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.
The main objective of this study is to evaluate the effectiveness of an iso-inertial
resistance training program on the eccentric muscle power of the lower limbs compared to the
same program executed with gravitational resistance in physically active older adults. The
secondary objectives are 1) to evaluate the effectiveness of an iso-inertial resistance
training program on the concentric muscle power, physical fitness (balance, walking speed and
functionality of the lower limbs), and risk of falls compared to the same program executed
with gravitational resistance in physically active older adults, and 2) to assess the
differences between men and women for the previous objectives.