The majority o f studies tend to use a minimum of 8-12 weeks o f training [Fiatarone et al, 1990; Jones & Rutherford, 1987]. In terms of completing such a relatively long-term exercise training program, subject adherence has to be taken into account. In the age group o f interest (postmenopausal), lack o f adherence is often based on social reasons. Therefore it is necessary, to simplify training programs as much as possible, and design them so as not to interfere with daily routine [Dishman, 1994; Mazzeo & Tanaka,
2001
].
Training the AP offers these advantages in that a training tool, which could be very cheap and easy to make, could be used whilst for instance, in conversation with friends or watching TV. As for the safety issue, because elderly populations respond to relatively low intensity training [Jonsson et al, 1992; Skelton et al, 1993; Skelton & McLaughlin, 1996; Bemben et al, 2000], the AP will not need to be overloaded, i.e. a high intensity regime will not be necessary to obtain a training effect. With this, is coupled the fact that as the training may need to be home-based, the risk o f training related injuries as might occur with a “whole body program” are less when using a smaller muscle group.
In short, we propose that because AP is small, it is not difficult or dangerous to train it. Simple portable apparatus can be used and training can be home based. This makes adherence more likely.
Chapter 2: Thesis aims
Studies on training the AP have previously been carried out successfully (table 7). AP
shows increased (17-30%) resistance to fatigue after 6-12 weeks of training [Duchateau & Hainaut, 1984a; Rutherford & Jones, 1988]. Various training modes have been found to increase AP strength (see table), and whether the mode o f stimulus is electrical or voluntary, does not have any bearing on the final strengthening outcome to any significant level (provided that muscle load is equivalent in the two regimes). For instance Lyle & Rutherford (1998) found that voluntary training at 50% MVF, and electrical stimulus at a rate which produces 30-50% MVF cause strength increases o f 79 and 74%, respectively, which are not significantly different statistically.
Table 7. AP response to various types of training stimuli
Isometric 12 50% (3x pw) 79 Lyle & Rutherford, 1998
isotonic 6 60-65% (7x pw) 20.6 Duchateau & Hainaut, 1988
Voluntary Isometric 5 80% (3x pw) 15 Cannon & Cafarelli, 1987
Isometric 12 30-40% (7x pw) 13 Duchateau & Hainaut,1984a
Isometric 11 70% (3x pw) 49.5 Mansell etal, 1997
Isometric (30Hz) 12 30-50% (3x pw) 74 Lyle & Rutherford, 1998
Isotonic (100Hz) 6 60-65% (7x pw) 17 Duchateau & Hainaut, 1988
Electrical Isometric (50Hz)Dynamic 5 80% (3x pw) 15 Cannon & Cafarelli, 1987
(100Hz) 12 30-40% (7x pw) 11 Duchateau & Hainaut, 1984 Isometric
(100Hz) 12 100% (7x pw) 20 Duchateau & Hainaut, 1984
Note. Intensity refers to the level of force at which the training was carried
2.2. DETERMINATION OF AREAS TO INVESTIGATE
2.2.1. Measurement o f skeletal muscle force
2.2.1.1. Stimulated versus voluntary tests
Skeletal muscle can be stimulated artificially via surface or needle electrodes. Whilst, the surface electrodes are usually placed on the skin area above the nerve supplying the muscle of interest or on the belly o f the muscle of interest, the needle electrodes are usually placed on the belly of the muscle of interest. The stimulus, usually 50-200ps and with a depth of penetration proportional to the current intensity, in all cases, activates the motoneurone. It should also be noted here that the threshold of the
Chapter 2: Thesis aims
motoneurone declines as the duration o f each stimulus increases, and motorunit recruitment is influenced by the distance between electrodes and the motor axon [Hultman et al, 1983].
Stimulus rate required for maximal force development is usually higher when using artificial means (50-125Hz) [Davies et al, 1985; Kramer, 1987], than when voluntary activation is employed (20-50Hz) [Bigland-Ritchie et al, 1992; Connelly et al, 1999]. The reason for this discrepancy has been suggested to raise the possibility that high threshold motor units cannot be fully activated voluntarily [Erim et al, 1996; Enoka & Fuglevand, 2001]. Alas we are not nearer to determining whether in fact the nervous system can/does/needs to, fully activate motor units (see section 1.4.3.a).
Moreover, the main difficulty to overcome with artificial stimulation experiments, is that, although type IV afferent neurones that transmit pain signals to the central nervous system have a higher recruitment threshold than motoneurones, electrical stimuli needed to elicit maximal contractions are high enough so that a considerable proportion of type IV afferent are activated, hence lending electrical stimulation its uncomfortable, even painful quality. Hence not all experimental subjects are able/willing to tolerate the intensity o f electrical stimuli necessary to fully activate their muscles, particularly big ones [Hortobayi et al, 1992]. Still, many investigators have been able to successfully carry out studies using maximal electrical stimuli on big and on small muscles [Merton (1954)].
The evidence that the maximum force generating capacity o f a muscle can be determined by supramaximal tetanic (be it electrical or magnetic) stimulation is not straightforward. Electrical stimulation o f the tibial nerve, for instance, has been reported to induce triceps surae contractions 20-30% stronger than maximal voluntary efforts, and this has often been taken to be an indication o f incomplete activation o f the voluntarily contracting muscle [Koryak, 1998 & 1999]. There is, however, also evidence for both electrically and voluntarily evoked contractions to yield the same amount o f force. Examples include the quadriceps [Bigland-Ritchie et al, 1978; James
et al, 1994], and adductor pollicis muscles [Merton, 1954]. This is therefore evidence that different muscles are controlled in different ways. Some like the triceps surae are difficult to activate maximally [Koryak, 1998 & 1999], whereas others like the first
Chapter 2: Thesis aims
dorsal interosseous [Rutherford & Jones, 1988] are easy to maximally activate (lOOHz tetanic stimulation cause 95-98% of voluntarily evoked force).
A muscle contracting in vivo is not active alone, and in fact, there is evidence that agonist muscles also come into play. Hence, it is suggested that when studies report that the maximum force evoked electrically is smaller than that evoked voluntarily [Davies
et al, 1985; Herbert & Gandevia, 1999; James et al, 1994; Kent-Braun & Le Blanc, 1996], it is possible that the electrical stimulus to the target muscle did not activate the synergistic muscles.