http://informahealthcare.com/ahb ISSN: 0301-4460 (print), 1464-5033 (electronic) Ann Hum Biol, 2015; 42(3): 199–209 ! 2014 Informa UK Ltd. DOI: 10.3109/03014460.2014.934920

REVIEW PAPER

Age as a moderator of the secular trend for grip strength in Canada and the United States Irwin W. Silverman

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Department of Psychology, Bowling Green State University, Bowling Green, OH, USA

Abstract

Keywords

Purpose: To determine whether grip strength changed on average over recent decades at each of two age levels (children and adolescents versus adults) in Canada and the US. Methods: For each sex, weighted least squares regression analyses were performed on mean grip strength values as reported in studies conducted from the 1960s onwards. Results: Grip strength did not change significantly as a function of year tested in children and adolescents, whereas it declined as a negatively accelerated function of year tested in adults. Conclusion: The results are contrary to what might be expected given that body weight has increased in both countries in recent decades and given that grip strength has been found to be positively correlated with body weight. It is suggested that there was a concurrent change in a factor that operated to counteract the effect of increased body weight on grip strength, a prime candidate being a decrease in levels of serum vitamin D. It is also suggested that the secular decline in adult grip strength can be explained by a factor that affects muscular function but which has a long latency period, a prime candidate here being obesity history.

Age differences, Canada, grip strength, secular trend, United States

Introduction There is a large amount of evidence indicating that over the entire age span, grip strength increases as a linear function of body weight (e.g. Bohannon et al., 2012; Butler, 1997; Clement, 1974; Gu¨nther et al., 2008; Ha¨ger-Ross & Ro¨sblad, 2002; Milliken et al., 2008; but see Kuh et al., 2005). As body weight has increased dramatically over the past few decades in both Canada and the US (MMWR 14 January 2011; Ogden & Carroll n.d.; Shields, 2006; Shields et al., 2011; Torrance et al., 2002), it might be expected that grip strength increased concurrently in these two countries. However, when Silverman (2011) put this expectation to the test, he found in two meta-analyses no evidence for a secular increase in grip strength in Canadian and US children and adolescents. More specifically, these meta-analyses revealed that, from the 1960s onwards, grip strength had slightly declined in boys but had not changed in girls. As the secular decline in boys accounted for far less than 1% of the variance in their grip strength scores, Silverman suggested that this result was due to chance. If this suggestion is correct, the conclusion follows that grip strength remained essentially unchanged in Canadian and US children and adolescents over recent decades. However, the foregoing conclusion may be in error. The basis for doubt in this regard is a critique directed by

Correspondence: Irwin W. Silverman, Department of Psychology, Bowling Green State University, Bowling Green, OH 43403, USA. Tel: 561-752-9150. E-mail: [email protected]

History Received 27 September 2013 Revised 2 April 2014 Accepted 11 April 2014 Published online 21 July 2014

Trzesniewski & Donnellan (2010) to the use of meta-analysis to appraise population characteristics. As noted by these authors, meta-analyses are based on the results of studies that typically involve convenience rather than probability samples. Whereas probability samples are selected to be representative of particular populations, convenience samples may or may not match the population of interest. Consequently, metaanalyses are liable to yield results that are misleading with respect to population characteristics, including secular trends. The studies included in the meta-analysis conducted by Silverman (2011) used, with one exception, convenience samples. Thus, given the uncertainties that attend convenience samples, it might well be that the secular trends found by Silverman are not real. Contrary to Trzesniewski & Donnellan (2010), the present study is based on the premise that a secular trend identified through a meta-analysis can be demonstrated as being likely valid. Moreover, it can be demonstrated whether or not the studies included in the meta-analysis used convenience samples. The way to demonstrate this is by showing that the secular trend in question holds up as more and more studies are added to the dataset. The underlying assumption here is that if a condition has a widespread effect on some outcome, we should observe a change on the outcome in diverse study samples. Alternatively, if a condition does not have a widespread effect on the outcome in question – or if the effect of the condition is counteracted by another condition – then we should not observe a change on the outcome in all (or most all) study samples. Another underlying assumption is

