2.1.1. Experimental importance o f upper limbs
A possible contributing cause o f the strength loss with age is decline in physical activity [Bassey et al, 1988; Rutherford & Jones, 1992; Avlund et al, 1994]. This could lead to loss o f muscle bulk [Phillips et al, 1992], fatty infiltration [Brooks & Faulkner, 1988], and also to change in muscle quality [Phillips et al, 1992]. All o f these contribute to strength loss but do not explain all o f the strength loss seen [Bruce et al, 1989b; Hughes
et al, 2001]. The contribution o f other factors (sometimes termed neural factors) to decline in specific force, is being further explored in various laboratories (see chapter
1).
Studies o f age-related strength declines are found to show differential effects o f ageing between muscle groups [Grimby & Saltin, 1983; Anianson et al, 1983; McDonagh et al,
1984]. For instance, in compiling the results o f the studies detailed in table 2 (Literature Review) it is apparent that whilst
• The quadriceps muscles lose strength at an average rate of 0.75% per year (0.70% in females and 0.80% in males),
• and the triceps surae muscles lose strength at an average rate o f 0.69% per year (0.67% in females and 0.70% in males),
• The yearly rate o f strength decline in the AP muscle is smaller at -0.59% (0.63% in females and 0.55% in males)
Table 6 shows that in the upper limb muscles, there tends to be a smaller decline in voluntary strength compared to the decline seen in lower limb muscles. The table also shows that aged muscle is generally weaker, slower, tetanises at lower fusion frequencies but is often more resistant to static fatigue. It is likely that the increased fatigue resistance is due to reduced capacity for energy usage. Indeed aged muscle shows decreased ATP concentrations [Pastoris et al, 2000]. Moreover, after fatiguing contractions, elderly muscle demonstrates a slower return to resting levels o f twitch
Chapter 2: Thesis aims
parameters [Klein et al, 1988], as well as decreased firing threshold rate [24% lower, Newton et al, 1988] compared to young.
Table 6. Comparison of ageing effects on muscle parameters of upper versus lower limbs.
Newton & Yemm, First dorsal
interosseous
Not given 142-
164% Not given 73% Not given
1986 & Newton e t a l , 1988 & Erim e t a l. 1999 Elbow flexors group 76% n s 107% n s 69% (at 40 Hz) 80% 111% n s McDonagh e l a l , 1984 Triceps surae 63% 124% 50% (at 20 Hz) 59% 81% n s McDonagh e t a l ,1984 - 6 7 ( a t 1 0 H z ) Triceps surae 78-80% n s 108- 131% -6 2 % ( a t 2 0 H z ) -6 3 % ( a t 5 0 H z ) 59-72% 60-78% Davies e t a l , 1983 & 1986
Notes. Data shown is the ratio of old/young force parameter expressed as percent
values. The fatigue index is the ratio of the force at 2min divided by force at time 0. ns
stands for non significant difference between young and old.
The age-related preferential loss of fibre type II is also greater in lower limbs. Grimby et al, (1982) report that this preferential loss is greater in vastus lateralis compared to
biceps brachii. This is probably due to postural activity of the leg muscles. Perhaps because walking becomes slower and less common, a large change in activity of type II fibres occurs in the legs and therefore a large loss of these fibres and a large loss of strength is seen. Indeed suggestions are that at moderate speeds, type I fibres are recruited [Ross et al, 2001] and since elderly seldom walk fast, they would therefore use type II fibres less and less.
This interpretation is supported by observations that strength training reverses preferential type II loss [Hakkinen et al, 1985a & b; Hortobagyi et al, 1996]. The increased type I versus type II fibres ratio, would also account, at least in part, for the increased fatigue resistance sometimes seen in elderly triceps surae [Davies et al, 1983
Chapter 2: Thesis aims
& 1986]. This may be because type I fibres are found to have greater volumes of mitochondria compared with type II fibres.
We propose that a reason for the differential ageing effect lies in the difference in amount o f usage since customary activities around the home involve the upper more than the lower body. The smaller atrophy of type II in the upper limbs may be linked to the fact that rapid movements, which are suggested to preferentially recruit type II [Ross
et al, 2001], are still carried out. With this, is combined the reluctance o f a substantial proportion o f the elderly population to “get out and about”. Anxiety, fear o f falling and lack o f an adequate social circle, it is thought, are some of the factors that contribute to the elderly not making enough use o f the lower limbs. Hence, it may therefore be useful to study upper limbs rather than lower limbs when trying to separate ageing effects on muscle strength with minimum input from disuse effects.
