Relationship of strength training lifetime exposure with functional outcomes and mobility over 4 years: Data from the Osteoarthritis Initiative

Highlights

• •Among 3192 individuals at risk for, or who had symptoms of knee osteoarthritis, lifelong participation in strength training activity can boost sport and recreational participation levels in later life.
• •Individuals who did not engage in any strength training during life had the highest prevalence of mobility loss and assistive device use after 50 years of age.
• •Participation in some strength training during different life periods or participation in all life periods is related to lower knee pain severity compared to no strength training participation.
• •Strength training throughout life may help people with or at risk for knee osteoarthritis maintain functional levels and activity in sport and recreational activities in later life, while keeping knee pain levels relatively lower.

Abstract

Background

This study compared knee osteoarthritis (OA) outcomes specific to pain, physical function, and quality of life in later life based on strength training (ST) participation over a lifetime.

Methods

Participants from the Osteoarthritis Initiative (n = 3192) were grouped by ST engagement during ages 12–18 years, 19–34 years, 35–49 years, and 50+ years. Participants were categorized as: No ST (no ST at any point; 61.7 ± 9.0 years (mean ± SD)), Some ST (engaged in ST during 1–3 life stages; 58.9 ± 8.7 years), and Lifelong ST (consistently engaged in ST across all life stages; 55.6 ± 8.1 years). Measures were collected at baseline and Year 4: Western Ontario and McMaster Universities Osteoarthritis Index Scores (WOMAC; pain, daily activities), Knee Injury and Osteoarthritis Outcome Score (KOOS; sports, recreation), Physical Activity Score for the Elderly (PASE), Short Form-12 Physical Component Score (SF-12 PCS), mobility disability, chair rise time, and walking speed (20 m and 400 m).

Results

At Year 4, the Lifelong ST group reported better WOMAC activity scores in the right knee along with better WOMAC pain, KOOS sports/recreation, and PASE scores compared to other groups (p < 0.05). The Lifelong ST group had the lowest incidence of mobility disability of all groups (0.8% vs. 2.3%–4.1%; p = 0.015) and maintained the fastest walking speeds in Year 4.

Conclusion

For those with knee OA, ST throughout life may help preserve function and mobility, allowing for greater physical activity engagement while keeping pain levels relatively lower.

Graphical abstract

1. Introduction

Osteoarthritis (OA) is a debilitating condition and the seventh leading cause of disability throughout the world.¹ The Global Burden of Disease Study has estimated that OA has affected over 595 million individuals, or approximately 7.6% of the global population as of 2020.² As the average life expectancy continues to rise, the prevalence of OA is expected to follow suit in the aging population.² Primary knee OA is the predominant form of arthritis and the leading contributor to the worldwide OA burden compared to other joint sites.³ Individuals who experience moderate-to-severe OA pain display strength loss,⁴ a withdrawal from physical activities,5, 6, 7 multisite pain,⁸ decline in mental well-being,⁵ loss of mobility,⁹ poor work productivity, and premature unemployment.¹⁰ Thus, long-term management of OA using accessible lifestyle interventions may help counteract disease symptoms, improve function, preserve healthcare resources, and minimize monetary cost. Patient-driven activity, such as engagement in exercise, could be the first step to achieving this goal.

Systematic reviews and meta-analyses have provided extensive evidence that strength training (ST) programs ranging from weeks to months are effective in decreasing OA pain.11, 12, 13 Additionally, Lawford et al.¹⁴ found that over the course of a 6-month ST program, adults with OA showed greater muscular strength, lower risk of mortality, lower percent body fat, and lower cardiometabolic disease risk than sedentary adults. Modifying the intensity of activity also plays an important role in benefiting those with knee OA. While high intensity exercise has been linked to an increase in knee OA, moderate intensity activities, including sports involvement and walking for pleasure, have been associated with a protective effect in those with knee OA.¹⁵ The Strength Training for Arthritis Trial (START) trial reinforced the relevance of low-to-moderate intensity activities, demonstrating that high intensity strength training did not provide greater benefit than low intensity strength training for adults with knee OA.¹⁶ Patients who participate in ST over the short term experience many benefits. Moderate intensity whole body ST interventions performed 2–3 times per week for a few months can improve overall health, increase muscle strength, and reduce functional pain during ambulation and daily activities.^17,18 Additional research shows that engaging in ST at any point in life reduces the risk of OA-related knee pain as well as radiographic evidence of knee OA.¹⁹ However, it is not well established whether ST over a life course might promote greater quality of life and physical function while simultaneously decreasing pain levels and pain medication use in later life. The Osteoarthritis Initiative (OAI) provides long-term data on people at risk for OA or those already diagnosed, making it useful for understanding the role of ST across the life course.

