Running Mechanics and Variability with Aging ¨ GGEMANN3, JULIA FREEDMAN SILVERNAIL1, KATHERINE BOYER1, ERIC ROHR2, GERT-PETER BRU and JOSEPH HAMILL1 1

Department of Kinesiology, Biomechanics Laboratory, University of Massachusetts, Amherst, MA; 2Brooks Sports, Inc., Bothell, WA; and 3Institute for Biomechanics and Orthopedics, German Sports University, Cologne, GERMANY

ABSTRACT ¨ GGEMANN, and J. HAMILL. Running Mechanics and Variability with FREEDMAN SILVERNAIL, J., K. BOYER, E. ROHR, G. BRU Aging. Med. Sci. Sports Exerc., Vol. 47, No. 10, pp. 2175–2180, 2015. Introduction: As the elderly population in the United States continues to grow, issues related to maintenance of health become increasingly important. Physical activity has positive benefits for healthy aging. Running, a popular form of exercise, is associated with the risk of developing injury, especially in older runners. Initial differences between older and younger runners have been observed, but these were observed without consideration of other differences between groups, such as running mileage. Purpose: This study aims to compare running mechanics and lower-extremity coordination variability in matched groups of healthy younger and healthy older runners. Methods: Three-dimensional kinetics and kinematics were collected while 14 older adults (45–65 yr) and younger adults (18–35 yr) ran overground at 3.5 mIsj1. Knee, ankle, and hip joint angles and moments were determined. Discrete measures at foot strike (maximum and minimum) were determined and compared between groups. Segment angles during stance were utilized to calculate segment coordination variability between pelvis and thigh, thigh and shank, and shank and foot, using a modified vector coding technique. Results: Knee and ankle joint angles were similar between groups (P 9 0.05). Older runners had greater hip range of motion (P = 0.01) and peak hip flexion (P = 0.001) at a more extended hip position than younger runners. Older runners had smaller ankle plantarflexion moment (P = 0.04) and hip rotational moment (P = 0.005) than younger runners. There were no between-group differences in any of the variability measures (P 9 0.05). Conclusions: Runners appear to maintain movement patterns and variability during running with increasing age, indicating that running itself may be contributing to maintenance of health among older runners in the current study. Key Words: COORDINATION, GAIT MECHANICS, INJURY RISK, OLDER RUNNERS

A

Address for correspondence: Julia Freedman Silvernail, Ph.D., University of Nevada, Las Vegas, Box 3034, 4505 South Maryland Parkway, Las Vegas, NV 89154-3034; E-mail: [email protected]. Submitted for publication April 2014. Accepted for publication January 2015. 0195-9131/15/4710-2175/0 MEDICINE & SCIENCE IN SPORTS & EXERCISEÒ Copyright Ó 2015 by the American College of Sports Medicine DOI: 10.1249/MSS.0000000000000633

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a marathon and found that older runners were more protected from calf injuries than were younger runners. However, Wen et al. (26) observed that runners with hamstring injuries were older than those without and that hamstring injuries were more common in older runners. Although prospective and retrospective studies have investigated the influence of age on injury risk, the underlying mechanics contributing to injury risk remain unclear. There is a paucity of literature investigating differences in running mechanics as individuals age. Initial studies on aging and running have found differences in running form (2,5,7,13). These differences have primarily been decreased knee flexion range of motion and shorter stride length in older runners (2,5,7). Although these studies provide initial insight into differences between groups, there were limitations in the matching of groups that could impact the comparison of older and younger runners. Runners are injured at alarming rates; in spite of ongoing research on running, the overall incidence of running injuries has not decreased over the last 30 yr (12,23,24). Although direct causes of running injuries remain difficult to ascertain, some researchers have identified aspects of gait that can differentiate injured runners from uninjured runners. Most of the prior studies focusing on aging and running have been limited by their use of a single joint or segment analysis (2,5,7). However, other researchers have used a systems approach based on dynamical systems theory to study injured versus uninjured runners and have suggested that multisegment coordination allows a more sensitive measure of joint mechanics (10).

