It is known that there are two main strategies for increasing force. The first is by increased recruitment and the second by increased firing frequency. With increased
C h a p te r 5: E ffe c ts o f stre tc h velo city a iu i v o lu n ta ry c o n tr a c tio n on th e E /l ra tio
recruitment, more motor units are active but the frequency o f action potentials in each is about the same, whereas with increased firing frequency, the number of action potentials is increased, and so is the concentration of free calcium in muscle fibres ([Ca^^fibre) without there being increased number of active motor units.
The E/I ratio is lower in slow (e.g. 1.76 in rabbit soleus [Stienen et al; 1992]) compared to fast fibres (e.g. 2.10 in rabbit psoas [Stienen et al; 1992]) stretched at similar velocities. During steady force development, slower fibres are recruited first [McComas, 1996]. Therefore if force increase is due to increased recruitment, then E/T should be approximately constant since there is no a-priori reason to suppose that motor units (of the same fibre type) do not all have the same E/I ratio. If force increases due to increased frequency, the [Ca^^]obre will increase and E/I vdll also increase as shown in skinned fibres experiments. Stienen et al 1992 found that at levels o f [Ca^^] that produce 50% activation, the E/I ratio is increased by 20% (see Figure 60 below). What our own experiment shows is that throughout a range of activation levels (30-100%), E/I ratio falls exponentially with increased activation following the relationship described by:
Y = 2.181©’^’°®^* (equation 3)
Where y = increase in E/l & x = activation level on a scale of 0 to 1
The fact that E/I ratio increases with decreasing activation is very interesting and surprising. As above mentioned, our results would suggest that force increment is possibly by either changing the proportion o f fibre types, or simply by changing the frequency coding. To investigate these possibilities it might be interesting to compare the voluntary force E/I vs. F/Fmax relationship to the electrically induced responses (whereby stimulus is supramaximal).
To summarise our observations we have added our data to Figure 6 previously shown in chapter 1 (p42), and the result is shown in Figure 60 below. The figure suggests that, as higher force levels are achieved in human voluntary contractions (our own results shown as a red line) the effect on E/I is similar to the regression line describing the effects of Ca^^ on the E/I ratio. This therefore suggests that increased force development in the AP is done through higher [Ca^^]f,bre, implying higher frequencies of action potentials. A result which is in agreement with studies showing, from fine needle
C lu ip ter 5: E ffe c ts o f s tre tc h velo city a n d v o iu ttta ry c o n tr a c tio n o n th e E /l ra tio
electrodes inserted into the muscle, that in the AP no additional motor units are recruited at forces greater than 50% of maximal [Kukulka & Clamann, 1981].
60% 40% « 2 0% 0% 40% 0% 2 0% 60%
R e d u c tio n in Isom etric f o rce
A 15mM Pi in rabbit soieus [Stienen et ai, 1992] ■ Fatigued AP at 36.8 [De Ruiter & De Haan, 2000 &
2 0 0 1]
e Fatigued AP at 22.2 [De Ruiter & De Haan, 2000 &
2 0 0 1]
A 14.7degs Drop in AP temp [De Ruiter & De Haan,
2 0 0 0 & 2 0 0 1]
O Aged mouse soieus [Phiilips e ta l, 1991]
• Post m enopausai women AP [Phillips et ai, 1993 &1993b]
□ Ovariectom ised mice [Phillips et ai, 1993b]
X Reduced Ca2+ in rabbit soieus [Stienen et ai, 1992]
a Acidified pHi at pHe 6.5 Frog muscle [Curtin & Edman, 1989]
□ Acidified pHi at pHe 7.0 Frog muscle [Curtin & Edman, 1989]
Our result: Stretching at various F/Fm ax levels (y = 2.319e-0.019x)
A 15mM Pi in rabbit psoas [Stienen et al, 1992] a Reduced Ca2+ in rabbit psoas [Stienen et ai, 1992]
Line of identity
Figure 60. Effects of P|, pH, Ca^*, fatigue, temperature, age, oestrogen and voluntary activation on the E/l ratio. The results obtained In our experiments
are shown in red. The blackregression includes all data points except for pH
and studies. The greyregression includes only pH and 2+studies
Our results are not close to the line of identity in Figure 60. That line corresponds to a situation where although the E/I ratio changes, the maximum stretch force stays constant (i.e. only isometric force would be affected by the differences in activation level). Consider a model where the force enhancement due to stretch stays unchanged throughout various levels of F/Fmax, and only E/I varies (figure 61).