The aim of this study was to investigate whether different levels of exposure to ST over a lifetime contribute to lower OA pain and improved functional outcomes. Compared to no ST and some exposure to ST in life, we hypothesized that lifelong ST exposure would enhance preservation of physical function in later life based on performance measures (20-m and 400-m walk velocities, repeated chair rise time) and self-reported pain and function (Knee Injury and Osteoarthritis Outcome Score (KOOS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and Physical Activity Score for the Elderly (PASE)).

2. Methods

2.1. Study design

This was a longitudinal analysis of retrospective data from OAI, a multicenter prospective longitudinal observational study. Data were obtained from the publicly-available OAI dataset Version 0.2.3 (release created 9.0401M4). These data are available for public access (https://nda.nih.gov/oai/). The OAI was originally described as a multicenter, observational population-based study of knee OA and is composed of 3 subgroups (n = 4796): the incidence cohort, the progression cohort, and the control group. Data were previously collected at 4 OAI clinical sites: Memorial Hospital of Rhode Island, Ohio State University, the University of Pittsburgh, and the University of Maryland/Johns Hopkins. The OAI was originally designed to longitudinally observe people over time to better understand the knee OA disease process and treatment impact in people at risk for or with the disease. The manuscript follows the recommended format for observational studies described by the statement on Strengthening the Reporting of Observational Studies in Epidemiology (STROBE).²⁰ Given that the investigators on this study obtained these data as an anonymized dataset, that there was no direct interaction with any of the subjects in the OAI, that we did not obtain consent or collect any data directly, and that the dataset was deidentified, this study was deemed “nonhuman” by the University of Florida Institutional Review Board and was exempt from board review (Protocol # NH00046709).

2.2. Participants

For this analysis, participants who had undergone a total knee or hip replacement at any time in the study, or who had incomplete 20 m walk data at baseline and 48 months were excluded from this dataset. There was a subset of patients (n = 38) aged 45–49 years old who performed ST in all their life epochs but were not of age to record activity in the 50+ life epoch. These individuals were placed in the lifetime ST group despite being engaged in ST in only 3 total life epochs because, for them, ST participation did occur consistently over their life span. Some supportive evidence suggests that engagement of physical activities earlier in life predicts activity levels in later life,21, 22, 23 and that baseline physical activity levels in other studies predict activity levels at later time points.²⁴ Thus, a final total of 3192 participants were included in these analyses to study pain, function, and gait. The study flow diagram is shown in Fig. 1.

Fig. 1.

The strengthening the reporting of observational studies in epidemiology study flow diagram. OAI = The Osteoarthritis Initiative.

2.3. Time schedule of assessments

Data from the OAI were extracted from the baseline and 4-year visits. Data from participants who completed self-reported pain and functional measures and mobility testing at baseline were then analyzed by the study team to determine the effect of lifetime ST exposure on study outcomes.

2.4. Strength training exposure throughout life

Life exposure to ST was characterized using an OAI participant questionnaire. In this questionnaire, participants were asked to review and report the recalled engagement in 37 different leisure physical activities over the course of the life span, including “strength or weight training”. Specifically, participants were asked whether they performed ST for “at least 20 minutes within a given day for at least 10 times” during the following age epochs in life: 12–18 years, 19–34 years, 35–49 years, and at or after 50 years. The investigators tallied the number of life epochs each participant reported engaging in ST throughout life. Finally, participants were grouped for statistical analysis into three categories by their life ST exposure as reported on the questionnaire at baseline: No ST (0 epochs; n = 2033), Some ST (1–3 epochs at various life stages; n = 1017), and Lifelong ST (4 epochs; n = 142).