s the population continues to increase in both age and level of obesity, exercise research expands on these topics and becomes increasingly important. Exercise, in general, has been linked with lowered comorbidities of obesity, lowered risk of becoming obese, and maintenance of a healthy life with advancing age (27). Running is a popular form of exercise that requires little equipment, making it accessible to people at varying income levels. Unlike group sports, running does not require the organization of other individuals to take place. Running is also not age-dependent, making it a universally available exercise for all healthy, active individuals. The relationship between age and running can be viewed from more than one perspective. Although physical activity has positive benefits for healthy aging, older runners have been shown to be at increased risk for developing injury (14,23). Satterthwaite et al. (19) investigated injuries incurred during

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Movement coordination represents the organization or selection of movements from available degrees of freedom to perform a task (17). In a healthy state, one has many available individual degrees of freedom that can be combined or coordinated to achieve a movement task (9). Therefore, analysis of the coordination variability of movement provides insight to the health of the system and allows for a higher-level understanding of the movement. Rather than looking at the movement of the joint itself, segment coordination represents the relationship between the segments that make up a given joint. Joint angles during running are key parameters in describing the angular relationship between adjacent segments. However, it is possible to have similar joint angles but different movements and positions of the segments that make up a given joint. Therefore, coordination may provide more information on the stresses placed on the joint. The variability of segment coordination has been tied to injuries in runners, with individuals suffering from patellofemoral syndrome, low back pain, and tibial stress fractures, exhibiting less coordination variability than healthy individuals (8–11,22). It has been suggested that low variability indicates less flexibility of the body to adapt to changing situations (10). Less variability may also lead to greater repetitions of stress on lower-extremity joints. When the body has more flexibility of movement patterns, joint stress from loading can be spread across the joint rather than loading repeatedly on the same tissues. Initial research has shown that aging decreases coordination variability in walking (25), but the influence of age on coordination variability in active runners remains unknown. Therefore, the purpose of this study was to compare running mechanics and lower-extremity coordination in matched groups of healthy younger and healthy older runners. Factors other than age, including weekly mileage, running experience, and body mass index, may also contribute to differences in coordination patterns in running. For the current study, the two groups were matched on height, body mass, and weekly running mileage. Strength declines with age (4,21); as this decline in strength is often accelerated for muscles crossing the ankle joint (4), it was also hypothesized that older runners would exhibit a smaller plantarflexor moment and a smaller ankle range of motion compared with younger runners. Owing to increased injury rates in older runners (14,23), it was also hypothesized that younger runners would exhibit greater coordination variability at the ankle, knee, and hip compared with older runners.

METHODS Participants. Data from the literature were used to estimate sample size for a minimum statistical power of 80%

with an alpha level of 0.05 (2). Sagittal-plane-dependent variables utilized in the power analysis included ankle range of motion and knee range of motion. As a result, 28 injuryfree recreational runners age between 18 and 65 yr were included in this analysis. Participants of this study were selected as a subgroup from a larger study of 110 runners. Participants were recruited from the university community. All participants provided a written informed consent form, as approved by the University of Massachusetts’s institutional review board. In order to participate in the study, participants had to be healthy runners who were free of lowerextremity injury and did not wear orthotics. Runners selected for this study were required to run a minimum of 10 miles per week. Fourteen younger adult runners age between 18 and 35 yr were matched on gender, height, mass, and weekly mileage run with 14 older adult runners age between 45 and 65 yr (Table 1). Experimental setup. The laboratory consists of a 25-m runway surrounded by an eight-camera three-dimensional motion capture system (Oqus; Qualysis, Inc., Gothenburg, Sweden) with a force platform (AMTI, Watertown, MA, USA) flush-mounted with the floor at the center of the runway. Running velocity was monitored by two photoelectric sensors (Lafayette Instrument Company, Lafayette, IN, USA) placed 6 m apart on either side of the force platform. Strength measures were completed using the isometric mode of a Biodex dynamometer (Biodex, Shirley, NY, USA). Protocol. Participants completed questionnaires, providing information on their injury history, use of orthotics, running training, and running history. The information included weekly mileage, typical run length, preferred pace, and number of years in current training routine. Isometric knee strength measures were taken for the right shank of all subjects. During knee extension and flexion strength measures, participants were placed so that their knee was fixed at a 120- angle. Subsequently, participants were provided laboratory shoes, wore tight-fitting clothes, and were fitted with retroreflective markers attached to the pelvis and right thigh, shank, and foot (15). Participants were allowed to practice running trials so that they were comfortable running within 5% of 3.5 mIsj1 while contacting a force platform with their right foot. Participants completed five running trials while kinematic and kinetic data were collected at 240 and 1200 Hz, respectively. Data processing. Data were filtered with a low-pass Butterworth filter with a 12-Hz cutoff for kinematic data and a 50-Hz cutoff for kinetic data. The stance phase of running was determined using vertical ground reaction force. Touchdown was identified when the vertical force component was greater than 20 N. Joint angles and moments were calculated

