Intertrial Rest for Maximum Grip and Key Pinch Strength in Japanese Young Adults

Participants

Study participants came from a pool of approximately 180 students majoring in occupational therapy at the Health Science University (Yamanashi, Japan). To estimate the target sample size, effect size (Cohen’s d) was calculated from the mean (standard deviation) values of two grip strength values as described by Trossman and Li (1989) and Dunwoody et al. (1996). This value was input into the estimated effect size, and the sample size (power level, .80; probability level, .05) was calculated. The range of most of the sample size results was between 3 and 23. However, because one value exceeding 40 was detected, the sample size was set at 40. The exclusion criteria were a previous history of neuromuscular or orthopedic dysfunction that would significantly affect hand strength and any condition that would restrict movement distal to the shoulder joint. This study was approved by the ethics committees of the Shinshu University (Nagano, Japan) Postgraduate School and Health Science University.

A total of 40 male and 40 female participants were included in this study. Mean age was 20.9 yr for the men and 21.2 yr for the women. The age range for both sexes was 20–22 yr. Mean (standard deviation) for heights and weights, respectively, for men were 171.1 cm (6.4) and 65.4 kg (15.9) and for women, 157.8 cm (5.5) and 55.0 kg (8.8). All participants except 3 women were right-hand dominant. No participant met the criteria for exclusion. All participants provided informed consent after receiving a written explanation of the study.

Instruments

To avoid reading errors by the researcher, equipment that provided a digital readout was used. A Jamar Plus+ digital hand dynamometer (Sammons Preston, Bolingbrook, IL) was used to measure grip strength, and a Jamar Plus+ digital pinch gauge (Sammons Preston) was used to measure key pinch strength. All participants used the hand dynamometer in the second handle position. Both instruments were newly purchased and displayed values in 0.1-kg increments.

Procedure

A repeated-measures design was used. After signing an informed consent form, participants provided information on their age, height, and weight, and the researcher ascertained their sex, dominant hand, and upper limb functionality through an interview.

The participants were seated with their shoulders adducted and neutrally rotated, elbows flexed at 90°, forearms in a neutral position, and wrists between 0° and 30° of extension, with between 0° and 15° of ulnar deviation, as recommended by Mathiowetz et al. (1984). A standardized script was used for providing instructions (Mathiowetz et al., 1984). To become familiar with the instruments and mitigate against a learning curve, participants performed two or more practice trials before each session. Practice was repeated until the attained value showed a decrease. After practice, participants were allowed to rest for ≥5 min, after which recorded trials were performed.

In a single session, three trials each for the right and left hands were performed. The grip and key pinch strength trials were performed using the same protocol, with a ≥30-min rest between the trials. Each participant was provided with standardized instructions at each assessment and was asked to apply maximum force to the device for 5 consecutive seconds for each measurement. Six intertrial rest periods were investigated—15 s, 30 s, 60 s, 90 s, 120 s, and 150 s—one per day on 6 different days, with an interval of ≥2 days between sessions. For each participant, the order of the six sessions and the hand to be tested first were determined randomly using a table of random numbers. It was not until all measurements were completed that a participant knew the results. All strength measurements were obtained by one examiner.

Statistical Analysis

Before statistical analyses, all data were separated into data groups according to sex, the hand tested, and the test type (grip or key pinch). The data for each rest interval of each data group were checked for normality using the Kolmogorov–Smirnov test. The mean and standard deviation difference (maximum − minimum) within each session and the effect size (partial η²) were then calculated.

A repeated-measures analysis of variance (ANOVA) or Friedman test was used to assess the presence of significant differences between the first trials of the sessions and differences between the three trials in a single session for each data group. Paired t tests were performed on the data (mean) from all possible combinations of the six conditions for each data group. The intraclass correlation coefficient (ICC_1,3) and 95% confidence interval (CI) of ICC were used to evaluate the correspondence between the three trials for each rest interval. All statistical analyses were performed using IBM SPSS Statistics (Version 24.0; IBM Corp., Armonk, NY). A p of <.05 was considered statistically significant.

Grip Strength

All data sets, except one (the second trial in men involving the left hand with a 120-s rest), were normally distributed. Using ANOVA, we found the differences to be negligible between any of the first trials of the data groups: p = .051 for women’s right hand and p = .848 for men’s right hand. Table 1 presents the results of the grip strength measurements. Using ANOVA or the Friedman test, we found significant decreases in grip strength over the three trials of all data groups, except two (the trials with women involving the right hand with a 120-s rest and with a 150-s rest). The mean value was the highest in the first trial of all data groups, and as the rest interval increased, the difference between each session’s minimum and maximum values tended to decrease. For both hands, effect sizes of <0.5 (partial η²) were found for ≥90 s in men and ≥60 s in women. Comparing the sessions using the paired t test, we found no significant differences in men at ≥60 s in the right hand or ≥90 s in the left hand. In women, no significant differences were noted in either hand at ≥90 s.

Table 2 presents the results of the ICC analysis, which showed a high degree of consistency (≥.8) for all test conditions, except for the condition involving men with a 15-s rest. ICC was high (≥.9) for both hands at ≥90 s in men and ≥30 s in women.

Key Pinch Strength

All data sets, except one (the first trial in men involving the left hand with a 60-s rest), were normally distributed. Using ANOVA or the Friedman test, we found no significant differences between any of the first trials of the data groups: p = .195 for men’s left hand and p = .802 for women’s left hand. Table 3 presents the results of the key pinch strength measurements. Using ANOVA or the Friedman test, we found significant decreases in pinch strength over the three trials in all data groups, except one (the trial in women involving the right hand with a 150-s rest). The mean value was highest in the first trial of all data groups, and the difference between each session’s minimum and maximum values decreased until 60 s, after which the difference did not fluctuate much. For both hands, effect sizes of <0.5 (partial η²) were found for ≥60 s in both male and female participants. Comparing the sessions using the paired t test, we found no significant differences for either sex at ≥60 s in either hand.

Table 2 presents the results of the ICC analysis, which showed a high degree of consistency (≥.8) for all test conditions. ICC was high (≥.9) for both hands at rest intervals of ≥90 s in men and for both hands at all rest intervals in women.