2.5. Participant characteristics and covariates

Age, anthropometrics (waist, weight, and height), demographics (race, ethnicity, and sex), current physical activity engagement, and pain medication use (“taken any pain medication today (include both prescription and over-the-counter medications for any type of pain)”) represented baseline health status. The age-adjusted Charlson Comorbidity Index (CCI) was calculated as a comorbid burden estimate, where age adjustment consisted of assigning each decade of life a comorbidity score of 1 point in addition to the presence of 19 conditions.²⁵ Educational status (stratified into less than high school or not), household income (< or ≥ USD 50,000), home living situation (living alone or not), the presence of radiographic knee OA, OA symptoms, and “aching or stiffness > half days per month for the last 12 months” were collected at baseline. Using the 37-item questionnaire of physical activities (described in the previous Section 2.4. Strength training exposure throughout life), the total number of other sports played or regular exercise sessions performed in addition to participation in ST was collected at baseline and tallied for each participant. The variables were used to describe the sample and evaluated as covariates. Finally, the available initial Kellgren Lawrence (KL) scores were extracted from the data files associated with radiographic imaging scores for baseline and Year 4 to obtain an estimate of OA severity and severity change over the 4-year period.

2.6. Performance-based physical functional measures

Objective, valid performance measures of physical function were extracted from the dataset and included 3 timed tests: the 20-m walk test, the 400-m walk test (included whether a cane was used for assistance to complete the test), and 5 chair stand time. Gait velocity was determined by dividing the walk test distance by the time required to complete the walk and is expressed in m/s. Gait velocities of <1 m/s are associated with greater risk for self-reported mobility disability among older adults,²⁶ and individuals who demonstrated velocities <1 m/s by Year 4 were categorized with mobility loss. Long 400-m walk test velocities are associated with mobility limitations, physical disability, and elevated rates of mortality.^27,28 Gait speed loss is related to loss of independence and quality of life.^29,30

2.7. Patient-reported pain and functional outcome measures

Several self-reported measures were selected to represent pain, functional capacity, and physical activity in daily life.

2.7.1. Short form (SF)-12

The SF-12 instrument captures self-reported impact of health on everyday life.³¹ Eight health-related domains with 1 or 2 questions per domain are included (limitations in physical activities, social activities and usual role activities, bodily pain, general mental health, vitality, and general health perceptions). A Physical Component Score (PCS) was determined using scores from the 12 questions that range from 0 to 100, where 0 = the lowest level of health and 100 = the highest level of health. The minimally clinical important difference in SF-12 PCS scores is 1.8 points for individuals with knee OA and surgical treatment.³² The Bodily Pain domain (“During the past 4 weeks, how much did pain interfere with your normal work (including work outside the home and housework?) 1 = not at all, 2 = a little bit, 3 = moderately, 4 = quite a bit, 5 = extremely”) was used to represent functional pain impact on daily life.³²

2.7.2. KOOS

The KOOS evaluates short- and long-term symptoms in individuals with knee OA or injury. This is a valid and reliable instrument that consists of 5 different subscales. Given that reporting of the total KOOS may mask effects of a medical condition on subscore domains and therefore reduce validity of the instrument,³³ we report one key subscore domain: Function in Sports and Recreation (KOOS FSR includes items about difficulty with tasks such as jumping, running, squatting, or kneeling as part of function, sports, and recreation).³⁴ Scores are percentages ranging from 0 to 100, where 0 = extreme symptoms and 100 = no symptoms. The minimum detectable change in FSR is 19.6 points in people with OA. Internal consistency for these KOOS subscales ranges from 0.71 to 0.98.³⁵

2.7.3. WOMAC, pain, and function

The WOMAC instrument is a 24-item questionnaire comprised of 3 subscales: pain, stiffness, and physical function.³⁶ The pain subscale score is derived from pain ratings during 5 activities (flat surface walking, stair climb, night sitting or lying, standing). The physical function subscale consists of 17 items relating to activities of daily living (ADL; e.g., rising from sitting, standing, walking, dressing, transfers). Each of the subscore items are scored from 0 to 4, where 0 = no difficulty and 4 = extremely difficult. Higher scores indicate greater functional limitation.

