Single-leg disuse decreases skeletal muscle strength, size, and power in uninjured adults: A systematic review and meta-analysis

Abstract

We aimed to quantify declines from baseline in lower limb skeletal muscle size and strength of uninjured adults following single-leg disuse. We searched EMBASE, Medline, CINAHL, and CCRCT up to 30 January 2022. Studies were included in the systematic review if they (1) recruited uninjured participants; (2) were an original experimental study; (3) employed a single-leg disuse model; and (4) reported muscle strength, size, or power data following a period of single-leg disuse for at least one group without a countermeasure. Studies were excluded if they (1) did not meet all inclusion criteria; (2) were not in English; (3) reported previously published muscle strength, size, or power data; or (4) could not be sourced from two different libraries, repeated online searches, and the authors. We used the Cochrane Risk of Bias Assessment Tool to assess risk of bias. We then performed random-effects meta-analyses on studies reporting measures of leg extension strength and extensor size. Our search revealed 6548 studies, and 86 were included in our systematic review. Data from 35 and 20 studies were then included in the meta-analyses for measures of leg extensor strength and size, respectively (40 different studies). No meta-analysis for muscle power was performed due to insufficient homogenous data. Effect sizes (Hedges' g_av ) with 95% confidence intervals for leg extensor strength were all durations = -0.80 [-0.92, -0.68] (n = 429 participants; n = 68 aged 40 years or older; n ≥ 78 females); ≤7 days of disuse = -0.57 [-0.75, -0.40] (n = 151); >7 days and ≤14 days = -0.93 [-1.12, -0.74] (n = 206); and >14 days = -0.95 [-1.20, -0.70] (n = 72). Effect sizes for measures of leg extensor size were all durations = -0.41 [-0.51, -0.31] (n = 233; n = 32 aged 40 years or older; n ≥ 42 females); ≤7 days = -0.26 [-0.36, -0.16] (n = 84); >7 days and ≤14 days = -0.49 [-0.67, -0.30] (n = 102); and >14 days = -0.52 [-0.74, -0.30] (n = 47). Decreases in leg extensor strength (cast: -0.94 [-1.30, -0.59] (n = 73); brace: -0.90 [-1.18, -0.63] (n = 106)) and size (cast: -0.61[-0.87, -0.35] (n = 41); brace: (-0.48 [-1.04, 0.07] (n = 41)) following 14 days of disuse did not differ for cast and brace disuse models. Single-leg disuse in adults resulted in a decline in leg extensor strength and size that reached a nadir beyond 14 days. Bracing and casting led to similar declines in leg extensor strength and size following 14 days of disuse. Studies including females and males and adults over 40 years of age are lacking.

Keywords: Atrophy; Immobilization; Muscle disuse; Single-leg disuse; Skeletal muscle; Unloading.

Figure 1

Flow diagram of the study selection process. Abbreviations: CCRCT, Cochrane Central Register of Controlled Trials; CINAHL, Cumulative Index to Nursing and Allied Health Literature; CSA, cross‐sectional area

Figure 2

Forest plot of a random effects meta‐analysis comparing changes in measures of leg extensor strength following single‐leg disuse. Because effect sizes were calculated as baseline minus post‐disuse values, negative Hedge's g values with larger magnitudes reflect larger declines in muscle strength. The red circle represents a combined effect size with 95% confidence intervals. Outcomes reported in N, kg, and nm were categorized as MVC, 1RM, and peak torque, respectively (unless reported otherwise). Table S5 in the supporting information includes all extracted outcome data; n = 9 studies ≤7 days, n = 17 studies >7 to ≤14 days, n = 9 studies >14 days; N = 35 unique studies. (a) trained subgroup; (b) untrained subgroup; (c) old subgroup; (d) young subgroup; (e) male subgroup; (f) female subgroup; * subgroups reported in the same publication. AJCN, American Journal of Clinical Nutrition; CI, confidence interval; MSSE, Medicine & Science in Sports & Exercise; MVC, maximum voluntary contraction; NR, not reported; 1RM, one‐repetition maximum

Figure 3

Forest plot of a random effects meta‐analysis comparing changes in measures of leg extensor size following single‐leg disuse. Because effect sizes were calculated as baseline minus post‐disuse values, negative Hedge's g values with larger magnitudes reflect larger declines in muscle size. The red circle represents a combined effect size with 95% confidence intervals. N = 20 unique studies. Sheet S5 in the supporting information includes all extracted outcome data; (a) female subgroup; (b) male subgroup; (c) old subgroup; (d) young subgroup; * subgroups reported in the same publication. AJCN, American Journal of Clinical Nutrition; AJPEM, American Journal of Physiology, Endocrinology, and Metabolism; CI, confidence interval; CSA, cross‐sectional area; NR, not reported; VL, vastus lateralis

Figure 4

Subgroup analyses for changes in measures of knee extensor strength and size following single‐leg disuse. Red circles represent combined effect sizes with 95% confidence intervals. Sheet S9 in the supporting information provides subgroup analyses with individual studies. Knee extensor strength: N = 35 separate studies; n = 429 observations; I ² = 0% for all. ANOVA for subgroup differences (between/model): Sum of squares = 5.98, df = 2, P = 0.05. Knee extensor strength (cast vs. brace following 14 days of disuse): N = 14 separate studies; n = 179 observations; I ² = 0% for all. ANOVA for subgroup differences (between/model): Sum of squares = 0.03, df = 1, P = 0.86. Knee extensor size: N = 20 separate studies; n = 233 observations; I ² = 0% for all. ANOVA for subgroup differences (between/model): Sum of squares = 1.56, df = 2, P = 0.46. Knee extensor size (cast vs. brace following 14 days of disuse): N = 6 separate studies; n = 82 observations; I ² = 0% for all. ANOVA for subgroup differences (between/model): Sum of squares = 0.17, df = 1, P = 0.68