The muscle-bone unit in adolescent swimmers.

Area of Science:

• Sports Medicine
• Orthopedics
• Human Physiology

Background:

• Adolescent swimmers' muscle-bone relationships are not fully understood.
• Previous studies show contradictory results regarding bone mineral density (BMC) in swimmers.
• The functional and structural muscle-bone unit requires examination in this population.

Purpose of the Study:

• To investigate the functional and structural muscle-bone unit in adolescent swimmers.
• To compare muscle-bone associations in swimmers versus non-swimmers (controls).
• To determine if standard adjustments for lean mass are appropriate for swimmers' bone evaluations.

Main Methods:

• Sixty-five swimmers and 119 controls underwent peripheral quantitative computed tomography (pQCT) for muscle cross-sectional area (MCSA), BMC, and polar strength-strain index (SSIPOL).
• Dual-energy X-ray absorptiometry (DXA) assessed subtotal BMC and lean mass.
• Muscle force was measured via handgrip and isometric knee extension (IKE) tests.

Main Results:

• Muscle size and force similarly influenced BMC and SSIPOL in both groups.
• Swimmers showed higher lean mass and MCSA for a given BMC or SSIPOL compared to controls (structural unit).
• Swimmers exhibited higher muscle force for a given BMC in the lower limbs, but not the upper limbs (functional unit).

Conclusions:

• The muscle-bone unit is uncoupled in swimmers, invalidating standard lean mass adjustments for bone assessment.
• Swimming may not be an effective sport for improving bone mass.
• Findings explain discrepancies in previous BMC studies on swimmers.