2.7.4. PASE

The PASE is a valid, 12-item self-administered questionnaire developed for the use in adults over 65 years of age to estimate the amount of physical activity performed over the last 7 days.³⁷ Physical activities include walking outside, household chores (light or heavy), sports and recreational activities (light, moderate, and strenuous), and work hours. The intensity level, duration, and frequency are used to create a score ranging from 0 to 793 points, with higher scores representing higher activity levels.³⁷ This instrument has good test–retest reliability (r = 0.75) and has recently been recommended for measurement of total physical activity in older adults.³⁸ Internal consistency values (Chronbach’s ɑ) for the total PASE score among adults with OA aged 45 years and older is 0.68.³⁹ The subscore for “Muscle strengthening and endurance activity, hours per day over the last 7 days” was chosen to indicate current participation in this type of exercise during the 4-year period (subdomain coefficient r = 0.56; p < 0.001).³⁹

2.8. Statistical analyses

Statistics were conducted in IBM SPSS Version 29.0 (IBM, Armonk, NY, USA). Normality of distributions were first examined for each outcome variable. For outcomes where skewness and kurtosis existed (400-m walk velocity, 20-m walk velocity, repeated chair rise time, SF-12 PCS subscores), Log₁₀ transformations were performed. Given that the analyses of the transformed variables yielded the same results as the raw data, the raw data are presented here. Levene’s test was used to test heterogeneity of variance; in cases of unequal variance by strength training group, Kruskal–Wallis tests were used. Repeated measures analysis of covariance were conducted where pain and functional scores were the dependent variables (KOOS FSR scores, WOMAC scores, PASE scores, chair rise times, gait velocities, and SF-12 scores) and ST exposure group was the independent variable (No ST, Some ST, and Lifelong ST). Separate analysis of covariance models were generated for each functional and pain-dependent variable. Covariates were obtained from the baseline visit and included sex, age, body mass index (BMI), marital status, ethnicity, CCI, current engagement in ST, taking medications for knee pain, number of other sports played, and presence of radiographic and symptomatic OA. In the cases of missing data at follow-up, complete case analysis was performed.⁴⁰ Kruskal–Wallis tests were used to determine whether differences existed among groups for categorical variables. Significance was established at p < 0.05 for all tests. The eta squared (η²) were provided to show the effect sizes for continuous variables; values of 0.01, 0.06, and 0.14 represented negligible-to-small, medium, and large effects, respectively.⁴¹

3. Results

3.1. Baseline characteristics

Table 1 provides a summary of the baseline characteristics of participants. Compared to males, a lower percentage of females participated in Lifelong ST (p < 0.001). Participants with some ST had a lower body BMI and waist circumference compared to those classified as No ST (p < 0.001). Participants who engaged in Lifelong ST had greater body weight (p < 0.001), used less pain medication (p = 0.002), and engaged in a higher number of sports and exercise sessions (p < 0.001). Furthermore, the extent of radiographic and symptomatic OA in patients engaged in Some ST was significantly lower than in No ST (p < 0.001). The Lifelong ST group had the fewest comorbidities, the lowest CCI score, and reported using fewer pain medications (p < 0.05).

Table 1.

Baseline characteristics of study groups.