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that biases due to sampling error are likely to be balanced as more and more diverse study samples are added to the dataset. The major impediment to implementing the approach just outlined is finding studies to add to the dataset. However, this may be less of a problem than what appears at first sight. There are several sources for finding additional studies: published studies that were overlooked in the literature search; unpublished studies that were left untapped in the literature search; and newly published studies. With respect to the dataset meta-analysed by Silverman (2011), it so happens that it can be augmented by studies from all three sources mentioned. Thus, the present study had as its first goal to test the validity of the secular trend found by Silverman with Canadian and US children and adolescents. The present metaanalyses were conducted on 28 studies for both sexes, whereas the meta-analyses by Silverman were conducted on 19 studies for boys and 18 studies for girls. (Silverman incorrectly stated that the dataset analysed by him consisted of 18 studies for boys and 17 studies for girls.) One limitation of the meta-analysis by Silverman (2011) is that it did not include studies with adults. This owed to Silverman not having found enough studies with adults to provide a fine-grained picture of changes in grip strength over time. However, as grip strength studies are scattered in a variety of research literatures, it seemed worthwhile to undertake another search for relevant studies with adults. The search for grip strength studies with adult samples living in Canada and the US proved to be fruitful, as it yielded 22 and 23 studies for men and women, respectively. Thus, the present study had as its second goal to assess the secular trend in grip strength in Canadian and US adults. Two predictions were tendered. The first was that grip strength would show no change over recent decades in children and adolescents living in Canada and the US. This prediction was based on the results obtained in the metaanalysis performed by Silverman (2011), assuming that the secular trend for boys was the same as that for girls. The second prediction was that grip strength would show a decline over recent decades in adults living in Canada and the US. This prediction was based on the results obtained in a study (Shields et al., 2010) that compared two nationally representative samples of Canadian adults, one having been tested in 1981 and the other in 2007–2009.

Methods Literature search The literature search proceeded in three steps. First, the following electronic databases were searched for pertinent articles: CINAHL, Medline, OregonPDF in Health and Performance, Physical Education Index, PubMed, PsycINFO and PQDT Open, SPORTDiscus. These databases were all searched by Silverman (2011) with the exception of the PQDT Open and OregonPDF in Health and Performance databases, both of which provide open access to unpublished theses and dissertations in the field of physical education. The search terms used were grip strength, grip force, hand grip, physical fitness, musculoskeletal fitness and neurological test battery. Second, the reference lists of the retrieved studies were read

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in order to locate additional relevant articles. Third, literature reviews by Bohannon et al. (2006) and Innes (1999) were checked for additional studies. Selection criteria To be included in the present meta-analyses, the data had to come from a study that met the following criteria: (1) The study was published from 1960 onwards, a criterion that was imposed because the literature search turned up no Canadian studies of grip strength that were conducted before the 1960s. (2) The means for the two sexes were reported separately or the senior author was able to provide the means on request. (3) The sample size for each sex was at least nine (this being the smallest sample in the meta-analysis conducted by Silverman (2011)). (4) The sample fell within the age range of 6–79 years. (5) The age range was no more than 5 years for participants less than 20 years old and no more than 20 years for participants 20 years old or older. Exceptions to these age ranges were made if the age range extended no more than 2 years at either end of the age range. If the sample was composed of college students and if no information about age was given, the average age was estimated as 19 years. (6) The study sample was recruited from the general population rather than a select population, say ‘athletes’. Included within the definition of general population were study participants belonging to a single ethnic or racial group. (7) The study sample had to be healthy and free of any injury that might affect grip strength. One exception is that, in older samples (60+ years), a small degree of disability was not considered to be a disqualifying factor. (8) Grip strength was measured in terms of force rather than pressure. (No retrieved study failed to meet this criterion.) (9) If the study results were reported graphically, means for grip strength had to be readable to an accuracy of 1 kg. The data for two of the retrieved studies (Nicolay & Walker, 2005; Sella, 2001) that met the above criteria were not retained because the grip strength values reported are highly discrepant from those that have been found in studies with participants of comparable ages. (It should be noted that the test instrument used by Nicolay and Walker differed in design from the instruments that have almost universally been used to measure grip strength.) With two exceptions, the studies included in the present meta-analyses used convenience samples. The two exceptions used nationally representative samples of Canadians, one involving children and adolescents (Tremblay et al., 2010) and the other, as mentioned above, involving adults (Shields et al., 2010). However, the convenience samples were diverse with respect to geographical areas represented; for example, the samples of US children and adolescents came from at least 15 states. Although grip strength has generally been measured in both hands, investigators have varied as to how they have reported the resulting data. Most often, they have reported the

