Age-related differences in lower-limb muscle cross-sectional area and torque production in boys with Duchenne muscular dystrophy

Abstract

Objective: To examine the relationship between lower-extremity muscle cross-sectional area, muscle strength, specific torque, and age in ambulatory boys with Duchenne muscular dystrophy (DMD) compared with controls.

Design: Observational cross-sectional study.

Setting: University research setting.

Participants: Volunteer sample of boys with DMD (n=22) and healthy control boys (n=10), ages 5 through 14 years.

Interventions: Not applicable.

Main outcome measures: Maximal muscle cross-sectional area (CSA(max)) assessed by magnetic resonance imaging of quadriceps, plantarflexors (PFs) and dorsiflexors (DFs), peak isometric torque from dynamometry, and timed functional tests.

Results: The average CSA(max) of the triceps surae muscle group was approximately 60% higher in boys with DMD compared with controls (39.1+/-13.6 cm(2) vs 24.5+/-9.3 cm(2); P=.002), while the tibialis anterior muscle showed age-appropriate increases in CSA(max). The increase in quadriceps CSA(max) was also distinctly different in boys with DMD compared with controls. Specific torque (ie, peak torque/CSA(max)) was impaired in all 3 muscles groups, with the knee extensor (KE) and PF muscles showing 4-fold, and the DF muscles 2-fold, higher values in controls compared with boys with DMD. Large age-related gains in specific torque were observed in all 3 muscle groups of control subjects, which were absent in ambulatory boys with DMD. Correlations were observed between performance on functional tasks and quadriceps and PF torque production (r=-.45 to -.57, P<.05), but not with DF strength.

Conclusions: Age-related changes in muscle cross-sectional area and specific torque production in lower-extremity muscles showed distinctly different patterns in the KE, PF, and DF muscles of boys with DMD compared with controls.

Fig 1

T1-weighted fat-suppressed transaxial images of the thigh (A) and lower leg (B) obtained from an 11-year-old boy with DMD. The rectus femoris and vasti muscles are outlined on the thigh, and the TA, soleus, MG, and LG are outlined on the lower leg. Note the presence of fatty infiltration to a greater extent in the thigh musculature than in the calf. Abbreviations: LG, lateral gastrocnemius; MG, medial gastrocnemius; RF, rectus femoris; TA, tibialis anterior.

Fig 2

Scatter plots of age versus peak torque (Nm) in boys with DMD (filled circles, solid line) and controls (open circles, dashed line): (A) PFs (n = 22 DMD, n = 10 control), (B) DFs (n = 21 DMD, n = 10 control), and (C) KEs (n = 22 DMD, n = 10 control). Please note that in A, 2 subjects with DMD had overlapping values for plantarflexion peak torque (age 6.2y, 33.2Nm and 33.4Nm); in B, 1 subject with DMD was unable to perform the dorsiflexion torque measurement.

Fig 3

Scatter plots of CSA_max (cm²) versus age in boys with DMD (filled circles, solid line) and controls (open circles, dashed line): (A) triceps surae (n = 22 DMD, n = 10 control), (B) tibialis anterior (n = 22 DMD, n = 10 control), and (C) quadriceps (n = 20 DMD, n = 8 control). Please note that in B, 2 subjects with DMD had overlapping values for tibialis anterior CSA_max (age 6.2y, 2.7cm ² and 2.8cm²).

Fig 4

Scatter plots of age versus specific torque (torque normalized to CSA_max of muscle [Nm/cm²]) in boys with DMD (filled circles, solid line) and controls (open circles, dashed line). (A) PFs (n = 22 DMD, n = 10 control), (B) DFs (n = 21 DMD, n = 10 control), and (C) KEs (n = 20 DMD, n = 8 control). Please note in C, there are 2 overlapping points in the DMD group (age 9y, KE specific torques of .56 and .53Nm/cm²).

Fig 5

Group means (± SD) of 4 timed functional tasks in boys with DMD and controls. *P<.001.

Fig 6

Relationships between peak torque of the KEs (A) and PFs (B) and time to walk 9m.