Dorsiflexion peak torque performance also showed a main effect by age group with a more curvilinear pattern than for men (Table 10 and Figure 5). At 1.05 rad·s−1, DFPT for 70 G was different from 40 G (Mdiff = 7.3 ± 2.9 N·m, p < 0.005), 30 G (Mdiff = 6.9 ± 2.8 N·m, p < 0.005), and 20 G (Mdiff = 8.5 ± 2.8 N·m, p < 0.005). Dorsiflexion average power also showed significant differences at 1.05 rad·s−1 between 50 and 20 G (Mdiff = 3.5 ± 1.4 W, p < 0.005).
The principal findings of this study were that men and women showed different patterns of decline in isokinetic strength and power across the 6 decades included in these analyses, and that the patterns of change varied by movements and speed. As noted above, statistically significant declines in men's KEPT and KEAP at all speeds occurred between 50 and 59 years of age; however, these declines began insidiously decades earlier. Knee flexion results followed the same pattern with declines earlier than expected, especially at 5.24 rad·s−1. Our data closely match those of Runnels et al. (60) for KEPT and KFPT across the same age groups at 1.05 and 3.14 rad·s−1 in 75 men. Figures 6–9 present age cross-sectional plots (a), scatter plots with best fit lines and 95% CI (b), and Bland-Altman plots (c) comparing our results with theirs for KEPT at 1.05 and 3.14 rad·s−1, and KFPT at the same speeds. Additionally, Table 11 provides intraclass correlations, Bland-Altman statistics, and Chronbach's Alpha scores for each comparison.
Another study reported declines between 10 and 22% in KEPT at speeds between 0.52 and 5.15 rad·s−1 in 73- to 83-year-old men over 7 years (3). These researchers then identified additional declines of 25–35% in KE performance at 0.52 rad·s−1, 11 years later (29). Our observed declines for these age groups across the 3 testing speeds averaged 28–47% for 7 and 11 years, respectively, somewhat higher than these researchers, respectively. As Raj et al. (54) note in their review, testing speeds and activity levels may affect the isokinetic test results. This may explain, in part, the somewhat greater declines seen as in our study, especially in the 9 subjects in the 11-year follow-up by Grimby (29) in which most subjects reported unchanged levels of physical activity. Additionally, our results are from a much larger sample and are cross sectional rather than longitudinal.
Although, to the best of our knowledge, no studies have looked at these variables in men and women across 6 decades, our findings are in line with other studies using isokinetic and other contractile testing conditions in both genders. For example, Hortobágyi et al. (34) tested 18 to 80-year-old men and 20 to 74-year-old women using isometric and isokinetic concentric and eccentric KE contractions at 1.05, 2.09, and 3.14 rad·s−1. Although direct comparisons are not possible given the differences in measurement units and dynamometers (Biodex vs. Kin-Com), these researchers reported approximately a 7 and 8% decline in KE torque production per decade for men and women, respectively, which compares favorably with the 7.4% in men and 7.6% in women by decade declines in this study. Unfortunately, the linear regression modeling in their study makes comparisons of patterns of decline impossible. In 2 separate studies, Goodpaster et al. (25,26) reported average values of 132.3 ± 34.5 and 81.4 ± 22.0 N·m for white and black men and women 73.7 ± 2.9 and 73.4 ± 2.8 years of age, which compare favorably with the values of 127.4 ± 4.8 and 73.3 ± 2.1 N·m for the men and women in our study. Declines across 3 years reported by these researchers for white and black men (15.4 ± 21.4, 19.7 ± 26.4 N·m) and white and black women (8.0 ± 22.5, 10.2 ± 19.8 N·m) compare favorably with the 7.4 and 7.6% declines by decade for men and women in our study. A longitudinal study by Hughes et al. (35) examining changes in isokinetic strength in 68 women and 52 men, 46–78 years of age, reported 11.8 ± 15.5 and 14.5 ± 15.6% declines in KEPT for women and men, respectively, across 9.7 ± 1.1 years. These declines are comparable with the 16.9, 18.9, and 13.4% declines in women and 21.1, 9.9, and 17.5% declines in men seen when across decades for 40, 50, and 60 G in this study. Additionally, our data show the same patterns of decline for KEPT of men and women 19–80 as reported in the Baltimore Longitudinal Study of Aging in 786 individuals aged 26–96 (48), similar KEPT values to those reported in the Health ABC study for men and women 72–83 years (70), and similar patterns for KEPT and KFPT in the MOST cohort study (61) incorporating 3,856 men and women aged 61.6 ± 8.1 and 62.2 ± 7.7, respectively.