Variable	No ST	Some ST	Lifelong ST	η2	p
	(n = 2033)	(n = 1017)	(n = 142)
Category
Risk factors only	42.2	46.7	30.5
Symptoms only	6.0	6.0	6.4
Symptoms and risk factors	23.8	26.2	34.8
Progression	28.0	21.0	28.4	0.096	<0.001
Sex (female)	60.8	56.2#	24.6†	0.151	<0.001
Age (year)	61.7 ± 9.0	58.9 ± 8.7	55.6 ± 8.1	0.035	<0.001
Height (m)	1.67 ± 0.91	1.69 ± 0.93	1.74 ± 0.84	0.019	<0.001
Weight (kg)	80.8 ± 16.2	80.3 ± 16.5	86.7 ± 13.8⁎	0.006	<0.001
BMI (kg/m2)	28.6 ± 4.8	27.9 ± 4.8#	28.5 ± 3.9	0.004	<0.001
Waist (cm)	103.0 ± 12.7	100.1 ± 12.9#	101.0 ± 10.9	0.011	<0.001
Race (%)
African-American	18.5	19.4	21.1
Caucasian	78.9	77.7	76.1
Other	2.6	2.9	2.8	0.017	0.650
Ethnicity (Hispanic)	0.9	2.2	2.8	0.054	0.009
Living alone	23.4	21.3	21.4	0.024	0.399
Income < USD50,000	38.1	39.3	40.7	0.014	0.729
Education
≤HS graduate	15.0	17.4	20.7	0.029	0.307
CCI (pts)	2.0 ± 1.3	1.7 ± 1.2#	1.4 ± 1.2†	0.019	0.029
Either knee aching or stiffness > half days/month past 12 months	29.9	22.9#	23.7	0.069	<0.001
Either knee
ROA	54.5	46.7#	48.6	0.073	<0.001
SOA	52.5	44.3#	46.5	0.077	<0.001
KL score (pts)	2.1 ± 0.6	2.1 ± 0.7	2.2 ± 0.5	0.023	0.285
Pain medication use for knee	11.5	8.2#	4.9#	0.063	0.002
Sports played or exercise activity (n)	1.3 ± 2.1	4.8 ± 2.8#	7.4 ± 3.5†	0.390	<0.001

Note: Values are presented as means ± SD or % of the group. Percentages add up not to 100% due to rounding.

Abbreviations: BMI = body mass index; CCI = Charlson Comorbidity Index; HS = high school; KL = Kellgren Lawrence; pts = points; ROA = radiographic osteoarthritis; SOA = symptomatic osteoarthritis on most days/month for past 12 months; ST = strength training.

^⁎p < 0.05 different than Some ST.

^#p < 0.01 different than No ST.

^†p < 0.05 different than all groups.

3.2. Knee pain, bodily pain subscores, and Kellgren Lawrence (KL) score change

Fig. 2 provides the WOMAC pain subscores at baseline and Year 4. There was a main effect of the ST group for the left limbs (p = 0.004); the No ST group reported higher pain scores than the groups with Some ST or Lifelong ST. The effect sizes for the ST group were negligible to small for these pain scores (η² range: 0.001–0.003). Fig. 3 provides the SF-12 Bodily Pain domain scores. There was a significant group × time interaction (p = 0.042), where the No ST group reported increased pain scores between baseline and 4 years and both ST groups reported less bodily pain by Year 4; the effect size was negligible to small (η² = 0.001). The KL score changes over 4 years for the No ST, Some ST, and Lifelong ST groups were 0.22 ± 0.50 points, 0.15 ± 0.39 points, and 0.15 ± 0.37 points, respectively (mean ± SD) (p = 0.184).

Fig. 2.

WOMAC scores at baseline and Year 4. Values are presented as mean ± SD. (A) Right knee scores; (B) left knee scores. ST = strength training; WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index scores.

Fig. 3.

The mean score of SF-12 bodily pain question response: “During the past 4 weeks, how much did pain interfere with your normal work (including work outside the home and housework?”. Values are presented as mean ± SD. SF-12 = Short Form-12 Physical Component Score; ST = strength training.

3.3. Patient reported functional outcomes

Table 2 describes functional outcome scores collected at baseline and after 4 years. There were significant group × time interactions for WOMAC ADL (right knee) and for KOOS FSR scores (both p < 0.05). Main effects existed for the ST group on WOMAC ADL, PASE daily muscle strengthening activity, and KOOS FSR scores. PASE scores were different by group. Overall, the Some ST and Lifelong ST groups showed higher daily muscle strengthening activity and better functional scores compared to the No ST group for these measures. The effect sizes for ST were negligible to small for these patient-reported tests (η² range: 0.000–0.007).

Table 2.

Patient-reported functional outcomes at baseline and Year 4.

