscle function is normal, but there is an excessive body mass. This may lead to prescribe a different treatment (e.g. achieve a negative energy balance by diet and exercise) compared to the one that should be prescribed to a patient with low lower limb muscle function (e.g. power-oriented resistance training). However, there exists an alternative to use the chair stand test to assess lower limb muscle function in older people. We validated an equation that transforms chair stand performance (derived from either 30 s or 5 chair stands) into muscle power (in Watts [W]) (3, 4). To make it easier and more suitable for the clinical setting we also developed a free smartphone app available both for Android and iOS devices (5). Most importantly, muscle power obtained from the so-called sitto-stand (STS) muscle power test was found to be more strongly associated to older people’s physical performance than handgrip strength, sarcopenia, traditional measures of chair stand performance and leg extension power obtained with a ‘gold standard’ device (3, 4, 6). In addition, low STS power was independently associated to disability, hospitalization and mortality (7-9), and an operational definition and algorithm for its identification in older people has been proposed (6, 10). Of note, Prof. McGrath used one of the studies that used this equation to, in that case, erroneously justify the use of chair stands as a relevant measure to predict health outcomes (reference 7 on his letter) (1). Therefore, we greatly appreciate the debate raised by Prof. McGrath on this relevant and hot topic, but we strongly believe that the chair stand test should not be considered a muscle function assessment per se. We rather consider that the STS muscle power test can be used as a muscle function test, and in fact there is enough evidence showing its adequacy for the clinical setting and its functional and clinical relevance in older people.