2.1.2. Human adductor pollicis (AP) and first dorsal interosseus
(FDI) muscles
2.1.2.1. Thumb adduction: anatomy and biomechanics
The muscles to be studied are the adductor pollicis {AP) and first dorsal interosseus
{FDI), two hand muscles supplied by the deep branch o f the ulnar nerve (8^ cervical and thoracic nerves), which is accessible at the wrist (see figure 10).
AP has two heads, one transversal and the other oblique, both having a distal attachment to the first metacarpal [Williams & Warwick, 1980]. The proximal attachment of the transverse head is to inter-metacarpal ligament between the 2"^ and the 3^** metacarpals. The oblique head attachment is from the shaft and from the base o f the 4^ metacarpal [Chang & Blair, 1985].
The interossei are a group of muscles that are arranged in both palmar and dorsal faces o f the hand. The FDI is the largest of the interossei and it is attached to the first and second metacarpals and to the lateral side of the proximal phalanx o f the index finger (see figure 10).
Chapter 2: Thesis aims ¥ 0 0 P w DC 1 rst dorsal Interosseus Ulnar nerve Pronator quadratuSS^yj j Flexor+metinaculum Thenar muscle^? thumb
Adductor qpllicis, Flexor pollicis longus
Extensor of index finger Long extensor of thumb Extensor of wrist Fourth l u m b n c a l Hypothenar muscles of little finger
Figure 10. Anatomy of the adductor pollicis and first dorsal interosseus muscles. On the left, are the muscles on the palmar surface, whilst on the right, the muscles of the palmar aspect of the hand are shown.
The muscle action is adduction of the thumb in which the proximal phalanx is caused to move in the sagittal plane (from the anatomical position) towards the plane of the palm (see figure 11 ). This principally is due to movement at the saddle joint between the trapezium and the first metacarpal, with the AP alone contributing -64% of the total thumb adduction force (as determined by the tetanus-to-maximum voluntary contraction ratio) [Belanger & Noel, 1995].
The muscles that may, to some extent affect the movement o f the first metacarpal in this plane (e.g. flexor pollicis longus and flexor pollicis brevis) or o f the thumb joint
{extensor pollicis longus and flexor hallucis longus), are not shown in this picture for simplification purposes.
Chapter 2: Thesis aims Adductor Dollicis (AP) First dorsal Interosseus (FDI) Phalanges
A .
r
Proximal Distal Middle THUMB 2 Metacarpal (AP is attached to 3'^'^ metacarpal on palmar face)Figure 11. Artistic im pression of thumb adduction in the plane of the hand. AP and FDI are highlighted in pink and red respectively. Strictly speaking, subjects are performing an adduction coupled with slight flexion of the thumb. This does not have any effect on the voluntary force as would be produced from a simple thumb adduction. This is t>ecause the force-generating capacity of thumb adductor muscles is similar in both the parallel and the perpendicular planes of adduction [Belanger & Noel, 1995].
• Contractile properties o f A P and FDI
The AP and FDI are frequently used in experimental physiology. Post-mortem examinations of the fibre type composition of the AP shows that approximately 80% of the fibres are of type I, the slow contracting fibre type. However the speed of contraction o f these fibres is faster than the speed o f type I fibres in other muscles (e.g. soleus (69% type I) and quadriceps (50% type I)) [Round et al, 1984]. The FDI on the other hand is more like the quadriceps in composition [Jonhson et al, 1973], i.e. -50% type II.
Chapter 2: Thesis aims
The two muscles {AP and FDI) have similar force-frequency relationships (electrically stimulated (lOOHz) tetanic force is -96% of maximum voluntary force, demonstrating that subjects are able to fully activate these muscles) [Merton 1954; Rutherford & Jones, 1988]. AP and FDI also share similar contractile properties including, twitch 63 and 58msec respectively), and lOOHz-tetanic (44 and 46msec) relaxation rates and fatigue indices (49.9 and 49.3% decline in force after 3mins o f electrically stimulated (40Hz) contraction) [Rutherford & Jones, 1988]. In fact, both prolonged voluntary and electrical stimuli cause AP strength to diminish (i.e. fatigue [Rutherford & Jones,
1988]). It was thus concluded that the early AP response to fatigue is mainly due to peripheral, rather than neural factors [Merton, 1954; Bigland-Ritchie et al, 1979; Duchateau & Hainaut, 1984a; Hainaut & Duchateau, 1989]. All these make the AP
suitable for study o f other factors causing change in muscle strength.
For simplicity (unless otherwise stated) the term AP will be used throughout to describe the combination o f AP and FDI.