Descripción del instrumento # 2 taller: Caballo de Troya

C h a p te r 6: S tr e tp ^ h tr a in in g stu d y

6.1. INTRODUCTION

We have based our training regime on previously published studies on strengthening elderly muscle and on training the adductor pollicis (see Chapter 2, table 7). Our decision on what the training regime ought to be was based on the following:

a) Lyle & Rutherford (1998) showed that voluntary and stimulated training regimes are equally effective on the AP muscle, for this reason and for simplicity, we chose to use voluntary contractions for training.

b) We opted for a 12 weeks training period because numerous studies have been able to show significant training effects after three months o f exercise (as illustrated in table 3 of the literature review)

c) We chose to ask subjects to carry out five seconds maximal contractions of the

adductor pollicis of the non-dominant hand based on the study by Duchateau & Hainaut (1984), where a significant training effect was reported using this method (though by electrical stimulation).

d) In a review paper. Porter & Vandervoort, (1995), showed that training studies on elderly subjects using thrice weekly sessions, show good subject compliance and optimal gains in strength, even in nonagenarians. Also, Mansell et al,

(1997) and Cannon & Cafarelli (1987) are two examples showing that training three times per week is effective in the AP, with strength increment raging from 15-70%. Duchateau & Hainaut (1984) on the other hand showed that a daily training regime was effective for training the AP muscle.

Therefore to optimize likelihood o f compliance in our training study, a voluntary training regime was used, and the choice was left to each subject to either train daily (for 10 minutes), or three times per week (for approximately 24 minutes).

6.2. METHODS

Measurements reported are muscle isometric, and stretch forces, muscle size, handgrip strength, functional torque and ACE genotype. In addition to this, we applied magnetic stimulation to the ulnar nerve in order to carry out the single twitch superimposition experiment to determine whether subjects were able to fully activate the muscle during

C h a p te r 6: S tr e n g th (rainin f; stu d y

maximal isometric voluntary contractions. Many of these techniques have been described in the previous chapters. Therefore in outlining the methods for this section, only the techniques not yet described will be given in detail.

6.2.1. Adductor pollicis strength training regime

The training apparatus consisted of a spring mounted on a metal bar (figure 63). The subjects were required to push hard against the spring, and to hold the pushed position for 5 seconds. As in Duchateau & Hainaut (1984), each of the training sessions consisted of one maximal voluntary contraction of the AP o ï 5 seconds duration at the beginning of each minute, followed by 55 seconds rest. Hence, the subject would contract 10 times every day in the daily regime, or 24 times in the thrice-weekly regime (the latter case to get a similar training stimulus to the daily regime). The strength of each subject was assessed during the preliminary testing session and the level of spring stiffness was set accordingly for the subject (figure 63).

Angle a

Figure 63. Training apparatus. Because torque Q = Ka, thus Q=DiFi=D2p2, and If

Di> D2. therefore p2> Fi. In other words, when we have large forces from a strong

person, we need to have a small distance D (from the centre of rotation (o) of the

force application point). The torque around o depends on angle a change. Here,

position 1 requires less force than position 3, with position 2 intermediate. 15cm separated the insertion points between spring positions.

C hapU ’r 6: S tr e n g th fruin ’m ^ stu d y

In order to decide what level of spring stiffness to be on, the subject was asked during the first session to apply a maximum push at level 1 (see figure). If they managed to collapse the spring completely (figure 64), spring position was moved to level 2 and the subject was asked to repeat the process. In fact, all subjects started at level 2. They were then asked to monitor themselves during training sessions at home and to check that as soon as they were able to collapse the spring at level 2, to move up to level 3. About 70% of the training population did this.

Figure 64. Change In spring stiffness for one subject. The first two pictures show a subject exerting an MVC on spring level 3 (top and side views), she is clearly able to collapse the spring The third picture shows the same subject pushing on level 2; she can no longer collapse the spring.

Calibration o f the training spring

Weights were added so that they would apply forces in the same direction as the thumb would during training. A calibration curve was subsequently drawn (figure 65 below).

3.5 3 2.5 2 O) 1.5 1 y = O.OSx + 0.03 = 0.99 0.5 0 30 0 10 20 40 Weight added (N)

Figure 65. Calibration of the training spring. Weight added in the direction of force application of the thumb, is plotted against displacement of spring from relaxed position.

C h a p te r 6: Streiv^lh tr a in in g stu d y

Since the distance between the uncompressed spring (spring with no force being applied to it) and the top o f the metal coat hook was -3.4cm at position 1, therefore 42N were required to maximally compress it. Note here however, that the effect o f moving the spring is mainly psychological. The subject can see the spring move and therefore can quantify that she is having an effect and is encouraged to push as hard as possible, thinking that the object is to end up being able to collapse the spring. As far as we are aware, isometric contractions and contractions against a spring are equally effective as training stimuli.

6.2.2. Assessment o f isometric strength, E/I, size and ACE

genotype

Maximum voluntary force (MVF) and cross-sectional area (CSA) of the AP were measured as described in Chapter 3, and ACE genotype was assessed in the manner described in Chapter 4 p i 38.

The stretch/isometric force ratio (E/I) measurement was carried out as described in Chapter 5, with the exception that the data acquisition set up was changed. Although perfectly adequate, using an oscilloscope to capture the data involved far too lengthy processing (three stages from acquisition to analysis). Thus, it was superseded by

TESTPOINT, an object-based programming package using A/D boards to process the

electrical signals. A TESTPOINT acquisition-analysis program which we wrote especially to deal with our data. Brief description of the key subroutines is shown in appendix V.

6.2.3. Magnetic stimulation o f the ulnar nerve

An accepted method for assessing maximal activation in a voluntary muscle contraction is the twitch interpolation technique [Merton, 1954]. It involves the application of stimuli (electrical/magnetic) to an active muscle during voluntary isometric contraction. The expectation is that in a muscle that is not fully activated, inactive motor units will respond with a twitch, so that the force trace shows an “interpolated twitch (IT)” where stimulus was applied. The IT height becomes small, and eventually becomes undetectable (ie. the number o f inactive motor units decreases) as activation increases to

C h a p te r 6 ; S t reniât h tr a iiiiiip stu d y maximum [Belanger & McComas, 1981]. Working on the assumption that a negative linear relationship exists between superimposed twitch size and muscle activation [Merton, 1954; Chapman et al, 1984], voluntary activation is frequently quantified in two different ways:

1- Voluntary activation (%) = 1- (IT/RT) x 100

Where IT is the force of the interpolated twitch and RT is the

twitch evoked in the relaxed muscle [Allen et al,1995].

2- Voluntary activation {%) = (OF/RF) x 100

Where OF is the observed maximal force and RF is the real maximal force. RF is obtained by extrapolating the relationship between evoked twitch and voluntary torque to the x-axis to provide an estimate of the force that would be generated if the

muscle was fully activated [Merton, 1954; Bulow et al, 1993;

Rutherford et ai, 1986].

The two methods do not necessarily provide the same results, which brings controversy as to which of these is the best [Dowling et al, 1994; Behm et al, 1996; Allen et al,

1998; Herbert & Gandevia, 1999]. In this thesis, voluntary activation is calculated as in both 1- and 2- and a discussion is made on the implications for interpreting interpolated twitch data.