**2. Weakness and muscles activation abnormalities**

It is well described that age-related muscle strength loss causes a reduction in maximal voluntary joint torque and power production, resulting in clinical implications for older adults, particularly when this strength loss involves weakness in the lower limbs (Bento et al., 2010, LaRoche et al., 2010). It is also clear that this age-related weakness is not fully explained by muscle mass loss (Clark & Fielding, 2012). Recent studies have demonstrated that the decline of muscle mass only explains 6-10% of strength impairments and that muscle mass gains in older adults do not prevent this age-related weakness (Clark et al., 2006a, Clark et al., 2006b, Delmonico et al., 2009). Explanations of these phenomena have proposed that age-related loss of muscle strength is associated with impaired intrinsic force generation capacity and abnor‐ malities in muscle fiber contractile and metabolic properties, excitation-contraction coupling and patterns of muscle activation (Clark & Fielding, 2012, Manini & Clark, 2012).

EMG is widely used to assess muscle activation and is used to highlight the relationship between muscle recruitment and age-related weakness (Clark et al., 2010, Ling et al., 2009, Watanabe et al., 2012, Wheeler et al., 2011). Muscle activation is a result of the excitation of motor neurons leading to force production in muscle fibers (Clark et al., 2010). Additionally, the quantity of motor units and the firing rates of these motor neurons play important roles in determining the intrinsic muscular force (Clark et al., 2010). Along these lines, age-related losses may be related to a suppressed ability of the central nervous system to maximize motor unit recruitment, resulting in a lower activation of agonist muscles (Clark et al., 2010). Other studies have proposed that age-related weakness is also associated with increased antagonist activation (Macaluso et al., 2002).

Recent studies showed that muscle strength is a good predictor of mobility and disability in older adults (Clark & Field, 2012). Clark et al. (2010) assessed the isometric strength of knee extensors (3 maximal trials of 3-5 seconds at 60º of knee flexion), the isokinetic strength of knee extensors (5 consecutive contractions at 60, 90, 180 and 240°.s-1) and the EMG activation of knee extensors (Vastus Medialis, Vastus Lateralis and Rectus Femoris) and knee flexors (Biceps Femoris and Semimembranosus) in older adults with normal and impaired mobility. These authors identified that older adults with impaired mobility had lower activation of knee extensor muscles in all maximal isokinetic voluntary contractions. Additionally, the lower activation of knee extensor muscles was associated with lower torque and power in all isokinetic trials. Thus, the most novel result of this study is the demonstration that agonist muscle activation deficits may contribute to reduced lower limb strength. However, the findings of this study did not support the hypothesis that increases in antagonist coactivation leads in strength deficits during fast contractions (Clark et al., 2010).

contraction of knee extensors was a significantly higher antagonist coactivation found in older women. Thus, antagonist coactivation may contribute to decreased strength in older adults and, in agreement with Clark et al (2010), Macaluso et al. (2002) also proposed that decreased neural activation of the agonist muscles is another potential explanation for age-related

**Figure 1.** The relationship between knee extensor maximum voluntary torque and rectus femoris activation during a

p < 0.05 (Cardozo

Age-Related Neuromuscular Adjustments Assessed by EMG

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

115

knee extensor isokinetic concentric movement in older women with impaired or normal mobility. \*

Ling et al. (2009) compared the surface-represented motor unit size and firing rate of the vastus medialis (VM) during knee extension at 10, 20, 30 and 50% of maximal voluntary contraction in young and old adults. These authors used EMG positioned at the VM motor point and discharged supramaximal stimulation on the femoral nerve. This study demonstrated that aging causes neuromuscular compensations that counteract Henneman's size principle (Henneman & Olson, 1965; Ling et al, 2009). According to this principle, the recruitment of larger motor units and the increase in their firing rates are progressive and consistent with increases in force level (Henneman & Olson, 1965, Ling et al., 2009). However, Ling et al. (2009) demonstrated that in contrast to young adults, old adults recruit larger motor units and

Figure 1 presents the relationship between knee extensor maximum voluntary torque and rectus femoris activation during a knee extensor isokinetic concentric movement in older

weakness.

et al., unpublished data).

have higher firing rates at low loads.

women with impaired or normal mobility.