TABLE 1. Subject demographics. Younger Older

Gender

Age (yr)

Height (m)

Mass (kg)

Run (milesIwkj1)

Self-reported Pace (minImilej1)

4 Female; 11 male 4 Female; 11 male

21.2 (3.1) 54.6 (6.4)

1.73 (0.07) 1.71 (7.3)

68.6 (7.9) 68.3 (7.8)

25.3 (15.8) 26.7 (13.1)

7.7 (1.2) 8.9 (0.9)

Data are presented as mean (SD).

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for the ankle, knee, and hip during stance. Vertical ground reaction force characteristics (impact peak and vertical loading rate) were calculated. Angle and moment peaks, values at foot strike, and range of motion were averaged for all trials for each participant. A modified vector coding technique was used to investigate segment coordination variability for each participant (3). The modified vector coordination technique consisted of creating angle–angle plots of the stance phase of the segments of interest. The SD (calculated using circular statistics) (1) of the vector created between each consecutive two time points throughout stance represented the variability of the coordination of the segments in question. Segment couplings were selected to represent movement at the joint and movements that may stress the tissues that make up the joint. For each participant, average variability was calculated for the duration of stance. Coordination variability was calculated for the following couplings between thigh and shank: thigh flexion and shank internal rotation, thigh and shank flexion, and thigh and shank internal rotation. Thigh flexion– shank internal rotation and thigh–shank flexion couplings were selected to investigate the knee as it moves to absorb impact associated with running. Owing to the bony structure of the knee, normal knee motion incorporates the combination of thigh flexion and shank internal rotation. Therefore, to provide further insight, we investigated the coupling consisting of the combination of thigh flexion and shank internal rotation. As the knee is supported largely by soft tissue rather than by bony structure, movements that place additional stress on soft tissue can provide insight to injury risk. Excessive rotation at the knee joint may lead to greater stress on soft tissue; therefore, the coupling of thigh–shank internal rotation was also included. Similarly to the knee, coordination variability was calculated for shank and foot couplings (shank internal rotation and foot eversion, shank and foot internal rotation, and shank adduction and foot eversion) and pelvis and thigh couplings (pelvis and thigh flexion, and pelvis and thigh internal rotation). Statistical analysis. Between-group differences in joint kinematic and kinetic parameters were determined using independent t-tests and effect sizes (ES). The specific dependent measures at the ankle knee and hip were angle at foot strike, peak angle, joint range of motion, and peak joint moment.

In order to assess differences in coordination variability, we calculated independent t-tests and ES on mean variability during stance.

