The Cumulative Trauma Epidemic
Repetitive strain injuries cost the global economy over $80 billion annually in medical treatment, lost productivity, and disability compensation. Unlike acute injuries with clear causes, RSI develops gradually over weeks to months, making it difficult to recognize early. By the time symptoms appear — pain, tingling, weakness, or reduced range of motion — significant tissue damage has often accumulated. Prevention through ergonomic job design is far more effective than treatment.
The Three Risk Factors
RSI risk is driven by the interaction of three primary factors: force (how hard), repetition (how often), and posture (in what position). Each factor alone at moderate levels is tolerable, but their combination creates multiplicative risk. A task requiring 20% MVC force at 15 repetitions per minute with wrist deviation is far more dangerous than any single factor would suggest. The Strain Index captures this interaction quantitatively.
Muscle Fatigue and Recovery
Muscles generate force by consuming ATP and producing metabolic waste (lactate, hydrogen ions). At low force levels, blood flow clears waste faster than it accumulates. Above about 15% of maximum voluntary contraction, intramuscular pressure begins restricting blood flow, and fatigue accumulates. The Rohmert endurance curve shows this relationship: at 50% MVC, a muscle can sustain contraction for only about 1 minute before failure. Recovery requires both time and reduced loading.
Job Redesign Strategies
When the Strain Index identifies hazardous tasks, systematic redesign follows a hierarchy: eliminate the hazardous motion entirely (automation), reduce force requirements (better tools, mechanical advantage), reduce repetition (job rotation, pace variation), improve posture (workstation redesign), and increase recovery time (micro-breaks, task rotation). This simulation helps identify which factor contributes most to the overall risk and where intervention will be most effective.