Musculoskeletal System
The scaffold and motors. Bones give you a frame, muscles move it, and both are more hormonally active than the textbook suggests.
The musculoskeletal system is your shape and your motion. Bones carry load and store minerals. Muscles convert chemical energy into force. Tendons couple them. Cartilage cushions. Ligaments connect bone to bone. The whole thing is alive and constantly being remodeled on signals you can influence.
At a glance
What it does
Supports the body, protects organs, produces blood cells (in red marrow), stores calcium and phosphate, converts chemical energy into mechanical work, and maintains posture. It is also an endocrine organ in its own right — bone releases osteocalcin and FGF23, muscle releases myokines that act systemically during and after exercise.
Bones
Bone is a living composite of collagen (tension-tolerant protein) and hydroxyapatite (compression-tolerant mineral). It is remodeled continuously by osteoblasts (build) and osteoclasts (resorb). Under load it thickens; under disuse it thins, which is why astronauts and bedridden patients lose bone fast. Weight-bearing exercise and resistance training drive accrual; sedentary life drives loss.
Bone is also a mineral bank. Parathyroid hormone, vitamin D, and calcitonin tune how much calcium is held versus released into blood. Sex hormones tune the bank's capacity; estrogen in particular is bone-protective, and its collapse at menopause is why postmenopausal women account for most osteoporosis.
Muscles
Three types: skeletal (voluntary, striated), cardiac (heart, striated, involuntary), smooth (gut, vessels, uterus, involuntary). Skeletal muscle is what you usually mean. It is built from fibers made of myofibrils made of sarcomeres, the actual contractile unit.
Muscle grows through mechanical tension and metabolic stress, interpreted by signaling pathways (mTOR, satellite cell activation) that increase protein synthesis for roughly 24-48 hours after a training bout. Net growth requires synthesis exceeding breakdown, which requires training plus adequate protein (roughly 1.6-2.2 g/kg/day for trained lifters) plus recovery. Muscle is also a metabolic sink for glucose — more lean mass means better insulin sensitivity, independent of fat mass.
Hormonal regulation
- Testosterone — the strongest natural driver of muscle hypertrophy in men; women have roughly 5-10% as much and still respond to training, just less dramatically.
- Estrogen — bone-preserving in both sexes; loss at menopause drives rapid bone density decline.
- Growth hormone and IGF-1 — support lean tissue maintenance; supraphysiologic doses drive abnormal growth but do not replicate testosterone's effects on muscle fiber size.
- Cortisol — catabolic when chronically elevated; eats lean tissue when sustained, especially under-fueled.
- Insulin — anabolic; fed state favors protein synthesis, fasted state favors breakdown.
- Thyroid — sets baseline turnover rate; hyperthyroidism wastes muscle, hypothyroidism causes weakness and cramps.
When it goes wrong
Osteoporosis affects roughly 200 million people globally, mostly postmenopausal women, and drives hip fractures that carry a 20-30% one-year mortality after a break. Sarcopenia — age-related muscle loss — starts accelerating in the fifth decade and is a strong independent predictor of mortality; you typically lose 3-8% of muscle mass per decade after 30 if you do not train against it. Osteoarthritis is wear-driven cartilage loss; rheumatoid arthritis is autoimmune and destroys joint tissue.
Low back pain is the world's leading cause of disability. It is usually a multifactorial mix of deconditioning, postural load, and occasional real pathology — imaging findings correlate badly with symptoms, which is why MRI-everyone-early strategies do more harm than good.
Honest take
Resistance training is the highest-leverage health intervention most adults are ignoring. It directly pushes back on sarcopenia, improves bone density, improves insulin sensitivity, improves cardiovascular risk, and the mortality signal in observational data is larger than most pharmacologic interventions. Two to four sessions a week of real lifting, done for decades, changes how you age. The protein intake literature has been crystal clear for years — most people still under-eat protein, especially older adults, and feeble old age is partly a protein-and-training failure, not destiny.
Sources
- Rosen, Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism — reference on bone biology.
- Phillips et al., British Journal of Sports Medicine — protein intake requirements for muscle protein synthesis.
- Cruz-Jentoft et al. (2019), Age and Ageing — EWGSOP2 consensus on sarcopenia.