Age and grip strength

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DOI: 10.3109/03014460.2014.934920

values obtained for the right and left hands separately, for the dominant and non-dominant hands separately or for the preferred and non-preferred hands separately. Less often, they have reported the values for the two hands summed, for the average of the two hands or for the best value for the two hands. In the present meta-analyses, grip strength was analysed in terms of the summed values for the two hands, as these values were either directly given or could be readily computed from the information given in the study report. Thus, no use was made of the data for the relatively small number of studies in which grip strength was measured in only one hand or was given in terms of the best value for the two hands. It should be noted that, when grip strength was measured in only the dominant or non-dominant hand or in only the preferred or non-preferred hand, the study report typically did not state the criteria by which hand dominance or preference was categorized, thus raising the question of the comparability of results from studies that ostensibly measured grip strength in the same hand. Measurement of grip strength could be affected by a number of methodological factors, including body position (standing or sitting), arm position (straight or bent), number of practice and test trials and, given that more than one test trial is used, whether the mean or maximum value is recorded. Modelling the effects of these variables was not possible, however, because the necessary information to do so was lacking in a substantial proportion of study reports. Relevant in this connection is Silverman’s (2011) observation that evidence has been mixed about the effects on results for two of the methodological variables—body position and whether the mean or maximum value is used to represent grip strength when grip strength was measured on more than one trial. Another methodological variable that could not be modelled is the type of test instrument used. For one thing, the test instrument used was not stated in a substantial number of study reports. For another, at least six different test instruments were used in the studies included in the present metaanalyses. (Curiously, the literature contains virtually no information about the comparability of the measurements yielded by the different instruments, known as dynamometers, used to test grip strength.) Another variable that could affect study results is the accuracy of the test instrument. Study reports rarely mention whether the test instrument used was calibrated against a known standard, but it seems likely that the errors in measurement for different test instruments vary randomly in direction and magnitude. This assumption is supported by results reported by Castro-Pin˜ero et al. (2010), who compared three test instruments produced by three different manufacturers against known standards ranging from 20–70 kg. They found that two of these instruments were negatively biased, one by 1.92 kg and the other by 1.43 kg, and that the third instrument was positively biased, by 0.49 kg. (Unfortunately, there are no data as to whether errors vary randomly for different copies of the same instrument.) Of particular interest here is the year in which the participants were tested. Year of testing was mentioned in only a few study reports; in the absence of this information, it was estimated as year of publication minus 2.

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The data were analysed for evidence of secular changes in grip strength according to age and sex. Specifically, the data for children and adolescents (aged 6–19) were analysed separately from the data for adults (aged 20–79) and, for each of these age levels, the data were analysed separately by sex. In these analyses, the data were subjected to a weighted least squares hierarchical multiple regression analysis with the weights being the sample sizes for each sex. The dependent variable was mean grip strength and the predictors were age, age squared, country (Canada ¼ 1, US ¼ 0), year tested and year tested squared, with the predictors being entered into the equation in the order just mentioned. To minimize multicollinearity, all of the predictor variables except country were mean-centred. The analyses were carried out using SPSS version 15.0 for Windows.

Results Children and adolescents The child and adolescent data analysed here consisted of the data given in Silverman (2011, Table 1) plus the data given here in Table 1. As seen in Table 2, there was a total of 79 means for boys and 80 means for girls, with the total number of boys and girls being 6814 and 6583, respectively. The regression analyses (see Table 2) show that grip strength in boys declined across year tested at a borderline level of significance, b ¼ 0.10, p ¼ 0.06, whereas in girls grip strength was not significantly related to year tested, b ¼ 0.04. Regarding boys, it should be emphasized that the per annum decline in grip strength was not only small, but that it accounted for far less than 1% of the variance in their grip strength scores. As expected, the regression analyses showed that grip strength increased with age over childhood and adolescence. Table 1. Descriptive statistics for additional studies of grip strength in children and adolescents by country. Males Study

Year Age Mean tested range age

Canada Katzmarzyk (1997)

1996

US Gallup et al. (2010)

5–9 7.0 10–14 12.0 15–19 17.0

2008 13–16 16–19 Garcia (1991) 1989 12–17 15–18 Gordon et al. (1981) 1979 – Isaacs & Frederick 1983 – (1985) Jackel (1972) 1972 9–12 Lirgg et al. (2011) 2009 – Lunde et al. (1972) 1970 – Siegel et al. (1989) 1987 – Svehla (1991) 1990 – – – Woodard (1988) 1988 –

14.2 17.0 13.8 16.4 19.7 10.0

n

n

M (kg)

