Effectiveness of Iso-inertial Resistance Training on the Muscle Power of the Lower Limbs, Physical Aptitude, and Risk of Falls in Physically Active Older Adults: a Randomised Controlled Trial

With the progressive increase in life expectancy of the population, attention to ageing has gained special interest in recent years. Actions aimed at promoting healthy ageing in the older adults are key to slow down the physiological progressive loss in skeletal muscle mass, quality and function as the person ages. These losses affect the individual's ability to carry out activities of daily living. More concretely, the muscle strength and power are reduced during aging, which has a negative impact on the functional capacity and quality of life of older adults. In fact, recent research has identified a significant decrease in muscle strength of 1-1.5% each year from the age of 50. While the muscle strength is defined as the ability to generate intramuscular tension when facing a resistance, regardless of whether it generates movement, the muscle power is defined as the maximum amount of force one can generate during a specific movement at a specified velocity. Previous studies report that muscle power is diminished to a greater extent than strength over time. Muscle power is considered a predictor of functional capacity, as it is associated with activities of daily living such as climbing stairs, standing up from a chair or walking. This fact has justified the use of strength and power training in different studies that aim to prevent the risk of falls, improve the balance or the walking capacity in older adults.

Resistance training (RT) is one of the main strategies to prevent the decrease in functional capacities and has demonstrated its effectiveness in combating age-induced muscle atrophy (sarcopenia), risk of falls, and fragility. RT has also been shown to improve cardiovascular health. Regarding the types of RT, previous studies have noted statistically significant differences in favor of eccentric training, when comparing it with concentric RT. Specifically, it has been observed that eccentric training leads to higher peak strength with lower muscle activation and lower metabolic cost, increased muscle mass and higher jumping performance. Thus, the values of muscle strength and power resulting from an eccentric RT are physiologically superior to those obtained when performing concentric RT.

The most traditionally used RT method in the field of community health is the one known as gravitational, in which a resistance is opposed through free weights or by blocks or disks in cable machines. One of the main restrictions of this method is that the workload applied during the shortening-longing cycle of a repetition is limited to concentric muscle capacity and does not allow a progression of the eccentric workload. This fact limits the potential of this method for generating the improvements associated with eccentric training mentioned above. In contrast, the iso-inertial (ISI) training method is based on the application of resistance generated by an iso-inertial device, where the workload is provided by the inertia of a rotating mass. Unlike the gravitational system, the ISI method can provide a resistance workload in the eccentric phase that is proportional to the concentric phase. Thanks to that, high workloads can be applied for both phases. Another benefit of ISI method is that, when the load increases or the fatigue appears, only the execution speed is reduced but the execution is not interrupted .

Different studies show that eccentric overload training protocols (i.e., with a higher force in the eccentric phase of a repetition) improve muscle hypertrophy and power, as well as and neuromuscular functions. Because of its force-generating system, the ISI method is ideal for eccentric overload compared to gravitational systems, where eccentric overload can only be achieved with external assistance. Also, the ISI method allows a fluid movement compared to the interrupted movements of gravitational systems.

Recent studies have reported improvements in postural control or maximal isometric strength with ISI training in older adults. Also for metabolic variables such as lipid profile or maximum oxygen volume consumption . However, no study has been found that compares the effects of ISI and gravitational training on muscle power and other relevant functional variables in older adults. This is of critical importance because of the direct implications that the improvements of muscle power has on activities of the daily live.