**5. Changes in postural control**

are especially difficult for older adults due to sensorimotor deficits related to age, exposing these older adults to fatal accidents and serious injuries (Korteling, 1994, Roeneker et al., 2003).

An age-related decline in the ability to perform physical tasks associated with daily living as well as in strength and muscle size may occur regardless of physical fitness or amount of training (Klitgaard et al., 1990, Schulz & Curnow, 1998). The decline in force and task per‐ formance may be related to alterations in the activation of motor units, decreases in muscle

Performance in daily tasks may be investigated by EMG in young and old persons. A study performed by Landers et al. (2001) analyzed integrated electromyography (IEMG) in two tasks of daily living: while the subjects sat down on a chair and while they carried a small load. The muscles analyzed were rectus femoris and biceps brachii. The raw EMG signal was recorded over six seconds for each collection point at a sample rate of 100 Hz. Subjects were given three practice trials, followed by three maximum isometric contractions at each test angle. The results showed that higher normalized integrated EMG values indicate greater muscular effort and, when combined with other tests, that biomechanical measures can provide information about

The ability to walk efficiently and safely is important to maintain independence (Callisaya et al., 2010). However, the energy cost of gait in the elderly is higher than in young people, which can cause early fatigue (Hortobagyi et al., 2009). However, little is known about what makes the elderly more prone to fatigue during the gait, but existing hypotheses are that this fatigue is related to neuromuscular mechanisms, such as increased muscle coactivation (Burnett et. al., 2000, Hortobagyi et al., 2009, Macaluso et al., 2002). Increased coactivation might be used to optimize power generation and compensate for aging-related decline of neuromotor functioning, as manifested by reduced strength and power of muscles, reduced proportions

Older adults also require greater effort relative to their available maximal capacity to execute daily motor tasks when compared with younger adults (Hortobagyi et al., 2003). This is due to a change in muscle fiber type with aging and a higher percentage of peak oxygen uptake required to perform daily tasks (Astrand et al., 1973, Waters et al., 1983). Higher physiological relative effort in elderly people may be the cause of premature fatigue associated with decline of motor function and, consequently, falls. Hortobagyi et al. (2003) tested the hypothesis that the relative effort to execute daily activities is higher in old adults compared with young adults. They assessed the vastus lateralis and biceps femoris muscles by EMG during the ascent and descent of stairs, the rise from a chair and the performance of maximal-effort isometric supine leg presses. The EMG signals were sampled at 1000 Hz, and the dependent variables included the average root mean square (RMS) EMG and EMG coactivity, expressed as a ratio of biceps femoris root mean square EMG with vastus lateralis RMS EMG activity. The results show that the relative vastus lateralis EMG activity is higher in old adults than young adults during some daily activities, and an association exists between the increased relative effort at the knee joint

of fast twitch muscle fibers and increased response times (Ishida et al., 2008).

mass and increases in fat mass (Lexell, 1995).

118 Electrodiagnosis in New Frontiers of Clinical Research

muscle function in older adults.

and increased muscle activation.

The capacity to maintain the body in an upright position in a stable state is critical to prevent falls in old people. This capacity requires the integration of visual feedback, the vestibular system, proprioception, reaction times and muscular responses. However, these mechanisms are negatively affected by aging, and therefore, the adaptive reflexes that respond to distur‐ bances of balance are damaged (Abreu & Caldas, 2008). As a result of these changes, the elderly become more prone to falls (Tinetti, 2003).

EMG can evaluate the response of muscles during postural control in different situations requiring the integrity of the neuromuscular system. Figure 3 presents the time delay between a perturbation (accelerometer signal) and the muscle activation (EMG onset) response obtained by EMG analyses of the tibialis anterior muscle. This time delay is negatively affected by the aging process, promoting slower responses in old adults. This behavior may increase the risk of falls in this population when the muscle activation may not be fast enough to maintain stability after a perturbation.

**6. Effects of physical activity on the neuromuscular system**

population.

life expectancy.

rate of falls among this population.

strength and endurance training.