Variable	No ST(n = 2033)	Some ST(n = 1017)	Lifelong ST(n = 142)	p
				η2	Group	Time	Interaction
WOMAC ADL (pts)
Right knee
Baseline	7.7 ± 10.2	5.0 ± 7.8	5.3 ± 7.2
Year 4	6.9 ± 10.2	5.0 ± 8.4	4.6 ± 6.4	0.007	<0.001	0.899	0.015
Missing	0	0	1
Left knee
Baseline	7.9 ± 11.1	5.1 ± 8.7	5.9 ± 8.8
Year 4	6.6 ± 10.3	4.8 ± 8.5	5.0 ± 7.3	0.001	<0.001	0.707	0.271
Missing	1	0	1
PASE score (pts)
Total score
Baseline	159 ± 80	172 ± 82	196 ± 89
Year 4	151 ± 82	165 ± 82	190 ± 78	0.000	0.568	0.322	0.993
Missing	29	17	2
PASE Muscle strengthening activity (h/day)
Baseline	0.48 ± 0.69	0.69 ± 0.75	0.90 ± 0.80
Year 4	0.51 ± 0.71	0.69 ± 0.77	0.87 ± 0.79	0.002	<0.001	0.255	0.050
Missing	48	18	3
KOOS FSR (pts)
Baseline	75.3 ± 24.3	80.7 ± 21.7	72.8 ± 23.4
Year 4	78.1 ± 23.9	81.2 ± 23.0	77.7 ± 22.1	0.004	0.014	0.157	0.013
Missing	61	14	0
SF-12 PCS (pts)
Baseline	48.8 ± 9.3	51.1 ± 8.2	52.6 ± 7.3
Year 4	47.7 ± 9.5	50.1 ± 8.3	51.4 ± 7.3	0.001	0.105	0.057	0.437
Missing	50	23	7

Note: Values are presented as means ± SD or n.

Abbreviations: KOOS FSR = Knee Injury and Osteoarthritis Outcome Score, Function in Sports and Recreation subscore; PASE = Physical Activity Score for the Elderly; pts = points; SF-12 PCS = Short Form-12 Physical Component Score; ST = strength training; WOMAC ADL = Western Ontario and McMaster Universities Osteoarthritis Index subscores.

3.4. Performance based functional outcomes

Table 3 provides performance-based physical function test scores. There was a significant group × time interaction for the 20 m walk velocity, where participants with Lifelong ST had an increase in velocity from baseline to Year 4 and the other two groups had a reduction in speed (p = 0.054). None of the participants engaged in Lifelong ST used a cane in the 400 m walk. For the 20-m walk speed, there was a significant main effect of time, where Yll participants exhibited speed change at Year 4 (p < 0.001). While the 400-m walk speed was faster at baseline and increased by Year 4 in the Lifelong ST group, the interaction of group × time did not achieve statistical significance (p = 0.088). The lowest percentage of mobility loss at Year 4 was observed in patients engaged in Lifelong ST (p = 0.015). The effect size of ST ranged from negligible to small for these performance tests (η² range: 0.001–0.056).

Table 3.

Performance-based physical function test scores at baseline and Year 4.

Variable	No ST	Some ST	Lifelong ST	p
				η2	Group	Time	Interaction
Gait velocity
20-m walk (m/s)
Baseline	1.32 ± 0.22	1.36 ± 0.20	1.39 ± 0.18
Year 4	1.30 ± 0.22	1.35 ± 0.20	1.41 ± 0.18	0.001	0.546	<0.001	0.054
Missing	3	0	1
400-m walk (m/s)
Baseline	1.34 ± 0.21	1.39 ± 0.20	1.43 ± 0.19
Year 4	1.32 ± 0.28	1.36 ± 0.27	1.46 ± 0.21	0.001	0.456	0.064	0.088
Missing	171	67	7
Five chair stand time (s)
Baseline	11.5 ± 3.8	10.5 ± 2.9	10.2 ± 2.9
Year 4	10.8 ± 3.2	10.0 ± 3.1	9.5 ± 2.5	0.001	0.163	0.694	0.320
Missing	60	5	3
Cane use during 400 m walk (Year 4; %)	1.5	0.5	0.0	0.047	0.044	—	—
Mobility loss (Year 4; %)	4.1	2.3	0.8	0.056	0.015	—	—

Values are presented as means ± SD, n, or % as indicated. Mobility loss = gait velocity of <1.0 m/s.

Abbreviation: ST = strength training.