37 25.50 22 48.90 16 102.30

23 20 15

22.26 44.60 63.60

68 69 – – 25 30

70 93 76 23 25 30

55.20 59.50 49.34 56.43 46.96 22.07

21.77 57 27.16 40 – 57 27.16 40 32.69 103 36.26 109 41.12 106 84.37 –

16.29 22.12 57.15 22.12 30.06 33.70 38.76 –

9.7 43 9.0 56 19.0 – 8.4 56 9.0 105 11.0 107 12.0 109 15.0 50

M (kg)

Females

72.68 92.85 – – 97.02 28.12

Grip strength measure is the sum of values for the left and right hands, preferred and non-preferred hands or dominant and non-dominant hands.

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Table 2. Summary of hierarchical regression analyses for predicting grip strength at each age level for each sex.

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Step/predictor

B

SEB



Children and adolescents: Males (k ¼ 79, n ¼ 6544) 1. Age 6.79 0.21 0.96** 2. Age squared 0.09 0.05 0.05 3. Country 0.03 0.04 0.02 4. Year tested 0.10 0.05 0.05y 5. Year tested squared 0.00 0.00 0.00 6. Constant 52.03 1.26 Children and adolescents: Females (k ¼ 80, n ¼ 6413) 1. Age 3.90 0.15 1.08** 2. Age squared 0.29 0.04 0.30** 3. Country 0.01 0.03 0.01 4. Year tested 0.00 0.04 0.04 5. Year tested squared 0.00 0.00 0.02 6. Constant 44.76 1.02 Adults: Males (k ¼ 84, n ¼ 12 224) 1. Age 0.42 0.02 0.71* 2. Age squared 0.01 0.00 0.31** 3. Country 0.02 0.03 0.03 4. Year tested 0.13 0.03 0.15** 5. Year tested squared 0.01 0.00 0.22** 6. Constant 102.16 0.70 Adults: Females (k ¼ 73, n ¼ 12 076) 1. Age 0.28 0.02 0.81** 2. Age squared 0.01 0.00 0.39** 3. Country 0.01 0.03 0.03 4. Year tested 0.13 0.03 0.21** 5. Year tested squared 0.02 0.01 0.22** 6. Constant 60.72 1.08

R2

DR2

0.954 0.955 0.956 0.958 0.958

0.954 0.001 0.000 0.003 0.000

0.841 0.906 0.906 0.908 0.909

0.841 0.065 0.000 0.002 0.001

0.711 0.837 0.841 0.871 0.912

0.711 0.125 0.004 0.030 0.041

Finally, the regression analyses found that grip strength did not differ between countries in either men, b ¼ 0.01, or women, b ¼ 0.02. Age changes Scatterplots of the data (see Figures 1 and 2) show that grip strength in each sex increased rapidly over the course of childhood and adolescence. Grip strength continued to increase during early adulthood, reaching a peak at 20–40 years, and then decreased gradually with age. The scatterplots also show that the grip strength scores within ages varied far more during adulthood than during childhood and adolescence. It should also be pointed out that the curvilinear change with age in females’ grip strength found when the data for childhood and adolescence were considered separately is no longer observable when the data for this age level are considered simultaneously with the adult data. From age 6 to about age 20, the increase in grip strength with age is linear in both sexes.

Discussion 0.631 0.793 0.801 0.871 0.892

0.631 0.162 0.008 0.069 0.021

B, SEB and are the values on the final step of the regression. k ¼ number of means. Country: Canada ¼ 1, USA ¼ 0. y p50.10. *p50.01. **p50.001.

However, as there is no set age at which adolescence ends and adulthood begins, age changes in grip strength should be examined in terms of age changes across adolescence and adulthood considered simultaneously. This examination will be undertaken after the results for the adults are presented. For both sexes, the regression analyses found that grip strength in children and adolescents did not differ between Canada and the US. Adults Table 3 lists the studies with adults. As seen in Table 2, these studies yielded 83 means for men and 73 means for women, with the total number of men and women being 12 100 and 12 050, respectively. The regression analyses (see Table 2) found that grip strength was significantly and negatively related to year tested in men and women, bs ¼ 0.14 and 0.12, respectively, as well as to year tested squared in men and women, bs ¼ 0.01 and 0.02, respectively. These results indicate that in both sexes grip strength declined from the 1960s on as a negatively accelerated function of year tested. The regression analyses also found that grip strength was curvilinearly related to age in both sexes. In the following section, age changes in grip strength in adulthood will be discussed along with age changes during childhood and adolescence.