RESULTS Older and younger runners did not differ in their peak knee flexion (P = 0.242) or extension (P = 0.061) strength measures. Upon investigation of joint motion, there were no significant differences at the ankle and knee, but significant differences between groups were observed at the hip (Table 2). Older adults ran with a more extended hip position throughout stance (Fig. 1A) and had a significantly greater sagittal hip range of motion than younger adults (P = 0.01, ES = 1.0). Maximum hip flexion, which occurred near foot strike, was greater in younger adults than in older adults (P = 0.001, ES = 1.4), but older adults reached a significantly more extended hip position at the end of stance than younger adults (P = 0.04, ES = 0.8). Older and younger groups differed in joint moments at both the ankle and the hip. Older adults had a smaller maximum ankle plantarflexion moment (Fig. 1B; P = 0.04, ES = 0.5) and a smaller maximum rotational hip moment (Fig. 1C; P = 0.005, ES = 0.8) than younger adults. There were no between-group differences in any of the investigated ground reaction force characteristics: loading rate (P = 0.33, ES = 0.3), impact peak (P = 0.31, ES = 0.29), or time to impact peak (P = 0.08, ES = 0.02). Coordination variabilities for thigh flexion–shank internal rotation, thigh–shank flexion, and thigh–shank internal rotation were similar between older and younger runners (Fig. 2A; P = 0.57, ES = 0.09). Coordination variabilities for shank– foot internal rotation (P = 0.07, ES = 0.05) and shank adduction–foot eversion (P = 0.23, ES = 0.03) were similar between older and younger runners (Fig. 2B). Coordination variabilities for pelvis–thigh flexion (P = 0.41, ES = 0.32) and pelvis–thigh internal rotation (P = 0.74, ES = 0.22) were similar between older and younger runners (Fig. 2C).

DISCUSSION The purpose of this study was to investigate differences in running form and lower-extremity segment coordination

Foot Strike Younger

Peak Angle Older

Younger

Older

Ankle plantarflexion–dorsiflexion 4.22 (10.36) 7.68 (9.04) 20.82 (3.51) 21.11 (2.47) Ankle eversion–inversion 7.56 (3.20) 5.19 (4.43) j8.25 (3.09) j8.53 (2.70) Ankle internal–external rotation j9.45 (4.78) j10.64 (4.49) j16.74 (3.94) j16.35 (3.62) Knee flexion–extension j10.77 (5.46) j11.16 (5.13) j39.17 (4.73) j38.06 (4.29) Knee abduction–adduction 0.51 (2.75) j0.07 (3.71) 3.27 (2.80) 3.24 (3.83) Knee internal–external rotation j8.48 (4.22) j10.28 (7.89) 7.04 (4.47) 4.08 (4.47) Hip flexion–extension 33.19 (6.24) 27.14 (10.65) j6.98 (6.02) j18.52 (10.54)* Hip abduction–adduction 5.08 (4.34) 5.44 (4.34) 11.1 (4.93) 12.69 (4.89) Hip internal–external rotation 4.01 (4.94) 4.07 (6.33) 6.73 (4.42) 7.14 (4.29)

Range of Motion Younger 16.60 15.81 7.29 28.4 2.76 15.52 40.17 6.02 2.72

(8.51) (3.06) (2.28) (6.15) (1.46) (3.41) (6.34)* (2.30) (4.10)

Older

Joint Moment Younger

Older

13.44 (8.06) j186.74 (32.71)* j156.94 (42.99)* 13.72 (3.79) 56.00 (18.90) 51.63 (18.46) 5.70 (2.52) j28.70 (8.15) j23.63 (8.65) 26.91 (5.17) 156.93 (44.28) 152.73 (35.65) 3.31 (2.55) j72.46 (25.72) j67.97 (23.99) 14.37 (7.90) 26.02 (13.01) 25.22 (14.39) 45.65 (4.44)* j135.14 (29.68) j110.53 (34.83) 7.25 (3.05) 144.79 (30.22) 140.79 (28.27) 3.08 (3.16) j52.38 (18.62)* j34.02 (14.00)

Data are presented as mean (SD). *Significant group differences (P G 0.05).