4. Discussion

This longitudinal analysis of retrospective data compared the functional and pain outcomes among individuals with primary knee OA who had different life ST exposure. Although there is a plethora of research on the short-term role of strength training, we investigated the less understood relationships between varying lifetime exposure levels of muscle strengthening exercises and functional benefits in later life. Findings partially support the hypothesis that ST participation throughout life is associated with preserved performance-based functional outcomes, self-reported physical function, and a lower mobility disability after 50 years of age. These findings suggest that consistent ST engagement throughout life is a valuable non-pharmacological intervention that may help mitigate pain and declines in physical functioning and quality of life associated with knee OA in later life.

4.1. Pain

Participants who never engaged in ST reported significantly more pain-limited days at baseline compared to those with Some or Lifelong ST participation. SF-12 bodily pain domain scores increased over the 4-year period in the No ST group, whereas participants with ST experience reported stable or lower pain levels over the 4-year period. Similarly, WOMAC pain subscores were consistently higher in the No ST group for both knees across the 4-year period. These findings align with systematic reviews demonstrating that resistance exercise interventions can alleviate OA-related pain^12,42,43 and maintain some pain relief over many years.¹⁶ Some evidence shows that all types of ST (involving isokinetic, isometric, and isotonic contractions) can reduce knee OA pain,^11,12 thereby providing options for patients to find activities that are sustainable. Moreover, some evidence shows that pain relief can occur irrespective of intensity of strengthening; even low intensity ST has been shown to reduce WOMAC pain similarly to high intensity ST.¹⁶ Studies among patients with knee OA have shown that increased grip strength and knee extensor strength correspond to better WOMAC function scores and less functional deterioration over time.44, 45, 46 Strengthening activities can improve proprioception, muscle support around the joints, and load distribution within the joint (creating better joint stability and biomechanics), while inflammation and cartilage deterioration are mitigated (inhibiting chondrocyte apoptosis) and synovial lubrication is enhanced.^47,48 These changes are associated with reductions in pain.¹⁴ Within the current study, fewer participants in the Lifelong ST group reported pain medication at baseline, suggesting a potential long-term analgesic benefit of sustained ST participation. Future studies on pain processing by strength-trained and non-strength trained individuals who have knee OA may help clarify whether analgesic effects are truly occurring or if individuals are experiencing pain differentially.

4.2. Disability and functional implications

The Lifelong ST participants had the fastest walking velocities for the 20 m and 400 m distances and better WOMAC and KOOS scores. Notably, fewer participants in the Some and Lifelong ST groups developed mobility disability. An umbrella review of ST activity revealed that, irrespective of strengthening exercise prescriptions, ST can effectively increase skeletal muscle mass, strength, and physical function compared to no exercise.⁴⁹ While the OAI does not collect details of ST elements (muscle contraction type, volume, frequency, and intensity), other interventional studies in knee OA have shown that strength and physical function can improve with a variety of ST activities. Specifically, strengthening/endurance programs preserve lower limb function (WOMAC subscores, chair rise, timed-up-and-go, and ambulation tasks)⁵⁰ and delay the onset of mobility limitations.^16,51 Given that gait speed is a key predictor of independence and overall health in older adults,³⁸ the faster walking velocity among Lifelong ST participants underscores the importance of consistent engagement in strength-based activities. Compared to previous interventional studies with rigorously designed ST exercise programs, the OAI data indicates that performing general muscle strengthening/endurance activities is related to functional benefits. Moreover, low and high intensity ST have been shown to produce similar functional benefits in this population⁵² and to enable some individuals who cannot engage in intensive activity to gain some benefit with less demanding exercise. ST involving both concentric and eccentric contractions improve muscle strength, physical function, and reduce functional pain during daily tasks such as walking, rising from a chair, or climbing stairs.^18,47 While we are unable to provide insight into the specific types of ST that are more strongly related to functional differences in this analysis, participation in any muscle strengthening over the long term—irrespective of type, intensity, or specific exercises performed—was associated with some aspects of greater function and mobility in later life.

