**1. Introduction**

From the many joints exposed to muscle-skeletal injuries, the knee joint is the one that more suffers consuming in the daily life, for both athletes and non-athletes [1], once for the maintenance of the corporal stability, it is necessary for the muscles of this joint to be the strongest as possible [2]. Such strengthening may be obtained through an isometric force training [3], which range from numbers of repetitions up to weekly frequencies [4,5].

The hamstrings muscle group is objeto de estudo devido seu papel como músculo biarticular, bem como a sua função na insuficiência mecânica [6]. This group is composed of the *semitendinosus* (ST), *semimembranosus* (SM) and *biceps femoris caput longum* (BFCL) all of which are active during knee flexion. The activity of these muscles is often examined using surface electromyography (sEMG) [7,8,9].

The efficiency of muscle contraction depends on factors such as the fiber cross-section, the number of muscle fibers, the degree of fiber stretching, the traction angle and the type of contraction required [5,10].

Isometric exercise is one of several forms of exercise used to develop muscle force in humans. Isometric contraction occurs without any appreciable change in muscle length, such that although there is tension in the muscle there is little muscle movement for most of the time, hence the term static contraction [6,11].

A important fact that be associated with a force output is the neuromuscular fatigue. This can under certain conditions be reflected in a decreased performance and/or the failure point at wich the muscle is no longer able to sustain the requeried force or work output level [12,13,14].

© 2012 Marson, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Marson, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Research by [15] Dimitrova & Dimitrov (2002) related that Muscle fatigue is recognized as a decline in force, or failure to maintain the required or expected force. It may occur at any point from the nervous centers and conducting pathways to the contractile mechanism of muscle fibers.

Relationships Between Surface Electromyography and Strength During Isometric Ramp Contractions 55

Twenty female healthy adults (age 19.5 ± 0.8 yrs, body mass 63.4 ± 1.5 kg, height: 1.65 ± 0.05 m), without muscle skeletal disorders and similar anthropometric measurement, subject in this study. Subjects were all right leg dominant. All subjects signed a written informed

Surface electromyography activity was collected by an eight-channel unit (EMG System do Brazil Ltda®) consisting of a band pass filter of 20–500Hz, an amplifier gain of 1000, and a common rejection mode ratio >100dB. All data were acquired and processed using a 16-bit analog to digital converter (EMG System do Brazil Ltda®), with a sampling frequency 1024 Hz. A channel of the acquisition system was enabled for the utilization of the load cell (SF01 - EMG System do Brazil Ltda®) having an output between 0 and 20mV and a range up to 5kN. The biosignals from the *semitendinosus* (ST), *semimembranosus* (SM) and *biceps femoris caput longum* (BFCL) muscles were recorded with pairs of bipolar silver–silver chloride surface electrodes (10 mm electrode diameter, fixed inter-electrode distance of 20 mm). Following skin abrasion with an alcohol soaked cotton pad, electrodes were placed with the

This protocol was had increase of maximal voluntary contraction (MVC) (10, 20 30 e 40%). The knee was flexion to 90º and isometric contractions were done by pulling on a cable fixed to the ankle which was kept at 90º relative to the longitudinal axis of the leg. The cable

The load cell traction was performed initial with 10% MVC during 20s, immediately

Initially the participant with the knee flexed 90°, is a traction against the cell load corresponding to 10% maintaining that drift for 20 seconds. Immediately the participant was

Continuous samples were collected these traction. These collections take place without the participants to rest between them. The RMS-sEMG values there is a change in load has been

The sEMG signals were amplified with gain 1000. The analog channel band pass was set to

20-500 Hz and the sampling rate for analog-to-digital conversion was 1024 Hz.

increased to 20% MVC during 20s. This characteristic was used until 40% of MVC.

consent. The study was approved by the AESA Ethics Committee protocol 344/10.

**2. Material and methods** 

recommendation of Marson [6].

length was adjusted to the size of the subject's leg.

asked to traction 20% and so on for 30 and 40% MVC (Figure 1).

**2.3. Ramp contractions** 

discarded (Figure 2).

**2.4. Signal processing** 

**2.2. Equipment and electrode placement** 

**2.1. Subject** 

Study by [16] Moritani & Yoshitake (1998)Such changes have been shown to be related to hydrogen ion and metabolite accumulation and to sodium and potassium ion concentration shifts. These changes would in turn affect the muscle excitation traction coupling including the muscle membrane properties and muscle action potential propagation, leading to sEMG manifestations of muscle fatigue distinct from mechanical manifestations.

The increase in amplitude of the sEMG signal as an empirical measure of localized muscle fatigue or as an indicator of muscle fatigue [9,17]. The RMS-sEMG values tended increase with decreasing force as a function of the number of repetitions [6] phenomenon that determines the neuromuscular fatigue process.

The active motor units also discharge with increasing speed to compensate for the fall in the force of contraction of the fatigued fibers [13,18].

The surface electromyography behavior at different force levels is of particular importance. This can be either achieved by performing multiple isometric contractions at various force levels or using ramp contractions.

A ramp contraction is defined as a progressive linear increase in force over time and relationships sEMG-force is linear or quadratic [19], and then, sEMG parameters and phsyiologicasl events used ramp contraction in investigation, as well, motor unit recruitments, force produce and gender influence [20].

Study by [19] Bilodeau et al (2003), [20] Pérot et al (1996) and [21] Stulen and DeLuca (1981) related the relationship of curve in ramp contraction were this behavior can be confirmed between of recruitment of large, type II muscles fibers, with a higher muscle action potential conduction velocity, is associated with an increase in the median frequency or mean power frequency values of the power spectrum of sEMG [22].

In the decade 80 researches showed that the ramp contraction procedure might be replaced by procedure comprising a number of distinct and constant force contraction. Ramp contractions involve the registration of sEMG while the strength performance gradually increases his or her level of effort up to maximal or submaximal levels. Although the latter procedure seems to be easier to use in some investigation, the ramp procedure has gained a wider acceptance since then. They have been extensively applied to examine muscle activation strategies as well as in new protocols of electromyography analysis. [19,23]

The purpose of the present study was to investigate the relationships of sEMG and time during isometric strength ramp contraction in the hamstrings muscle group. Hamstrings muscle group is composed of the *semitendinosus* (ST), *semimembranosus* (SM) and *biceps femoris caput longum* (BFCL) all of which are active during knee flexion.