To summarize, we believe this study is the first to show the patterns of decline for KE and KF for men and women across 6 decades and to demonstrate that declines begin during the fourth and fifth decade for women and men, respectively. This suggests that preventative exercise programming should begin earlier in a person's life span than previously suggested.
Men's PF declines differed from patterns for knee joint tests beginning later, exhibiting a more curvilinear pattern, and affecting AP more than PT. Additionally, there were no significant differences among age groups for PFPT or PFAP at 5.24 rad·s−1. The differences between KF and KE results and PF results may be attributable to a number of factors. First, PF incorporates a second class lever designed for force rather than velocity. This limits individuals' capacities to attain the necessary movement speeds to meet isokinetic testing demands at higher testing speeds. Second, the compliance of the Achilles tendon slows the rate of force development for the triceps surae compared with other muscles with similar fiber type distributions (2). Finally, our tests were performed at a 2.36 rad knee angle favoring the contribution of the slower soleus muscle over the faster gastrocnemius (64,72).
Although PFPT and PFAP studies are limited, our PFPT values for men are consistent with those of other studies. Our PFPT values (20 G = 76.68 ± 7.32 N·m, 30 G = 78.51 ± 8.18 N·m) are similar to preintervention values of 17 healthy men (aged 26–41 years) who participated in a 90-day bed rest study (1). Cunningham et al. (11) examined PFPT on a Cybex II (Lumex Inc., New York, USA) dynamometer in 25 older sedentary (n = 6; 64.0 ± 3.0 years), older active (n = 7, 63.0 ± 3.0 years), younger sedentary (n = 6, 20.8 ± 1.9 years), and younger active (n = 6, 22.5 ± 1.8 years) men. Peak torque at 1.05 rad·s−1 was approximately 36, 43, 49, and 64 N·m for older sedentary and active, and younger sedentary and active groups, respectively. At 3.14 rad·s−1, these values were approximately 8, 12, 18, and 21 N·m for the same groups, respectively. These values are considerably below the 59.03 ± 4.99 and 43.62 ± 4.98 N·m seen at 1.05 and 3.14 rad·s−1 for our 60 G, and the 76.68 ± 7.32 and 52.60 ± 7.65 N·m seen at these speeds for our 20 G. A number of factors may have produced the differences between these results and those reported in our study and the study by Alkner and Tesch (1). First, differences in dynamometers may have affected the results. We used a Biodex System 2, Alkner and Tesch (1) used a Cybex 6000, and Cunningham et al. (11) used a Cybex II. Second, the testing protocols varied considerably. Our subjects were seated in the Biodex accessory chair (angles: seatback = 0.26 rad, footplate = 0.79 rad, knee = 2.16 rad, hiprad) and performed 3 repetitions at 3 speeds alternating PF and DF. Additionally, speed specific warm-ups were performed at 50, 75, and 100% of perceived effort. Subjects in the Alkner and Tesch (1) study performed concentric and eccentric movements at 0.52 and 1.05 rad·s−1 while supine with hips semi-flexed at a 1.57 rad knee angle. These researchers provided no indications of a warm-up. Finally, Cunningham et al. (11) tested their subjects prone with the knee in full extension using 5 repetitions at 5 speeds ranging from 0.52 to 3.14 rad·s−1. Unfortunately, no information was provided concerning the method of returning the foot to the original position after each contraction. Third, data selection and analyses methods differed. Fourth, verbal encouragement was provided by the testers in our study and the study Alkner and Tesch; it is unclear whether verbal encouragement was provided by Cunningham et al. (11). Finally, differences existed in data sampling and analysis. We recorded 3 repetitions at each speed with the highest being used for analysis. Data in our study were windowed to reduce acceleration and deceleration artifacts, especially at 5.24 rad·s−1. Alkner and Tesch (1) used the higher of 2 contractions for analyses, and no indications of filtering were provided. Cunningham et al. (11) collected undamped signals on an analog oscilloscope and used an average of the 5 repetitions at each speed.
Women's ankle PF results again indicate that declines begin to decline earlier for women than men. Peak torque and AP in men were maintained (except for 5.24 rad·s−1) until 50 G, whereas women showed a nearly linear decline in these variables across age groups. Clearly, women's results show a greater number of differences across age groups and more overlaps among groups.