4.3. Physical activity and quality of life

PASE scores, which reflect overall physical activity levels, were highest in the Lifelong ST group and lowest in the No ST group over time, and this pattern corresponded to participation in sport activities. Individuals who engaged in more ST also participated in more sports. This suggests that individuals who engage in Lifelong ST are more likely to sustain an active lifestyle in later years. Increased activity levels have been linked to improved cardiovascular health, reduced comorbidity burden, and enhanced mental well-being.⁵³

We postulate that there are differential long-term mechanisms underlying the distinct findings of Some ST vs. Lifelong ST. Evidence showing durable benefits of ST in other populations indicates that ST (especially compound movements with heavier loads) can increase type II fibers in older adults⁵⁴ as well as decrease motor unit thresholds and increase spinal cord output to motor unit discharge rate.⁵⁵ Neural and muscle adaptations, such as improved motor unit synchronization, reduced antagonist co-activation, and increased myonuclei, can persist even when training stops (e.g., Some ST group). These adaptations accelerate training responses when ST resumes. Cerebral blood flow increases with ST⁵⁶ and is an under-appreciated benefit that can improve brain health and cognitive function. Mental health outcomes and wellbeing also improve commensurate with ST participation level—even when individuals are not adherent;⁵⁷ greater engagement protects against anxiety and depression more effectively than lower engagement. Several psychological aspects improve with acute or short-term ST, including overall mood, confusion and anger, tension, vigor, and sleep quality.⁵⁸ With Lifelong ST participation, we surmise that these collective adaptations would be more pronounced and reflect the findings we observed in the present study with respect to functional scores, patient-reported outcomes, wellbeing, and pain. Long term ST participation may also impart reduction of systemic inflammation, maintenance of muscle power and torque, and preservation of specific myosin and tropomyosin isoforms similar to younger adults.⁵⁸ These multisystem adaptations have implications for the performance of the gait and chair-rise tests as well as the responses given on the quality-of-life surveys.

4.4. Limitations and future directions

Several limitations in this study must be acknowledged. First, the self-reported nature of ST participation in the OAI methods introduces the potential for recall bias at baseline and Year 4. The lack of detail on the ST prescriptive elements may contribute to inconsistent self-reporting. Including data regarding types of exercise and quantity of exposure and following patients who completed formal exercise programs would be beneficial for future longitudinal studies. Objective measures to confirm ST engagement levels, such as accelerometry or direct fitness assessments, would also be informative. Second, the significant age difference between the no ST and Lifelong ST groups might have contributed to differences in comorbidities and the outcome measures. Modeling approaches with a matched age cohort may provide additional insights. Third, while efforts were made to control for confounding variables, unmeasured factors such as diet, genetic predisposition, hormonal supplementation, and other various non-pharmacological pain management strategies may have contributed to the results. Fourth, the sample of individuals engaged in Lifelong ST was relatively small (n = 142), limiting generalizability. Finally, the OAI cohort may contain several biases that could also affect the generalizability of these findings; these include volunteer and self-selection biases (thereby reflecting a cohort of people who are health conscious and motivated), recall bias (which can impact the volumes and time frame during which ST was performed by each participant), and geographic bias (participants represent select areas of the USA and may not broadly represent the typical profile of patients with knee OA).

Regarding future directions, longitudinal interventional studies could further clarify the causal relationship between Lifelong ST and OA-related outcomes, although this would be challenging and expensive. Data from more diverse demographic and clinical patient populations would increase clinical applicability of findings. Finally, the potential to combine existing datasets, resulting in larger sample sizes, warrants consideration.

5. Conclusion

The findings from this study emphasize the importance of ST across the lifespan for reducing pain, preserving mobility, and maintaining higher physical activity levels in individuals at risk for or with progressive knee OA. Engaging in regular ST may serve as a viable strategy to mitigate the functional decline associated with OA and enhance overall quality of life. Healthcare providers and public health initiatives should continue to emphasize patient education and encourage involvement in ST exercise as an integral component of OA management.

Authors’ contributions

DK, ZT, AJ, KS were responsible for visualization and formal analysis the study; KV was responsible for resources to complete the study; HV was the project administrator. All authors participated in the conception and design of the study, data curation, methodology, the interpretation of results, and manuscript writing. All authors have read and approved the final version of the manuscript, and agree with the order of presentation of the authors.

Declaration of competing interest

The authors declare that they have no competing interests.