As predicted, the present study found that age level moderated the secular trend for grip strength in Canada and the US. More specifically, grip strength remained essentially unchanged from the 1960s on in children and adolescents, but declined over that time period in adults. For children and adolescents, it will be recalled, Silverman (2011) found that grip strength had declined significantly over time in boys. In contrast, although the present study found a decline in boys’ grip strength over time, the decline fell short of the 0.05 level of significance. More important, however, year tested accounted for far less than 1% of the variance in boys’ grip strength with both the original and the present datasets. Thus, it may be concluded that boys’ grip strength changed negligibly (if at all) over recent decades, which is consistent with the non-significant change over time in grip strength found in girls with both the original and present datasets. The present results contradict the findings reported by Tremblay et al. (2010) with representative samples of Canadian children and adolescents. Their findings show that, in both sexes, grip strength significantly declined over the period from 1981–2009. In view of these results, weighted least squares regression analyses parallel to those reported above were performed on only the data analysed herein that came from the studies of Canadian children and adolescents. There were 12 such studies conducted over the period from 1967–2009 and they yielded 30 means for each sex. The regression analyses conducted on the data found that year tested was not significant in either boys (b ¼ 0.02) or girls (b ¼ 0.08) and that year tested squared was also not significant in either boys (b ¼ 0.01) or girls (b ¼ 0.01). These analyses further showed that age and age squared taken together accounted for 97.0% of the variance in grip strength in each sex, thus indicating the extreme unlikelihood that grip strength declined significantly as a function of time. Why the secular decline found by Tremblay et al. did not prove to be replicable using just the Canadian studies is not readily explainable, especially given the fact that the data analysed by

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Table 3. Descriptive statistics for studies of grip strength in adults by country. Males

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Study

Year tested

Canada Bornstein (1985)

1983

Desrosiers et al. (1995)

1993

Fortier et al. (2001) Fromm-Auch & Yeudell (1983)

1988 1981

Katzmarzyk (1997)

1996

Payne et al. (2000)

1998

Shephard (1986)

1981

Shields et al. (2010)

2006–2008

Trudeau et al. (2003) Yeudall et al. (1987)

1996–1998 1985

United States Bear-Lehman et al. (2003)

2001

Brennan et al. (2004)

2002

Fiebert et al. (1995) Gallup et al. (2007) Hanten et al. (1999)

1993 2005 1997

Hinson & Gench (1989)

1987

Horowitz et al. (1997) Jansen et al. (2008)

1995 2006

Kallman et al. (1990)

Koster et al. (2010) Kurina et al. (2004) Mathiowetz et al. (1985)

1961–1974

1998 1999 1999 1983

Females

Age range

Mean age

n

M (kg)

n

M (kg)

20–39 40–59 60–69 60–69 70–79 19–29 18–23 24–32 33–40 20–29 30–39 40–49 50–59 60–69 70–75 20–39 30–39 40–49 50–59 60–69 20–29 30–39 40–49 50–59 60–69 20–39 40–59 60–69 21–25 26–30 31–40

29.5 49.5 64.5 64.5 74.5 24.0 20.5 28.0 36.5 24.5 34.5 44.5 54.5 64.5 72.5 24.5 34.5 44.5 54.5 64.5 24.5 34.5 44.5 54.5 64.5 29.5 49.5 64.5 35.0 23.0 28.0 35.5

107 30 38 61 60 171 43 31 12 34 39 45 34 15 12 73 44 27 36 25 1631 1402 882 640 446 517 581 336 51 37 32 26

96.73 87.41 80.47 89.20 82.90 100.70 96.30 101.40 104.10 117.80 116.90 112.90 104.80 98.00 79.00 99.00 99.00 92.00 87.00 87.00 106.86 106.86 102.99 96.46 86.68 97.00 93.00 81.00 108.09 100.05 99.40 102.43

63 65 56 59 60 161 29 24 – 19 54 39 32 23 10 83 56 47 47 20 1843 1576 986 803 522 656 648 335 55 36 16 16

60.57 55.31 48.51 48.90 45.70 58.20 55.20 64.60 – 64.00 67.00 65.70 58.90 56.20 49.00 59.00 60.00 57.00 50.00 48.00 61.38 62.61 61.39 55.68 51.60 56.00 54.00 48.00 62.61 57.01 64.13 63.08