Decreases in maximal isometric, concentric, and eccentric forces, force development rate and muscle power are all age-related effects (Granacher et al., 2010, Petrella et al., 2005, Skelton et al., 1994). Regular physical exercise for the elderly population has been identified as an important intervention in the treatment and recovery of some diseases (Bassey, 1997). As the functional benefit of exercise may be greatest in older adults, in recent years, there have been several studies about the effects of physical activity on the neuromuscular system of this

Age-Related Neuromuscular Adjustments Assessed by EMG

http://dx.doi.org/10.5772/55053

121

Traditional strength training protocols can still be recommended to improve muscle strength and voluntary neural activity in older adults (Fung & Hughey, 2005, Runge et. al., 1998). However, other types of training have been shown to develop strength, power and balance in this population. Resistance training with power training and ballistic strength training may be effective for improving explosive force production and functional performance in old age (Granacher et al., 2011). Orr et al. (2006) show that power training at low intensities can improve balance, power, strength and endurance in the lower limb muscles of older adults. Recent studies have also shown that whole body vibration and resistance exercises combined with vascular occlusion may improve muscle strength (Granacher et al., 2012, Rabert et al., 2011, Takarada et al., 2000). Figure 4 shows the influence of an active lifestyle on increasing healthy

The assessment of lower limb muscle activity provides important information about neuro‐ muscular behavior before and after physical activities (Schmitz et al., 2009). EMG can identify changes in the motor skills of older adults and help create prevention strategies for ageassociated changes in neuromuscular factors that can impair daily activities and increase the

A recent study investigated the effects of strength and endurance exercises over the course of 12 weeks in older adults. The maximal neuromuscular activity of agonist muscles was evaluated using EMG (RMS) in the vastus lateralis and rectus femoris and antagonist coactivation in the biceps femoris long head. The sampling frequency was 2000 Hz, and the data were filtered using a Butterworth band-pass filter of the fourth order with a cutoff frequency between 20 and 500 Hz. The RMS values of the antagonist biceps femoris muscle were normalized by the maximum RMS values of this muscle. After determination of the maximal neuromuscular activity, the submaximal neuromuscular activity was evaluated to determine the isometric neuromuscular economy. The results show that training in older adults resulted in greater changes in neuromuscular economy as assessed by EMG (Cardore et al., 2012). Similarly, Cardore et al. (2011) investigated the effects of concurrent training on endurance capacity and dynamic neuromuscular economy in elderly men. During the maximal test, muscle activation was measured at each intensity by means of electromyographic signals from the vastus lateralis, rectus femoris, biceps femoris long head, and gastrocnemius lateralis to determine the dynamic neuromuscular economy. Changes in the myoelectric activity of the Rectus Femoris and Vastus Lateralis muscles were observed as an adaptative response after

**Figure 3.** EMG response due to perturbation (Cardozo et al., unpublished data).

Older people have different strategies to maintain posture in balance situations: the ankle strategy responds to slow disturbances; the hip strategy is used on larger and faster displace‐ ments of the center of pressure (COP); and the step strategy is used when the others are not able to return the COP to the support base, using quick jumps or steps (Vanicek et al., 2009).

Another strategy used by older adults is an increase in antagonistic muscle activation during balance recovery (Mixco et al., 2012). This coactivation can be a necessary change to compensate for the decline in postural control associated with aging (Nagai et al., 2011). Additionally, Freitas et al. (2009) have shown that older adults activated their muscles and were able to reach the peak of activation. However, they retained a higher level of activation longer than younger adults.

As a result of the aging process, reaction time tends to increase due to the atrophy of fast twitch fibers with aging. This atrophy contributes to a lower power output, slower sensory feedback and slower muscle onset, resulting in ineffectiveness of equilibrium recovery after disturban‐ ces (Pijnappels et al., 2008).

Due to physiological changes resulting from the aging process, recovery strategies are slower and therefore less effective in old adults (Mian et al., 2007). Thus, to minimize these changes, physical activity is highly recommended and widely used as an intervention to prevent falls.