Higher antagonist coactivation may not limit strength in older adults with different levels of mobility (Clark et al., 2010). However, age-related weakness may be influenced by increased antagonist coactivation (Macaluso et al., 2002). Macaluso et al. (2002) assessed vastus lateralis and biceps femoris activation during isometric contractions of knee extensors and knee flexors in young and older women. This study demonstrated that older women were on average 45% weaker than young women in knee flexor and extensor maximal torque. However, only in the

**2. Weakness and muscles activation abnormalities**

114 Electrodiagnosis in New Frontiers of Clinical Research

activation (Macaluso et al., 2002).

It is well described that age-related muscle strength loss causes a reduction in maximal voluntary joint torque and power production, resulting in clinical implications for older adults, particularly when this strength loss involves weakness in the lower limbs (Bento et al., 2010, LaRoche et al., 2010). It is also clear that this age-related weakness is not fully explained by muscle mass loss (Clark & Fielding, 2012). Recent studies have demonstrated that the decline of muscle mass only explains 6-10% of strength impairments and that muscle mass gains in older adults do not prevent this age-related weakness (Clark et al., 2006a, Clark et al., 2006b, Delmonico et al., 2009). Explanations of these phenomena have proposed that age-related loss of muscle strength is associated with impaired intrinsic force generation capacity and abnor‐ malities in muscle fiber contractile and metabolic properties, excitation-contraction coupling

and patterns of muscle activation (Clark & Fielding, 2012, Manini & Clark, 2012).

EMG is widely used to assess muscle activation and is used to highlight the relationship between muscle recruitment and age-related weakness (Clark et al., 2010, Ling et al., 2009, Watanabe et al., 2012, Wheeler et al., 2011). Muscle activation is a result of the excitation of motor neurons leading to force production in muscle fibers (Clark et al., 2010). Additionally, the quantity of motor units and the firing rates of these motor neurons play important roles in determining the intrinsic muscular force (Clark et al., 2010). Along these lines, age-related losses may be related to a suppressed ability of the central nervous system to maximize motor unit recruitment, resulting in a lower activation of agonist muscles (Clark et al., 2010). Other studies have proposed that age-related weakness is also associated with increased antagonist

Recent studies showed that muscle strength is a good predictor of mobility and disability in older adults (Clark & Field, 2012). Clark et al. (2010) assessed the isometric strength of knee extensors (3 maximal trials of 3-5 seconds at 60º of knee flexion), the isokinetic strength of knee extensors (5 consecutive contractions at 60, 90, 180 and 240°.s-1) and the EMG activation of knee extensors (Vastus Medialis, Vastus Lateralis and Rectus Femoris) and knee flexors (Biceps Femoris and Semimembranosus) in older adults with normal and impaired mobility. These authors identified that older adults with impaired mobility had lower activation of knee extensor muscles in all maximal isokinetic voluntary contractions. Additionally, the lower activation of knee extensor muscles was associated with lower torque and power in all isokinetic trials. Thus, the most novel result of this study is the demonstration that agonist muscle activation deficits may contribute to reduced lower limb strength. However, the findings of this study did not support the hypothesis that increases in antagonist coactivation

Higher antagonist coactivation may not limit strength in older adults with different levels of mobility (Clark et al., 2010). However, age-related weakness may be influenced by increased antagonist coactivation (Macaluso et al., 2002). Macaluso et al. (2002) assessed vastus lateralis and biceps femoris activation during isometric contractions of knee extensors and knee flexors in young and older women. This study demonstrated that older women were on average 45% weaker than young women in knee flexor and extensor maximal torque. However, only in the

leads in strength deficits during fast contractions (Clark et al., 2010).

**Figure 1.** The relationship between knee extensor maximum voluntary torque and rectus femoris activation during a knee extensor isokinetic concentric movement in older women with impaired or normal mobility. \* p < 0.05 (Cardozo et al., unpublished data).

contraction of knee extensors was a significantly higher antagonist coactivation found in older women. Thus, antagonist coactivation may contribute to decreased strength in older adults and, in agreement with Clark et al (2010), Macaluso et al. (2002) also proposed that decreased neural activation of the agonist muscles is another potential explanation for age-related weakness.