70–74 75–79 60–69 70–79 60–74 18–28 20–24 25–29 30–34 35–39 40–44 45–49 50–54 55–59 60–69 20–29 30–39 40–49 50–59 70–74 65–69 70–74 75–79 20–29 30–39 40–49 50–59 60–69 70–79 70–79 – – 20–24 25–29

72.0 77.0 64.5 74.5 68.3 20.0 22.0 27.0 32.0 37.0 42.0 47.0 52.0 57.0 64.5 24.5 34.5 44.5 54.5 72.0 67.0 72.0 77.0 27.2 34.7 45.9 55.0 64.4 74.5 74.2 48.0 47.6 22.0 27.0

– – – – 14 82 74 104 62 60 55 52 52 51 51 12 12 12 12 15 19 19 17 55 115 130 187 158 155 1429 – – 29 27

– – – – 79.31 96.09 104.38 102.51 101.15 104.33 101.61 102.51 101.61 83.46 81.65 104.78 108.86 99.64 87.85 81.56 81.51 75.11 72.21 100.20 104.30 101.00 95.20 88.00 75.40 77.26 – – 102.29 104.92

15 18 31 31 20 61 80 90 88 76 71 72 56 59 49 12 12 12 12 32 32 35 38 – – – – – – 1516 216 309 26 27

38.15 37.65 47.67 43.64 46.46 52.11 59.88 63.50 63.50 63.96 63.05 63.05 61.24 56.70 48.54 55.64 65.92 59.52 52.47 52.26 48.26 45.72 41.63 – – – – – – 47.02 61.6 60.4 59.60 62.60 (continued )

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Table 3. Continued

Males Study

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Pierson & O’Connell (1962) Schmidt & Toews (1970)

Year tested

Age range

Mean age

n

M (kg)

n

M (kg)

30–34 35–39 40–44 45–49 50–54 55–59 60–64 65–69 70–74

32.0 37.0 42.0 47.0 52.0 57.0 62.0 67.0 72.0 20.2 20 25 30 35 40 45 50 55 60 23.5 64.5 74.5 20.4 21.4

27 25 26 28 25 21 24 27 26 86 125 125 125 125 125 125 125 125 125 50 18 31 100 40

105.33 105.51 104.15 95.57 97.75 83.60 75.52 76.16 63.55 91.54 99.79 101.15 104.78 101.18 98.89 96.16 94.80 94.80 92.99 94.71 72.40 72.00 100.80 104.60

26 25 31 25 25 21 25 28 29 – – – – – – – – – – 50 45 90 105 34

66.54 63.69 60.19 53.62 55.84 47.45 45.72 41.10 41.32 – – – – – – – – – – 55.09 43.30 37.20 61.40 63.80

1960–1961 1968

Shechtman et al. (2005) Shechtman et al. (2004)

2003 2002

Vanderburgh et al. (1995)

1993

Females

20–40 60–69 70–79

Grip strength measure is the sum of values for the left and right hands, preferred and non-preferred hands or dominant and non-dominant hands.

Tremblay et al. were included in the foregoing analyses. However, it should be pointed out that systematic errors are likely to have a lesser effect on the results obtained when the data from multiple studies are analyzsed than when the data from only one study are analysed. Two possible sources of systematic error in the measurement of grip strength are the test instrument used (see above) and time of day (Souissi et al., 2010; Wright, 1959). The secular trends found here for Canadian and US children and adolescents should not be generalized to other countries. Indeed, studies conducted in other countries have produced results that are at odds with the present study’s findings. Specifically, secular declines in grip strength have been reported for Danish children (Heebøll-Nielson, 1982), English children (Cohen et al., 2011), Russian youth (Godina, 2011) and Spanish adolescents (Moliner-Urdinales et al., 2010). In addition, a study of indigenous rural Mexican youth (Malina et al., 2010) found secular gains in grip strength in some age/sex groups and no secular changes in other age/sex groups. These results, in combination with the present results, suggest that grip strength is determined by multiple variables and that these variables differ with respect to their magnitude in different countries. With regard to adults, the present results are in agreement with those obtained with representative samples of Canadian adults (Shields et al., 2010), but also those obtained in a study (Rode & Shephard, 1994) of the Inuits, an indigenous people living in Canada, which also found a secular decline in adult grip strength. However, as with children and adolescents, it seems likely that grip strength has followed different courses over time in different countries. The evidence for this comes from a study of the Zapotecs, an indigenous people living in rural Mexico, which found,