Hypothesis 2 (of fixed stretch force but changing E/I) was expressed mathematically, so that stretch force at MVF was the same as that at any value of F/Fmax- A further illustration that our results are different from this hypothesis is shown in Figure 61 where the theoretical line was drawn and found to be completely different from our observations (figure 61), thus Hypothesis 2 was rejected.
( ha p li’r S: E ffe c ts o f stretch v elo city a n il v o ln n ta ry c o n tra c tio n on th e E / l ra tio <+ 3 n 0) jO Q. c (0 tn c o u 0> ■D O S 4.0 3.2 2.4 1.6 0.8 0.0 60% 2 0 % 40% 80% 1 0 0% F/Fmax
Figure 61. Considering a hypothesis whereby E is fixed but E/l varies. The
observed average E/l value for each section of F / F m a x is shown in black.
Hypothesis 2 of a fixed stretch value is shown in bright blue (in this case, the constant stretch value was taken as the value of the stretch applied onto an MVF so that we could then observe the effects of decreased activation on E/l).
Figure 62 o f the cross-bridge cycle [Gordon et al, 2000] shows that because both calcium-dependent, and stretch-affected states are very close to one another, interactions between the two are expected.
Stretch of an active muscle
Actin + myosin I complex W eak binding
i
Stronger binding 6 8AM + A T P A M - A T P A-M • ADP • P , ► AM • ADP • P, . AHP. P. » AM**. ADP » AM
2 4 Î1 L ever arm motion and
r
8' /M + ATP «•— ► M -A T P M A D P .P , - * ^ M + ADP + P, Force production
Rapid re-association under physiological activation Thus under Ca regulation
Figure 62. The possible cause for the in vivo interactions between E/l and voluntary activation level. In green the effects of calcium and voluntary activation. In blue the key cross
bridges states. In redthe effect of stretch on the cross-bridge state.
C h a p te r 5: E ffects o f s tre tc h velo city a n d v o lu n ta ry c o n tra c tio n o n th e E /l ratio
Therefore, application o f our own observations and previous work conclusions onto the cross-bridge cycle suggests that:
a) At high firing frequencies i.e. when the contractile mechanism is powerfully activated, more cross-bridges are formed and if stretch occurs, the increased dissociation rate of cross-bridges does not significantly affect cross-bridge numbers due to increased cross-bridge re-association rate. This therefore results in a low E/I ratio since the stretch force would not be much higher than isometric tension.
b) During low voluntary activation however, the contractile mechanism requires sufficient time to form large numbers of cross-bridges. Should stretch occur, the increased dissociation rate o f cross-bridges is overcome by an even greater re association rate, or possibly increased force per cross-bridge and the result is a high E/I ratio.
•
Conclusions
Our findings have important implications for experimental human in vivo studies in that the stretch-induced force increase is a significant feature o f human muscle working in vivo. The in vivo E/I force ratio is proportional to both the velocity o f stretch at velocities below 435mm/sec, and the initial level o f preceding isometric contraction
(F/Fmax). E/I magnitude correlates more, though not significantly so, with actual values o f voluntary force (VF (N)), than it does with F/Fmax- The stretch-induced force enhancement is most probably due to effects at the cross-bridge level, and the various levels o f F/Fmax can be likened to the effects o f changing Ca^^ concentrations (pCa 5.0 causes MVF development (100%) in skinned rabbit psoas fibres [Fuchs & Wang,
1991]).