Women's PFPT results compare favorably results of some studies, but differ considerably from others. Plantar flexor peak torque values in a training study by Klentrou et al. (40) using weighted vests with women 44–62 years old (1.05 rad·s−1: controls, 47.3 ± 19.5 N·m and exercisers, 37.8 ± 8.4 N·m; 3.14 rad·s−1: controls, 30.9 ± 8.3 N·m and exercisers, 27.9 ± 7.0 N·m) are comparable with those of our 50 G group at these testing speeds. A training study by Eyigor et al. (20) examined the impact of a group exercise program on functional factors, including isokinetic strength, in 20 women aged 70.3 ± 6.5 years. They reported isokinetic pretest and posttest scores for subjects' dominant legs at 1.05 rad·s−1 of 20.40 ± 5.90 and 25.30 ± 8.05 N·m, respectively, and 9.95 ± 3.15 and 12.40 ± 3.17 N·m at 3.14 rad·s−1, which were somewhat lower than the 36.8 ± 3.4 and 23.4 ± 3.3 N·m values for our 70 G women at these testing speeds. In contrast, in a reliability study by Webber and Porter (75) involving 30 women (73.3 ± 4.7 years), PF strengths at 0.52 and 1.57 rad·s−1 averaged 66.7 ± 20.0 and 69.7 ± 20.2 N·m for day 1 and day 2, respectively, at 0.52 rad·s−1; and 61.4 ± 15.8 and 62.0 ± 18.5 N·m for day 1 and day 2, respectively, at 1.57 rad·s−1. These values are notably higher that the 36.8 ± 3.4 N·m average for the 70 G in our study. Again, differences may have been due to the dynamometers used (Biodex System 2, Cybex 2, and Biodex System 3), body positioning, passive return vs. alternating DF and PF movements, and variations in the testing protocols. Additionally, Webber and Porter (75) reported peak rather than AP values.
Studies incorporating men and women reported results similar to ours for PF. In a reliability study by Hartmann et al. (30) combining men and women (W: n = 18, age = 69.9 ± 5.0 years; M: n = 6, age = 75.0 ± 5.6 years), peak PF torques were 41.0 ± 15.8 and 42.6 ± 15.5 N·m for days 1 and 2, respectively. At the same testing speed, our 70 G produced 50.5 ± 4.4 N·m for men and 36.8 ± 3.4 N·m for women. In a second study by Trappe et al. (74), employing 14 men and 4 women, 23–51 years, isokinetic PTs at 1.05, 3.14, and 5.24 rad·s−1 were 88.5, 41.7, and 16.8 N·m, respectively. The lower speed values compare favorably with the 68.3 ± 9.6 to 82.4 ± 5.0 N·m and 50.7 ± 8.0 to 58.0 ± 5.2 N·m for the 20 G through 50 G in our study, but are considerably lower than the 44.4 ± 14.0 to 49.6 ± 10.8 N·m seen for this age range at 5.24 rad·s−1. Additionally, Buchner et al. (8) reported an average PFPT of 40 ± 22 N·m for men and women 60–96, which again mirrors our results for our 60 and 70 G men and women.
Plantar flexion results could have been affected by subjects' training histories. Krishnathasan and Vandervoort (41) reported that PF values at 0.52, 1.57, and 3.14 rad·s−1 varied 35–46% between endurance-trained vs. resistance-trained middle-aged individuals.
For studies involving men exclusively, our results are comparable with those of earlier studies. Our DFPT and DFAP results differed somewhat from those of Ranisavljev et al. (55) who examined the relationship between strength and power of the hip, knee, and ankle joints in relation to gait transition speed. For their DFPT and DF power values at 1.05 rad·s−1, using reported body weights and isokinetic values per unit body weight, we computed values of 39.26 ± 8.64 N·m and 25.91 ± 5.50 W; and at 3.14 rad·s−1, values were 33.76 ± 8.64 N·m and 31.40 ± 13.35 W. These values fell slightly outside the 95% CI for our PT values at each speed, whereas their power values were within the 95% CI for AP in our 20 G. The most obvious difference between the studies is the compressed age range (20–24 years) in their study compared with ours (20–29 years). The second factor was that our study used the Biodex 2 dynamometer, whereas they used a KinCom. And finally, the differences in the protocols for the Biodex and KinCom may have been a significant source of variance.