among adults, secular gains in grip strength in some age–sex groups, but no secular changes in other age–sex groups (Malina et al., 2011). As discussed in the Introduction, it would be expected, on the basis of the gain in body weight observed in Canada and the US in recent decades, to find that grip strength increased over that period of time. The fact that grip strength remained unchanged over time in children and adolescents and decreased over time in adults suggests that some factor counteracted the effect of body weight on grip strength. Several factors that might have had this effect will now be considered. One such factor is a lessening in strenuous physical activity, especially that involving the upper extremities. That physical activity of this type has a positive effect on grip strength has been shown in experimental studies with children (Bala´sˇ et al., 2009; Lirgg et al., 2011; Siegel et al., 1989) and young women (Thomas et al., 2008). Although there has been no experimental attempt to limit strenuous physical activity involving the upper extremities, it seems reasonable to believe this would result in decreased grip strength. On this argument, to explain the secular decline in grip strength, we should look for evidence that strenuous physical activity involving the upper body has decreased in recent decades. There is no direct evidence with regard to secular trends in strenuous physical activity involving the upper body. However, we do have evidence as to secular trends in physical activity that bear on aerobic fitness, such as walking, cycling and participation in sports. For children (see review by Dollman et al., 2005) and adolescents (Adams, 2006), the evidence indicates there was a decline in such activity over recent decades in Western children. For adults, studies have

Age and grip strength

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Figure 1. Scatterplot of female grip strength as a function of chronological age.

Figure 2. Scatterplot of male grip strength as a function of chronological age.

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shown that, in the 1980s and 1990s, physical activity increased substantially in Canada (Craig et al., 2004), but changed only slightly in the US (Macera & Pratt, 2000). If these findings, which reflect primarily aerobic fitness, can be generalized to anaerobic fitness, it is possible that, in children and adolescents, a decline in physical activity mitigated the impact of increased body weight on grip strength. On the other hand, they cannot explain the secular decline in grip strength in adults. One caveat here is that, as documented by Silverman (2011), research has produced mixed results as to whether physical activity and grip strength are correlated in children and adolescents. A second factor that might have counteracted the effect of increased body weight on grip strength is a secular decline in fat-free mass. There is very little evidence with respect to secular trends in fat-free mass. In children and adolescents, a study (Sun et al., 2012) conducted in the US found that fatfree mass (adjusted for height) was lower in boys born in the 1990s than in boys born in the 1960s, 1970s or 1980s, but there was no secular trend on this measure in girls. (According to Malina (2004), height in children in the US increased very slightly and only at the 5th and 10th percentiles, between 1963–1994; thus the adjustment for height should not have affected the across-time comparisons of fat-free mass in the study by Sun et al.) For adults, the only data available are for US Army recruits (Sharp et al., 2002). These data show that, over the period 1978–1998, fat-free mass increased in males, but that there was no systematic trend over this time period in females. The results of these two studies are in agreement for females only. As fat-free mass did not change over time systematically in females, this is contrary to the idea that a decrease in fat-free mass counteracted the effect of increased body weight on grip strength. A third factor that might have counteracted the effect of increased body weight on grip strength is a secular decline in serum levels of vitamin D. Two lines of research indicate that vitamin D plays a role in grip strength. One of these lines found that low serum levels of vitamin D are associated with higher levels of infiltration of fat in muscle in young women (Gilsanz et al., 2010). The other line of research found that low levels of serum vitamin D were associated with low levels of grip strength in adolescent and young adult females (Foo et al., 2009; von Hurst et al., 2013). Additionally, a study (Ward et al., 2009) of adolescent girls found positive correlations between serum levels of vitamin D and several measures of jump performance: velocity, height, power and force. Consistent with the idea that declining serum levels of vitamin D have counteracted the effect of increased body weight on grip strength, national surveys conducted in the US show that there was a marked decrease in serum levels of vitamin D in adolescents and adults over the period extending from 1988–1994 to 2001–2004 (Ginde et al., 2009). This downward trend was likely due to a change in lifestyle over this period of time. Evidence for such a change comes from a large panel study of adolescents and adults living in the US (University of Michigan News Service, 2004). This study found that, from 1981–1982 to 2001–2002, time spent in outdoor activities by children and adolescents declined by a

