A comparison of cardiac strain among drillers of two different age groups in underground manual coal mines in India.

Cardiac strain was evaluated in terms of working heart rate (WHR), relative cardiac cost (RCC), net cardiac cost (NCC) and other recovery indices among six younger (mean age 34.2 +/- 2.7 yr) and sixteen older (mean age 48.9 +/- 5.4 yr) drillers working in a manual underground coal mine over two spells of work. The mean WHR was within the range of 117-132 beats / min with corresponding mean relative cardiac cost between 44-48% of heart rate reserve for the younger group and 53-55% for their older counterparts. The mean NCC was above 50 beats/min for both age groups. It was seen that the workers surpassed the recommended limits of cardiac strain indices. The intensity of workload indicates the job to be "heavy" to "extremely heavy" in accordance with the heaviness scales based on WHR, NCC and recovery heart rates. Heat stress prevailing in the workplace in terms of effective temperature (ET) and wet bulb globe temperature (WBGT), was above the recommended limits as per the guidelines proposed by WHO and ACGIH. High physiological demands of the job which requires predominate static muscular exertions coupled with high heat stress were found to hinder the recovery process and may prove deleterious particularly for the older workers. Therefore, in the present context, the need of ergonomic interventions for job organization and quick reparation of environmental condition are strongly indicted.