Variability of grip strength during isometric contraction

Area of Science:

• Rehabilitation Medicine
• Biomechanics
• Ergonomics

Background:

• Measures of strength variability are increasingly used to assess sincerity of effort in medico-legal and rehabilitation contexts.
• The reliability and stability of these variability measures over time and across different conditions have not been well-established.

Purpose of the Study:

• To investigate the trial-to-trial variability of grip strength under maximal and submaximal effort conditions.
• To determine the test-retest reliability of the coefficient of variation (CV) for grip strength.
• To identify factors influencing grip strength variability, including effort level and gender.

Main Methods:

• Two studies were conducted involving 63 and 40 subjects, respectively, performing maximal (100%) and submaximal (50%) grip efforts.
• Grip strength was measured using a Jamar hand dynamometer, with the coefficient of variation (CV) calculated for every three repeat measures.
• Testing was performed on two separate occasions to assess interoccasion reliability of both grip strength and CV.

Main Results:

• Grip strength demonstrated very high interoccasion reliability, but the CV showed poor stability with reliability indices near zero.
• Submaximal effort levels resulted in significantly larger CVs compared to maximal effort.
• Females exhibited greater strength variability (higher CVs) than males.
• A 7.5% CV cut-off for indicating submaximal effort would misclassify 97% of females and 64% of males tested at maximal effort.

Conclusions:

• The coefficient of variation (CV) for grip strength is not a stable measure across test occasions.
• Current cut-off values for CV may unfairly discriminate against females due to inherent gender differences in strength variability.
• Further research is essential to understand factors influencing CV and to establish its validity as a clinical tool for assessing sincerity of effort.