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half, from an average of 1 hour and 40 minutes per week to 40 minutes per week. Thus, in 2001–2002, the average time spent in outdoor activities averaged less than 7 minutes per day, which suggests that some individuals spent almost no time in outdoor activities. The significance of time spent in outdoor activities is that it provides a ready explanation for the secular decline in serum vitamin D, because the action of ultraviolet B radiation in sunlight on the skin is the principle source of serum vitamin D (Cannell et al., 2009). Another factor that might have limited exposure to ultraviolet B radiation is the widespread use of sunscreen to prevent sunburn and skin cancer. Of the explanations discussed above for why grip strength did not increase in recent decades despite the increase in body weight over the same period of time, the most promising one is that the gain in body weight was counteracted by a decline in serum levels of vitamin D. However, more evidence is needed to show that vitamin D plays a role in grip strength. For example, there is no evidence that serum vitamin D plays a role in determining grip strength in males. In addition, data are needed to show that grip strength is related to grip strength across the entire age spectrum and that grip strength is increased by exposure to sunlight in people who are low in serum levels of vitamin D. The other question raised by the present findings relates to the fact that, whereas grip strength did not change over time in children and adolescents, it declined according to a negatively accelerated function over time in adults. A possible answer to this question is provided by a recent study (Stenholm et al., 2011) which examined the relationship between obesity history (or the age at which obesity begins) and grip strength in people aged 55 years and older. Results show that obesity history was negatively associated with grip strength after controlling for potential confounders such as current body weight and physical activity. Additional results show positive associations between obesity history and two variables postulated as contributing to the decline in grip strength, inflammation and insulin resistance. Other research (Ruiz et al., 2008) has shown that inflammation is negatively associated with muscle strength in adolescents. Thus, it might be that insulin resistance rather than inflammation is the reason that the obesity epidemic has had a negative effect on grip strength in adults but not in children and adolescents. Furthermore, these two sets of results suggest that there is a latency period between the onset of obesity and when the hormones secreted by adipose tissue begin to affect muscle strength. In future research, it would be of interest to determine whether obesity history is also related to grip strength in young and middle-aged adults and whether further evidence can be found for the hypothesized long-term effect of insulin resistance on muscle strength. Grip strength is correlated with a number of other measures of strength (Bohannon, 2008; Bohannon et al., 2012; Meyers et al., 1993; Wang et al., 2005; Wind et al., 2010). One would, therefore, expect that the secular trends found here for grip strength would be parallelled by the secular trends found with other tests of strength. Only one other study (Tomkinson, 2007) provides data relevant to this expectation. Contrary to the present results for children and

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adolescents, this study found that performance on tests of power (jumping) and speed (sprint running and agility running) increased over the period extending from 1958– 1989 in Canadian and US children and adolescents. One possible explanation for the difference in results on the different tests of strength is whether the specific test does or does not involve movement of the entire body. Incidental to the purposes of this study were the results obtained for age changes in grip strength. The main findings about this relationship are that grip strength increased from age 6 until about age 20, remained basically unchanged from about age 20 to about age 40 and declined gradually after about age 40. In a literature review, Innes (1999) concluded that the relationship between grip strength and age is curvilinear, but she also concluded that grip strength peaks within the age range of 30–45 years. To resolve the question of when grip strength peaks requires studying the same group of people longitudinally, beginning at age 20. To date, however, no such study has been reported. The scatterplots that related strength to age also show there was very little variability between studies in the grip strength values obtained in boys at the same age. This result suggests that differences in sample composition and differences in how grip strength was measured had little effect on the results obtained. If the latter inference is correct, it would also suggest that methodological factors likely had little effect on the entire set of results analysed here. In conclusion, the present results indicate that no improvement in grip strength attended the increase in body weight that has been observed in the Canadian and US populations over recent decades, contrary to what one would expect given that grip strength and body weight are positively associated when the two variables are studied contemporaneously. On the contrary, the present results indicate, for adults, that there was a decline in grip strength over recent decades. As discussed above, there exists some scepticism about the validity of secular trends found with meta-analyses conducted on convenience samples. Thus, it would be of particular interest to determine whether the secular trend found here for adults would hold up if new studies with this age group were added to the dataset. That grip strength declined over time in adults is an important result, as it may help explain why age-adjusted fatal falls increased by 55.3% in both sexes in US adults from 1993 to 2003 (MMWR 17 November 2006). Specifically, loss of grip strength may reflect a decline in general body strength, which in turn could impair balance control (Horlings et al., 2008), a known risk factor for falling in adults (Masud & Morris, 2001). Changes in grip strength may, thus, be predictive of changes in the rates of falling in adults.

Declaration of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.

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