Ling et al. (2009) compared the surface-represented motor unit size and firing rate of the vastus medialis (VM) during knee extension at 10, 20, 30 and 50% of maximal voluntary contraction in young and old adults. These authors used EMG positioned at the VM motor point and discharged supramaximal stimulation on the femoral nerve. This study demonstrated that aging causes neuromuscular compensations that counteract Henneman's size principle (Henneman & Olson, 1965; Ling et al, 2009). According to this principle, the recruitment of larger motor units and the increase in their firing rates are progressive and consistent with increases in force level (Henneman & Olson, 1965, Ling et al., 2009). However, Ling et al. (2009) demonstrated that in contrast to young adults, old adults recruit larger motor units and have higher firing rates at low loads.

Figure 1 presents the relationship between knee extensor maximum voluntary torque and rectus femoris activation during a knee extensor isokinetic concentric movement in older women with impaired or normal mobility.

Thus, we can see that age-related muscle strength loss decreases maximal joint torque and power production, yet the muscle activation mechanisms that promote this behavior are still not well described.

heart rate and rating of perceived exertion) and in EMG burst relative to younger adults. The authors speculated that changes in the EMG pattern were related to torque fluctuations. The authors concluded that motor unit activity increased most slowly during fatiguing submaxi‐ mal efforts in older adults, possibly leading to increases in the time of task failure (Hunter et

Age-Related Neuromuscular Adjustments Assessed by EMG

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

117

Lindstrom et al. (2006) assessed the EMG activation of the vastus lateralis and rectus femoris during 100 repeated maximum knee extension contractions at 90º.s-1 in young and old men and women. The authors found that older male adults were most fatigable according to the peak torque and EMG parameters (with a higher area based fatigue index and lower root mean square for the vastus lateralis in older men), but this group did not see the greatest fatigue according to the Borg scale. The authors suggested that the EMG amplitude revealed that fatigue is a combination of age-related changes in muscle and central activation failure

Aging leads to selective atrophy of type II fibers and increases the contribution of type I fibers to the generation of torque (Avin et al., 2011). However, even in low intensity activities (e.g., rising from a sitting position and walking) when torque is generated by the recruitment of type I fibers, older adults have a higher metabolic cost and higher fatigability than young subjects (Hortobágyi et al., 2011, Wert et al., 2010). This phenomenon is related to a declining VO2max (which occurs at a rate of approximately 8% per decade) and leads to older adults performing their daily activities at higher relative intensities (as measured by percentage of VO2max) than young people (Wilson and Tanaka, 2000). Additionally, recent studies have shown that the rate of consumption of VO2 during walking is also related to the EMG activation pattern

Peterson and Martin (2010) and Hotobágyi et al. (2011) found a moderate association between higher Cw and increased antagonist coactivation of the thigh and calf muscles in older adults (Peter & Martin, 2010, Hortobágyi et al., 2011). According to Hortobágyi et al. (2011), older adults had an 18.4% higher Cw than young adults and this higher Cw was associated with increased antagonist coactivation (Vastus Lateralis x Biceps Femoris and Tibialis Anterior x Gastrocnemius Lateralis). Peterson and Martin (2010) determined that antagonist coactivation of the thigh (vastus medialis, biceps femoris and semitendinosus) had a higher contribution to the increase in Cw than the contribution from the shank (tibialis anterior, lateral soleus and medial gastrocnemius). Both studies suggested that age-related neuromuscular adaptations in the lower limbs decrease the joint instability and that a higher antagonist coactivation is

required to maintain dynamic stability during a normal gait, which increases the Cw.

Everyday tasks are motor acts performed during a day that contribute to physical independ‐ ence, such as rising from a seated position, ascending or descending stairs, walking and taking a shower. Challenges encountered during daily activities, which are easily overcome by young adults, may represent a potential risk for falls among the elderly. Functional motor activities

al., 2004).

(Lindstrom et al., 2006).

(Peterson & Martin, 2010, Hortobágyi et al., 2011).

**4. Performance in daily activities**
