Optimization and construct validity of approaches to preclinical grip strength testing

Abstract

Grip strength is a robust biomarker showing good reliability^{1, 2} and prediction of negative health outcomes.^{3, 4} Low grip strength is associated with disability and premature death^5-9 and is more strongly associated with frailty than chronological age.¹⁰ Accordingly, recent updates to consensus definitions of sarcopenia focus on low grip strength as the primary characteristic as opposed to low muscle mass.^{11, 12} Because rodent models are indispensable tools in aging research, scientists have reverse-translated grip testing as a key outcome in the context of sarcopenia.^13-16 Serendipitous development of preclinical grip testing has resulted in a variety of protocols that have not been extensively examined and compared.^{13-15, 17} Variability, due to motivation, temperament, and other factors such as pain, is inherent in preclinical behavioural assessments.¹⁸ Limited research has focused on standardizing preclinical grip testing, validation of methods against other functional measures, and investigating how preclinical grip data compare to data from humans and whether these tests even measure the same construct. Additionally, prior work has not examined between-day reliability of grip testing in rodents. This work was undertaken to inform rigorous preclinical grip testing.

Differences between clinical and preclinical grip must be considered when reverse translating methods to mice. Clinical grip testing is volitional whereas preclinical testing depends on reflexive responses. Prior clinical studies have consistently shown ICC ≥ 0.80 for repeated grip strength testing.¹ No data is available regarding the reliability of grip strength methods in mouse models. One study tested grip strength across three successive trials at a single study timepoint (ICC ranging 0.363–0.803) but did not assess reliability across days.²⁴ Our study showed that preclinical grip testing methods are less reliable compared to prior clinical studies. Based on CV, all limb grip testing was the most reliable method; based on ICC, bilateral hindlimb and forelimb grip testing were the most reliable methods. Thus, when choosing a method for grip assessment in aged mice where hindlimb assessment is critical, both all limb and bilateral hindlimb methods appear to be the best options for repeatability. Of note, how mice are grasped, tail or scruffing, was not assessed herein, but might impact results. Thus, further work is needed to better refine preclinical grip testing protocols.

The relationships between grip strength and indices of muscle mass were explored in a clinical cohort to compare these same relationships in mice. The age-related differences in grip strength noted in both our clinical and preclinical age comparisons were more overt as compared with losses of lean/muscle mass. These findings are aligned with the continued evolution of sarcopenia diagnostic criteria, which are increasingly focused on loss of muscle function rather than size/mass.^{3, 11, 12} We found moderately strong relationships between clinical grip strength and DXA estimates of lean mass in humans. The muscles that are recruited during grip testing in mice have not been determined, and thus, we used soleus and gastrocnemius muscle mass as proxy measures of muscle mass. Associations in mice were considerably less robust than in the clinical cohort. However, for select grip methods, associations were comparable. Here, it is important to point out the possibility that large differences in sample sizes and size of mouse muscles versus lean mass in humans could impact correlations.

Our work provides insight into the construct validity of grip testing in rodents, which shows reasonable overlap with the conceptual elements of clinical grip testing. In humans, grip is a highly evolved, complex task that imposes great demands on the central nervous system.²⁵ Tests of maximum grip strength require that the human brain modulates recruitment of 19 muscles within the hand and another 20 muscles located in the forearm in a spatially and temporally differentiated pattern. Thus, the force measured during grip strength testing depends not only on activation of muscles that flex the fingers but also on the ability of the nervous system to engage muscles that orient the fingers as well as those that stabilize the hand and wrist. Impairments in the nervous system to fully activate the grip musculature become exacerbated with advancing age (for a review, see Clark and Carson²⁶). Neural impairment likely explains why ~60% of variability in grip strength was explained by muscle mass in humans. We speculate that in mice, the even lower association between grip strength and muscle mass is due to similar neural impairments but that broader issues related to animal motivation further impact the data.¹⁸ In addition to correlations with muscle mass, in our preclinical studies, we also used muscle contractility as a non-behavioural, proxy measure of muscle function. We previously showed that these measures are robust indicators of neuromuscular function in aged mouse models.^{20, 27} Muscle contractility showed correlations with all five testing methods, further supporting physiological validity of grip testing in mice.

In summary, grip strength is a standard method for assessing muscle function in preclinical and clinical studies. This work informs technique, study design, and implementation of preclinical aging studies. It also provides insight into the construct validity of grip strength testing in rodents having reasonable overlap with the conceptual elements of grip strength testing in humans. Our findings suggest a slight edge to the all limb over the bilateral hindlimb method in retest reliability and statistical significance between young and aged mice. Given the clinical significance of grip strength as an indicator of overall health outcomes in older adults, this optimization of grip strength testing techniques in preclinical studies is crucial to the validity and translatability of future preclinical studies exploring the mechanisms of sarcopenia and potential therapeutics to combat aged-related decline in motor function.

This work was supported by funding from NIA/NIH R56AG055795 and R03AG067387 to WDA, R01AG067758 to WDA and BCC, and R01AG044424 to BCC.

The authors declare no conflict of interest.