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TABLE 2. Kinematic and kinetic dependent variables.

maintained greater strength. DeVita and Hortobagyi (6) reported that there was redistribution of joint torques with increasing age in that older adults relied more on hip extensor torque and less on knee and ankle torques. The older adults in the current study appear to have maintained knee flexor and extensor strength, as their peak torque was not different from that of young runners. This lends evidence that a shift in support to the knee joint may be feasible. This shift in reliance on more proximal structures is also in line with the increased injury rates previously reported in the literature. Supporting this notion of a shift in injury risk is the observation that older runners are more protected from calf injuries than are younger runners, but individuals with hamstring injuries tend to be older than those without (19,26). Very few between-group differences in joint angles and moments were observed in the current study. Although previous investigations have observed decreased knee flexion in older runners, this was not observed in the current study. We contend that differences in group makeup may help to explain the inconsistent findings. Of two studies reporting on older runners with decreased knee flexion, one study did

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FIGURE 1—Joint angle and moment data. (A) Sagittal plane hip motion. (B) Sagittal plane ankle moment. (C) Transverse plane hip moment.

variability between older and younger runners. Although prior research has suggested that younger runners would have greater knee flexion peaks and range of motion than older runners (2), we observed no differences at the knee between groups. Contrary to our hypothesis that younger runners would exhibit greater coordination variability than older runners, variability was similar between groups. Our findings indicate that there are some differences in running mechanics— but not in the variability of the coordination of this movement— between healthy older and younger runners. Although there were very few differences between groups, the smaller plantarflexor moment and greater hip range of motion in older runners may provide some insight into how older runners move. These differences may indicate a shift of reliance from distal joints to more proximal joints in older runners. As we age, we lose both flexibility and muscle strength (4,21). With aging, the plantarflexor muscle group has been shown to suffer from strength deficits earlier than other muscle groups (4). This unequal reduction in strength with aging could lead to reliance on those muscle groups that have

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FIGURE 2—Coordination variability during stance. (A) Thigh–shank coupling. (B) Shank–foot coupling. (C) Pelvis–thigh coupling.

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not report how the groups were created (2). It specified that all runners who participated were highly trained but did not discuss current running schedules. It is possible that the older runners in their study were not as active as the younger runners. The second study investigated elderly (67–73 yr) runners (7); therefore, their older running group was significantly older than that of this study, thus making comparisons difficult. The older adults recruited for this study were highly active, healthy runners. The older adults did not differ from the younger adults in their maximum knee flexion and extension torques. It is possible that their high activity level and maintenance of strength help them to maintain the gait of younger runners, thereby potentially preventing injury. In this regard, running itself may be protecting older runners from injuries. This notion is also supported by the similar levels of segment coordination variability observed in the two groups. As decreased variability has been associated with an injured state, older runners would be anticipated to exhibit smaller amounts of variability if they were at increased risk of being injured. Marti et al. (16) reported that older runners had a lower reported rate of injury, but this improvement was also correlated with the number of years the participants had been running. Some investigations of injuries occurring during a running event have also observed a decreased risk of injury in older runners (18,19). However, these studies did not look at how running experience may alter injury risk. Therefore, it is unknown whether this decreased risk was a result of advancing age or, more likely, greater experience. Savelberg et al. (20) investigated the influence of running on maintaining a walking gait similar to that of young adults. By measuring gait in both active and inactive young and old adults, they strived to find differences that were related to both age and activity. They reported

that the support torque in their older group was affected by activity. The active older participants had a support torque similar to that of young participants, suggesting that some gait variables may be maintained with an active lifestyle. The current study adds to the increasing body of literature that supports the notion that activity, in this case running, acts to maintain health in aging individuals. The findings of this study are influenced by a few limitations. As this was not a longitudinal study, the differences detected were those between older and younger groups. This may not be indicative of what happens as an individual ages. Additionally, although there is a large body of literature supporting the notion that too little or too much variability is bad, a healthy window of variability has not been determined. An assumption of the current study is that young healthy adults exhibit healthy levels of variability and, therefore, any deviation from this in older adults would indicate a less optimal state.

CONCLUSIONS Runners appear to maintain joint mechanics during running with increasing age. This may indicate that running itself may be contributing to the maintenance of health of older runners in the current study. This is indicated by the fact that there were very few differences in running mechanics between older and younger runners from the perspectives of joint angles and moments and segment coordination variability.

This research was completed with support from Brooks Sports, Inc. The authors declare no conflicts of interest. The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Running Mechanics and Variability with Aging.

As the elderly population in the United States continues to grow, issues related to maintenance of health become increasingly important. Physical acti...
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