Women's results showed a loss of one third of DFAP at the lower speed of 1.05 rad·s−1 between 50 and 20 G. In contrast to women's PF results, DF values in our study compared favorably with nearly every other study. Webber and Porter (75) reported torque values of 14.0 ± 4.6 and 13.9 ± 4.8 N·m for day 1 and day 2 at 0.52 rad·s−1 and 10.5 ± 4.2 and 10.6 ± 4.1 N·m for day 1 and day 2 at 1.57 rad·s−1. These values compare favorably with the average value of 13.19 ± 1.34 N·m produced by our 70 G group. As would be expected, their peak power values (day 1: 7.2 ± 2.3 W, day 2: 7.1 ± 2.5 W at 0.52 rad·s−1; day1: 11.2 ± 4.5 W, day 2: 10.9 ± 4.4 W at 1.57 rad·s−1) were somewhat higher than the average value of 6.43 ± 0.90 reported for our 70 G. Our DFPT data also reflected those of Eyigor et al. (20) in women, 70.3 ± 6.5 years of age. Dorsiflexion peak torque at 1.05 and 3.14 rad·s−1 ranged from 3.8 ± 2.07 to 7.15 ± 2.75 N·m and 3.35 ± 2.18 to 6.10 ± 2.97 N·m, respectively, across their trained and untrained conditions. These results were slightly below those of our 70 G group at 1.05 rad·s−1 but fell near the midpoint of the 95% CI for this group at 3.14 rad·s−1.
Compared with studies involving both men and women, our results are also defensible. In a study, examining ankle DFPT in older men (78.2 ± 4.9 years) and women (74.7 ± 3.8 years) subjected to a 2-week isokinetic training program, Connelly and Vandervoort (10) illustrated DFPT at 3.14 rad·s−1 of approximately 15 N·m for pretest and posttest. As would be expected, this average value which included men and women fell between the 19.42 ± 6.37 N·m for men and 5.66 ± 0.80 N·m for women at 70 G. Our DFPT findings agree with those of Thomas et al. (73) for the control group in their study examining lower extremity strength after anterior cruciate ligament injury and reconstruction. Seven men and 8 women (24.73 ± 3.37 years) were tested on a Biodex System 3 dynamometer at 1.05 rad·s−1. Average DFPT was 20.09 ± 8.18 N·m, which compares well with the values of 28.24 ± 2.59 and 21.72 ± 1.81 N·m recorded for the 20 G men and women in our study, respectively. In a reliability study by Holmbäck et al. (33), DFPT at 1.05 rad·s−1 for men and women was 35.0 ± 7.5 and 28.8 ± 4.8 N·m, which was slightly above the 95% CI for each gender in our 20 G group. Our average results for men and women in the 60 and 70 G (19.12 ± 2.73 N·m) compared favorably with those of Buchner et al. (8) for men and women 60–90 years of age (15 ± 7 N·m), with the lower average attributable to the older upper age limit in their study. Our results differ from those reported by Hartman et al. (30), where DFPT values at 1.05 rad·s−1 were 10.4 ± 4.7 and 10.2 ± 4.2 N·m for days 1 and 2 compared with our values of 17.0 ± 2.0 N·m (60 G) and 13.2 ± 1.3 (70 G) for women and 25.8 ± 1.2 (60 G) and 20.5 ± 1.7 N·m (70 G) for men. The differences between their results and those reported by us and others are difficult to explain.
Among the strengths of this study were the inclusion of reciprocal KE, KF, PF, and DF data across 6 decades of life; the separation of results by age group and gender, the large sample size which allowed us to generate normative values, and the inclusion of knee and ankle test results allowing comparisons of varying patterns of change in PT and AP across decades for each joint movement. Limitations include the limited samples in some gender/age blocks, the inability to match these results with longitudinal results for activities of daily living performance, fall probability or other more practical measures of independence and injury risk, and the lack of specific information on activity levels, diet, and effort during testing, inherent to studies incorporating maximal testing.
This is of special importance in this population where lower-body neuromuscular performance is so important for reducing falls and maintaining independence.
There was no external funding associated with this study, the authors have no professional relationships with any company or manufacturer who could benefit from the results of this study and have no conflicts of interest to report. The results of this study do not constitute endorsement of any product by the authors or the NSCA. The authors thank all the students who over the past decade helped with the collection of these data and the subjects who graciously gave their time and effort.
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